BIRD User's Guide: Protocols

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## 6. Protocols

## 6.1 BFD

### Introduction

Bidirectional Forwarding Detection (BFD) is not a routing protocol itself, it is an independent tool providing liveness and failure detection. Routing protocols like OSPF and BGP use integrated periodic "hello" messages to monitor liveness of neighbors, but detection times of these mechanisms are high (e.g. 40 seconds by default in OSPF, could be set down to several seconds). BFD offers universal, fast and low-overhead mechanism for failure detection, which could be attached to any routing protocol in an advisory role.

BFD consists of mostly independent BFD sessions. Each session monitors an unicast bidirectional path between two BFD-enabled routers. This is done by periodically sending control packets in both directions. BFD does not handle neighbor discovery, BFD sessions are created on demand by request of other protocols (like OSPF or BGP), which supply appropriate information like IP addresses and associated interfaces. When a session changes its state, these protocols are notified and act accordingly (e.g. break an OSPF adjacency when the BFD session went down).

BIRD implements basic BFD behavior as defined in RFC 5880 ftp://ftp.rfc-editor.org/in-notes/rfc5880.txt (some advanced features like the echo mode or authentication are not implemented), IP transport for BFD as defined in RFC 5881 ftp://ftp.rfc-editor.org/in-notes/rfc5881.txt and RFC 5883 ftp://ftp.rfc-editor.org/in-notes/rfc5883.txt and interaction with client protocols as defined in RFC 5882 ftp://ftp.rfc-editor.org/in-notes/rfc5882.txt.

Note that BFD implementation in BIRD is currently a new feature in development, expect some rough edges and possible UI and configuration changes in the future. Also note that we currently support at most one protocol instance.

BFD packets are sent with a dynamic source port number. Linux systems use by default a bit different dynamic port range than the IANA approved one (49152-65535). If you experience problems with compatibility, please adjust `/proc/sys/net/ipv4/ip_local_port_range`

### Configuration

BFD configuration consists mainly of multiple definitions of interfaces. Most BFD config options are session specific. When a new session is requested and dynamically created, it is configured from one of these definitions. For sessions to directly connected neighbors, `interface` definitions are chosen based on the interface associated with the session, while `multihop` definition is used for multihop sessions. If no definition is relevant, the session is just created with the default configuration. Therefore, an empty BFD configuration is often sufficient.

Note that to use BFD for other protocols like OSPF or BGP, these protocols also have to be configured to request BFD sessions, usually by `bfd` option.

Some of BFD session options require time value, which has to be specified with the appropriate unit: num `s`|`ms`|`us`. Although microseconds are allowed as units, practical minimum values are usually in order of tens of milliseconds.

    protocol bfd [<name>] {
            interface <interface pattern> {
                    interval <time>;
                    min rx interval <time>;
                    min tx interval <time>;
                    idle tx interval <time>;
                    multiplier <num>;
                    passive <switch>;
            };
            multihop {
                    interval <time>;
                    min rx interval <time>;
                    min tx interval <time>;
                    idle tx interval <time>;
                    multiplier <num>;
                    passive <switch>;
            };
            neighbor <ip> [dev "<interface>"] [local <ip>] [multihop <switch>];
    }

`interface pattern [, ...] { options }`

  Interface definitions allow to specify options for sessions associated with such interfaces and also may contain interface specific options. See interface common option for a detailed description of interface patterns. Note that contrary to the behavior of `interface` definitions of other protocols, BFD protocol would accept sessions (in default configuration) even on interfaces not covered by such definitions.

`multihop { options }`

  Multihop definitions allow to specify options for multihop BFD sessions, in the same manner as `interface` definitions are used for directly connected sessions. Currently only one such definition (for all multihop sessions) could be used.

`neighbor ip [dev "interface"] [local ip] [multihop switch]`

  BFD sessions are usually created on demand as requested by other protocols (like OSPF or BGP). This option allows to explicitly add a BFD session to the specified neighbor regardless of such requests.

  The session is identified by the IP address of the neighbor, with optional specification of used interface and local IP. By default the neighbor must be directly connected, unless the the session is configured as multihop. Note that local IP must be specified for multihop sessions.

Session specific options (part of `interface` and `multihop` definitions):

`interval time`

  BFD ensures availability of the forwarding path associated with the session by periodically sending BFD control packets in both directions. The rate of such packets is controlled by two options, `min rx interval` and `min tx interval` (see below). This option is just a shorthand to set both of these options together.

`min rx interval time`

  This option specifies the minimum RX interval, which is announced to the neighbor and used there to limit the neighbor's rate of generated BFD control packets. Default: 10 ms.

`min tx interval time`

  This option specifies the desired TX interval, which controls the rate of generated BFD control packets (together with `min rx interval` announced by the neighbor). Note that this value is used only if the BFD session is up, otherwise the value of `idle tx interval` is used instead. Default: 100 ms.

`idle tx interval time`

  In order to limit unnecessary traffic in cases where a neighbor is not available or not running BFD, the rate of generated BFD control packets is lower when the BFD session is not up. This option specifies the desired TX interval in such cases instead of `min tx interval`. Default: 1 s.

`multiplier num`

  Failure detection time for BFD sessions is based on established rate of BFD control packets (`min rx/tx interval`) multiplied by this multiplier, which is essentially (ignoring jitter) a number of missed packets after which the session is declared down. Note that rates and multipliers could be different in each direction of a BFD session. Default: 5.

`passive switch`

  Generally, both BFD session endpoinds try to establish the session by sending control packets to the other side. This option allows to enable passive mode, which means that the router does not send BFD packets until it has received one from the other side. Default: disabled.

### Example

    protocol bfd {
            interface "eth*" {
                    min rx interval 20 ms;
                    min tx interval 50 ms;
                    idle tx interval 300 ms;
            };
            interface "gre*" {
                    interval 200 ms;
                    multiplier 10;
                    passive;
            };
            multihop {
                    interval 200 ms;
                    multiplier 10;
            };

            neighbor 192.168.1.10;
            neighbor 192.168.2.2 dev "eth2";
            neighbor 192.168.10.1 local 192.168.1.1 multihop;
    }

## 6.2 BGP

The Border Gateway Protocol is the routing protocol used for backbone level routing in the today's Internet. Contrary to other protocols, its convergence does not rely on all routers following the same rules for route selection, making it possible to implement any routing policy at any router in the network, the only restriction being that if a router advertises a route, it must accept and forward packets according to it.

BGP works in terms of autonomous systems (often abbreviated as AS). Each AS is a part of the network with common management and common routing policy. It is identified by a unique 16-bit number (ASN). Routers within each AS usually exchange AS-internal routing information with each other using an interior gateway protocol (IGP, such as OSPF or RIP). Boundary routers at the border of the AS communicate global (inter-AS) network reachability information with their neighbors in the neighboring AS'es via exterior BGP (eBGP) and redistribute received information to other routers in the AS via interior BGP (iBGP).

Each BGP router sends to its neighbors updates of the parts of its routing table it wishes to export along with complete path information (a list of AS'es the packet will travel through if it uses the particular route) in order to avoid routing loops.

BIRD supports all requirements of the BGP4 standard as defined in RFC 4271 ftp://ftp.rfc-editor.org/in-notes/rfc4271.txt It also supports the community attributes (RFC 1997 ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt), capability negotiation (RFC 3392 ftp://ftp.rfc-editor.org/in-notes/rfc3392.txt), MD5 password authentication (RFC 2385 ftp://ftp.rfc-editor.org/in-notes/rfc2385.txt), extended communities (RFC 4360 ftp://ftp.rfc-editor.org/in-notes/rfc4360.txt), route reflectors (RFC 4456 ftp://ftp.rfc-editor.org/in-notes/rfc4456.txt), graceful restart (RFC 4724 ftp://ftp.rfc-editor.org/in-notes/rfc4724.txt), multiprotocol extensions (RFC 4760 ftp://ftp.rfc-editor.org/in-notes/rfc4760.txt), 4B AS numbers (RFC 4893 ftp://ftp.rfc-editor.org/in-notes/rfc4893.txt), and 4B AS numbers in extended communities (RFC 5668 ftp://ftp.rfc-editor.org/in-notes/rfc5668.txt).

For IPv6, it uses the standard multiprotocol extensions defined in RFC 4760 ftp://ftp.rfc-editor.org/in-notes/rfc4760.txt and applied to IPv6 according to RFC 2545 ftp://ftp.rfc-editor.org/in-notes/rfc2545.txt.

### Route selection rules

BGP doesn't have any simple metric, so the rules for selection of an optimal route among multiple BGP routes with the same preference are a bit more complex and they are implemented according to the following algorithm. It starts the first rule, if there are more "best" routes, then it uses the second rule to choose among them and so on.

- Prefer route with the highest Local Preference attribute.
- Prefer route with the shortest AS path.
- Prefer IGP origin over EGP and EGP origin over incomplete.
- Prefer the lowest value of the Multiple Exit Discriminator.
- Prefer routes received via eBGP over ones received via iBGP.
- Prefer routes with lower internal distance to a boundary router.
- Prefer the route with the lowest value of router ID of the advertising router.

### IGP routing table

BGP is mainly concerned with global network reachability and with routes to other autonomous systems. When such routes are redistributed to routers in the AS via BGP, they contain IP addresses of a boundary routers (in route attribute NEXT_HOP). BGP depends on existing IGP routing table with AS-internal routes to determine immediate next hops for routes and to know their internal distances to boundary routers for the purpose of BGP route selection. In BIRD, there is usually one routing table used for both IGP routes and BGP routes.

### Configuration

Each instance of the BGP corresponds to one neighboring router. This allows to set routing policy and all the other parameters differently for each neighbor using the following configuration parameters:

`local [ip] as number`

  Define which AS we are part of. (Note that contrary to other IP routers, BIRD is able to act as a router located in multiple AS'es simultaneously, but in such cases you need to tweak the BGP paths manually in the filters to get consistent behavior.) Optional `ip` argument specifies a source address, equivalent to the `source address` option (see below). This parameter is mandatory.

`neighbor ip [port number] as number`

  Define neighboring router this instance will be talking to and what AS it's located in. In case the neighbor is in the same AS as we are, we automatically switch to iBGP. This parameter is mandatory.

`direct`

  Specify that the neighbor is directly connected. The IP address of the neighbor must be from a directly reachable IP range (i.e. associated with one of your router's interfaces), otherwise the BGP session wouldn't start but it would wait for such interface to appear. The alternative is the `multihop` option. Default: enabled for eBGP.

`multihop [number]`

  Configure multihop BGP session to a neighbor that isn't directly connected. Accurately, this option should be used if the configured neighbor IP address does not match with any local network subnets. Such IP address have to be reachable through system routing table. The alternative is the `direct` option. For multihop BGP it is recommended to explicitly configure the source address to have it stable. Optional `number` argument can be used to specify the number of hops (used for TTL). Note that the number of networks (edges) in a path is counted; i.e., if two BGP speakers are separated by one router, the number of hops is 2. Default: enabled for iBGP.

`source address ip`

  Define local address we should use for next hop calculation and as a source address for the BGP session. Default: the address of the local end of the interface our neighbor is connected to.

`next hop self`

  Avoid calculation of the Next Hop attribute and always advertise our own source address as a next hop. This needs to be used only occasionally to circumvent misconfigurations of other routers. Default: disabled.

`next hop keep`

  Forward the received Next Hop attribute even in situations where the local address should be used instead, like when the route is sent to an interface with a different subnet. Default: disabled.

`missing lladdr self|drop|ignore`

  Next Hop attribute in BGP-IPv6 sometimes contains just the global IPv6 address, but sometimes it has to contain both global and link-local IPv6 addresses. This option specifies what to do if BIRD have to send both addresses but does not know link-local address. This situation might happen when routes from other protocols are exported to BGP, or when improper updates are received from BGP peers. `self` means that BIRD advertises its own local address instead. `drop` means that BIRD skips that prefixes and logs error. `ignore` means that BIRD ignores the problem and sends just the global address (and therefore forms improper BGP update). Default: `self`, unless BIRD is configured as a route server (option `rs client`), in that case default is `ignore`, because route servers usually do not forward packets themselves.

`gateway direct|recursive`

  For received routes, their `gw` (immediate next hop) attribute is computed from received `bgp_next_hop` attribute. This option specifies how it is computed. Direct mode means that the IP address from `bgp_next_hop` is used if it is directly reachable, otherwise the neighbor IP address is used. Recursive mode means that the gateway is computed by an IGP routing table lookup for the IP address from `bgp_next_hop`. Recursive mode is the behavior specified by the BGP standard. Direct mode is simpler, does not require any routes in a routing table, and was used in older versions of BIRD, but does not handle well nontrivial iBGP setups and multihop. Recursive mode is incompatible with sorted tables. Default: `direct` for direct sessions, `recursive` for multihop sessions.

`igp table name`

  Specifies a table that is used as an IGP routing table. Default: the same as the table BGP is connected to.

`bfd switch`

  BGP could use BFD protocol as an advisory mechanism for neighbor liveness and failure detection. If enabled, BIRD setups a BFD session for the BGP neighbor and tracks its liveness by it. This has an advantage of an order of magnitude lower detection times in case of failure. Note that BFD protocol also has to be configured, see BFD section for details. Default: disabled.

`ttl security switch`

  Use GTSM (RFC 5082 - the generalized TTL security mechanism). GTSM protects against spoofed packets by ignoring received packets with a smaller than expected TTL. To work properly, GTSM have to be enabled on both sides of a BGP session. If both `ttl security` and `multihop` options are enabled, `multihop` option should specify proper hop value to compute expected TTL. Kernel support required: Linux: 2.6.34+ (IPv4), 2.6.35+ (IPv6), BSD: since long ago, IPv4 only. Note that full (ICMP protection, for example) RFC 5082 support is provided by Linux only. Default: disabled.

`password string`

  Use this password for MD5 authentication of BGP sessions. Default: no authentication. Password has to be set by external utility (e.g. setkey(8)) on BSD systems.

`passive switch`

  Standard BGP behavior is both initiating outgoing connections and accepting incoming connections. In passive mode, outgoing connections are not initiated. Default: off.

`rr client`

  Be a route reflector and treat the neighbor as a route reflection client. Default: disabled.

`rr cluster id IPv4 address`

  Route reflectors use cluster id to avoid route reflection loops. When there is one route reflector in a cluster it usually uses its router id as a cluster id, but when there are more route reflectors in a cluster, these need to be configured (using this option) to use a common cluster id. Clients in a cluster need not know their cluster id and this option is not allowed for them. Default: the same as router id.

`rs client`

  Be a route server and treat the neighbor as a route server client. A route server is used as a replacement for full mesh EBGP routing in Internet exchange points in a similar way to route reflectors used in IBGP routing. BIRD does not implement obsoleted RFC 1863, but uses ad-hoc implementation, which behaves like plain EBGP but reduces modifications to advertised route attributes to be transparent (for example does not prepend its AS number to AS PATH attribute and keeps MED attribute). Default: disabled.

`secondary switch`

  Usually, if an export filter rejects a selected route, no other route is propagated for that network. This option allows to try the next route in order until one that is accepted is found or all routes for that network are rejected. This can be used for route servers that need to propagate different tables to each client but do not want to have these tables explicitly (to conserve memory). This option requires that the connected routing table is sorted. Default: off.

`add paths switch|rx|tx`

  Standard BGP can propagate only one path (route) per destination network (usually the selected one). This option controls the add-path protocol extension, which allows to advertise any number of paths to a destination. Note that to be active, add-path has to be enabled on both sides of the BGP session, but it could be enabled separately for RX and TX direction. When active, all available routes accepted by the export filter are advertised to the neighbor. Default: off.

`allow local as [number]`

  BGP prevents routing loops by rejecting received routes with the local AS number in the AS path. This option allows to loose or disable the check. Optional `number` argument can be used to specify the maximum number of local ASNs in the AS path that is allowed for received routes. When the option is used without the argument, the check is completely disabled and you should ensure loop-free behavior by some other means. Default: 0 (no local AS number allowed).

`enable route refresh switch`

  When BGP speaker changes its import filter, it has to re-examine all routes received from its neighbor against the new filter. As these routes might not be available, there is a BGP protocol extension Route Refresh (specified in RFC 2918) that allows BGP speaker to request re-advertisement of all routes from its neighbor. This option specifies whether BIRD advertises this capability and accepts such requests. Even when disabled, BIRD can send route refresh requests. Default: on.

`graceful restart switch|aware`

  When a BGP speaker restarts or crashes, neighbors will discard all received paths from the speaker, which disrupts packet forwarding even when the forwarding plane of the speaker remains intact. RFC 4724 specifies an optional graceful restart mechanism to alleviate this issue. This option controls the mechanism. It has three states: Disabled, when no support is provided. Aware, when the graceful restart support is announced and the support for restarting neighbors is provided, but no local graceful restart is allowed (i.e. receiving-only role). Enabled, when the full graceful restart support is provided (i.e. both restarting and receiving role). Note that proper support for local graceful restart requires also configuration of other protocols. Default: aware.

`graceful restart time number`

  The restart time is announced in the BGP graceful restart capability and specifies how long the neighbor would wait for the BGP session to re-establish after a restart before deleting stale routes. Default: 120 seconds.

`interpret communities switch`

  RFC 1997 demands that BGP speaker should process well-known communities like no-export (65535, 65281) or no-advertise (65535, 65282). For example, received route carrying a no-adverise community should not be advertised to any of its neighbors. If this option is enabled (which is by default), BIRD has such behavior automatically (it is evaluated when a route is exported to the BGP protocol just before the export filter). Otherwise, this integrated processing of well-known communities is disabled. In that case, similar behavior can be implemented in the export filter. Default: on.

`enable as4 switch`

  BGP protocol was designed to use 2B AS numbers and was extended later to allow 4B AS number. BIRD supports 4B AS extension, but by disabling this option it can be persuaded not to advertise it and to maintain old-style sessions with its neighbors. This might be useful for circumventing bugs in neighbor's implementation of 4B AS extension. Even when disabled (off), BIRD behaves internally as AS4-aware BGP router. Default: on.

`capabilities switch`

  Use capability advertisement to advertise optional capabilities. This is standard behavior for newer BGP implementations, but there might be some older BGP implementations that reject such connection attempts. When disabled (off), features that request it (4B AS support) are also disabled. Default: on, with automatic fallback to off when received capability-related error.

`advertise ipv4 switch`

  Advertise IPv4 multiprotocol capability. This is not a correct behavior according to the strict interpretation of RFC 4760, but it is widespread and required by some BGP implementations (Cisco and Quagga). This option is relevant to IPv4 mode with enabled capability advertisement only. Default: on.

`route limit number`

  The maximal number of routes that may be imported from the protocol. If the route limit is exceeded, the connection is closed with an error. Limit is currently implemented as `import limit number action restart`. This option is obsolete and it is replaced by import limit option. Default: no limit.

`disable after error switch`

  When an error is encountered (either locally or by the other side), disable the instance automatically and wait for an administrator to fix the problem manually. Default: off.

`hold time number`

  Time in seconds to wait for a Keepalive message from the other side before considering the connection stale. Default: depends on agreement with the neighboring router, we prefer 240 seconds if the other side is willing to accept it.

`startup hold time number`

  Value of the hold timer used before the routers have a chance to exchange open messages and agree on the real value. Default: 240 seconds.

`keepalive time number`

  Delay in seconds between sending of two consecutive Keepalive messages. Default: One third of the hold time.

`connect retry time number`

  Time in seconds to wait before retrying a failed attempt to connect. Default: 120 seconds.

`start delay time number`

  Delay in seconds between protocol startup and the first attempt to connect. Default: 5 seconds.

`error wait time number,number`

  Minimum and maximum delay in seconds between a protocol failure (either local or reported by the peer) and automatic restart. Doesn't apply when `disable after error` is configured. If consecutive errors happen, the delay is increased exponentially until it reaches the maximum. Default: 60, 300.

`error forget time number`

  Maximum time in seconds between two protocol failures to treat them as a error sequence which makes `error wait time` increase exponentially. Default: 300 seconds.

`path metric switch`

  Enable comparison of path lengths when deciding which BGP route is the best one. Default: on.

`med metric switch`

  Enable comparison of MED attributes (during best route selection) even between routes received from different ASes. This may be useful if all MED attributes contain some consistent metric, perhaps enforced in import filters of AS boundary routers. If this option is disabled, MED attributes are compared only if routes are received from the same AS (which is the standard behavior). Default: off.

`deterministic med switch`

  BGP route selection algorithm is often viewed as a comparison between individual routes (e.g. if a new route appears and is better than the current best one, it is chosen as the new best one). But the proper route selection, as specified by RFC 4271, cannot be fully implemented in that way. The problem is mainly in handling the MED attribute. BIRD, by default, uses an simplification based on individual route comparison, which in some cases may lead to temporally dependent behavior (i.e. the selection is dependent on the order in which routes appeared). This option enables a different (and slower) algorithm implementing proper RFC 4271 route selection, which is deterministic. Alternative way how to get deterministic behavior is to use `med metric` option. This option is incompatible with sorted tables. Default: off.

`igp metric switch`

  Enable comparison of internal distances to boundary routers during best route selection. Default: on.

`prefer older switch`

  Standard route selection algorithm breaks ties by comparing router IDs. This changes the behavior to prefer older routes (when both are external and from different peer). For details, see RFC 5004. Default: off.

`default bgp_med number`

  Value of the Multiple Exit Discriminator to be used during route selection when the MED attribute is missing. Default: 0.

`default bgp_local_pref number`

  A default value for the Local Preference attribute. It is used when a new Local Preference attribute is attached to a route by the BGP protocol itself (for example, if a route is received through eBGP and therefore does not have such attribute). Default: 100 (0 in pre-1.2.0 versions of BIRD).

### Attributes

BGP defines several route attributes. Some of them (those marked with ``I`' in the table below) are available on internal BGP connections only, some of them (marked with ``O`') are optional.

`bgppath `bgp_path``

  Sequence of AS numbers describing the AS path the packet will travel through when forwarded according to the particular route. In case of internal BGP it doesn't contain the number of the local AS.

`int `bgp_local_pref` [I]`

  Local preference value used for selection among multiple BGP routes (see the selection rules above). It's used as an additional metric which is propagated through the whole local AS.

`int `bgp_med` [O]`

  The Multiple Exit Discriminator of the route is an optional attribute which is used on external (inter-AS) links to convey to an adjacent AS the optimal entry point into the local AS. The received attribute is also propagated over internal BGP links. The attribute value is zeroed when a route is exported to an external BGP instance to ensure that the attribute received from a neighboring AS is not propagated to other neighboring ASes. A new value might be set in the export filter of an external BGP instance. See RFC 4451 ftp://ftp.rfc-editor.org/in-notes/rfc4451.txt for further discussion of BGP MED attribute.

`enum `bgp_origin``

  Origin of the route: either `ORIGIN_IGP` if the route has originated in an interior routing protocol or `ORIGIN_EGP` if it's been imported from the `EGP` protocol (nowadays it seems to be obsolete) or `ORIGIN_INCOMPLETE` if the origin is unknown.

`ip `bgp_next_hop``

  Next hop to be used for forwarding of packets to this destination. On internal BGP connections, it's an address of the originating router if it's inside the local AS or a boundary router the packet will leave the AS through if it's an exterior route, so each BGP speaker within the AS has a chance to use the shortest interior path possible to this point.

`void `bgp_atomic_aggr` [O]`

  This is an optional attribute which carries no value, but the sole presence of which indicates that the route has been aggregated from multiple routes by some router on the path from the originator.

`clist `bgp_community` [O]`

  List of community values associated with the route. Each such value is a pair (represented as a `pair` data type inside the filters) of 16-bit integers, the first of them containing the number of the AS which defines the community and the second one being a per-AS identifier. There are lots of uses of the community mechanism, but generally they are used to carry policy information like "don't export to USA peers". As each AS can define its own routing policy, it also has a complete freedom about which community attributes it defines and what will their semantics be.

`eclist `bgp_ext_community` [O]`

  List of extended community values associated with the route. Extended communities have similar usage as plain communities, but they have an extended range (to allow 4B ASNs) and a nontrivial structure with a type field. Individual community values are represented using an `ec` data type inside the filters.

`quad `bgp_originator_id` [I, O]`

  This attribute is created by the route reflector when reflecting the route and contains the router ID of the originator of the route in the local AS.

`clist `bgp_cluster_list` [I, O]`

  This attribute contains a list of cluster IDs of route reflectors. Each route reflector prepends its cluster ID when reflecting the route.

### Example

    protocol bgp {
            local as 65000;                      # Use a private AS number
            neighbor 198.51.100.130 as 64496;    # Our neighbor ...
            multihop;                            # ... which is connected indirectly
            export filter {                      # We use non-trivial export rules
                    if source = RTS_STATIC then { # Export only static routes
                            # Assign our community
                            bgp_community.add((65000,64501));
                            # Artificially increase path length
                            # by advertising local AS number twice
                            if bgp_path ~ [= 65000 =] then
                                    bgp_path.prepend(65000);
                            accept;
                    }
                    reject;
            };
            import all;
            source address 198.51.100.14;   # Use a non-standard source address
    }

## 6.3 Device

The Device protocol is not a real routing protocol. It doesn't generate any routes and it only serves as a module for getting information about network interfaces from the kernel.

Except for very unusual circumstances, you probably should include this protocol in the configuration since almost all other protocols require network interfaces to be defined for them to work with.

### Configuration

`scan time number`

  Time in seconds between two scans of the network interface list. On systems where we are notified about interface status changes asynchronously (such as newer versions of Linux), we need to scan the list only in order to avoid confusion by lost notification messages, so the default time is set to a large value.

`primary [ "mask" ] prefix`

  If a network interface has more than one network address, BIRD has to choose one of them as a primary one. By default, BIRD chooses the lexicographically smallest address as the primary one.

  This option allows to specify which network address should be chosen as a primary one. Network addresses that match prefix are preferred to non-matching addresses. If more `primary` options are used, the first one has the highest preference. If "mask" is specified, then such `primary` option is relevant only to matching network interfaces.

  In all cases, an address marked by operating system as secondary cannot be chosen as the primary one.

As the Device protocol doesn't generate any routes, it cannot have any attributes. Example configuration looks like this:

    protocol device {
            scan time 10;           # Scan the interfaces often
            primary "eth0" 192.168.1.1;
            primary 192.168.0.0/16;
    }

## 6.4 Direct

The Direct protocol is a simple generator of device routes for all the directly connected networks according to the list of interfaces provided by the kernel via the Device protocol.

The question is whether it is a good idea to have such device routes in BIRD routing table. OS kernel usually handles device routes for directly connected networks by itself so we don't need (and don't want) to export these routes to the kernel protocol. OSPF protocol creates device routes for its interfaces itself and BGP protocol is usually used for exporting aggregate routes. Although there are some use cases that use the direct protocol (like abusing eBGP as an IGP routing protocol), in most cases it is not needed to have these device routes in BIRD routing table and to use the direct protocol.

There is one notable case when you definitely want to use the direct protocol -- running BIRD on BSD systems. Having high priority device routes for directly connected networks from the direct protocol protects kernel device routes from being overwritten or removed by IGP routes during some transient network conditions, because a lower priority IGP route for the same network is not exported to the kernel routing table. This is an issue on BSD systems only, as on Linux systems BIRD cannot change non-BIRD route in the kernel routing table.

The only configurable thing about direct is what interfaces it watches:

`interface pattern [, ...]`

  By default, the Direct protocol will generate device routes for all the interfaces available. If you want to restrict it to some subset of interfaces or addresses (e.g. if you're using multiple routing tables for policy routing and some of the policy domains don't contain all interfaces), just use this clause. See interface common option for detailed description. The Direct protocol uses extended interface clauses.

Direct device routes don't contain any specific attributes.

Example config might look like this:

    protocol direct {
            interface "-arc*", "*";         # Exclude the ARCnets
    }

## 6.5 Kernel

The Kernel protocol is not a real routing protocol. Instead of communicating with other routers in the network, it performs synchronization of BIRD's routing tables with the OS kernel. Basically, it sends all routing table updates to the kernel and from time to time it scans the kernel tables to see whether some routes have disappeared (for example due to unnoticed up/down transition of an interface) or whether an `alien' route has been added by someone else (depending on the `learn` switch, such routes are either ignored or accepted to our table).

Unfortunately, there is one thing that makes the routing table synchronization a bit more complicated. In the kernel routing table there are also device routes for directly connected networks. These routes are usually managed by OS itself (as a part of IP address configuration) and we don't want to touch that. They are completely ignored during the scan of the kernel tables and also the export of device routes from BIRD tables to kernel routing tables is restricted to prevent accidental interference. This restriction can be disabled using `device routes` switch.

If your OS supports only a single routing table, you can configure only one instance of the Kernel protocol. If it supports multiple tables (in order to allow policy routing; such an OS is for example Linux), you can run as many instances as you want, but each of them must be connected to a different BIRD routing table and to a different kernel table.

Because the kernel protocol is partially integrated with the connected routing table, there are two limitations - it is not possible to connect more kernel protocols to the same routing table and changing route destination (gateway) in an export filter of a kernel protocol does not work. Both limitations can be overcome using another routing table and the pipe protocol.

### Configuration

`persist switch`

  Tell BIRD to leave all its routes in the routing tables when it exits (instead of cleaning them up).

`scan time number`

  Time in seconds between two consecutive scans of the kernel routing table.

`learn switch`

  Enable learning of routes added to the kernel routing tables by other routing daemons or by the system administrator. This is possible only on systems which support identification of route authorship.

`device routes switch`

  Enable export of device routes to the kernel routing table. By default, such routes are rejected (with the exception of explicitly configured device routes from the static protocol) regardless of the export filter to protect device routes in kernel routing table (managed by OS itself) from accidental overwriting or erasing.

`kernel table number`

  Select which kernel table should this particular instance of the Kernel protocol work with. Available only on systems supporting multiple routing tables.

`graceful restart switch`

  Participate in graceful restart recovery. If this option is enabled and a graceful restart recovery is active, the Kernel protocol will defer synchronization of routing tables until the end of the recovery. Note that import of kernel routes to BIRD is not affected.

### Attributes

The Kernel protocol defines several attributes. These attributes are translated to appropriate system (and OS-specific) route attributes. We support these attributes:

`int `krt_source``

  The original source of the imported kernel route. The value is system-dependent. On Linux, it is a value of the protocol field of the route. See /etc/iproute2/rt_protos for common values. On BSD, it is based on STATIC and PROTOx flags. The attribute is read-only.

`int `krt_metric``

  The kernel metric of the route. When multiple same routes are in a kernel routing table, the Linux kernel chooses one with lower metric.

`ip `krt_prefsrc``

  (Linux) The preferred source address. Used in source address selection for outgoing packets. Have to be one of IP addresses of the router.

`int `krt_realm``

  (Linux) The realm of the route. Can be used for traffic classification.

### Example

A simple configuration can look this way:

    protocol kernel {
            export all;
    }

Or for a system with two routing tables:

    protocol kernel {               # Primary routing table
            learn;                  # Learn alien routes from the kernel
            persist;                # Don't remove routes on bird shutdown
            scan time 10;           # Scan kernel routing table every 10 seconds
            import all;
            export all;
    }

    protocol kernel {               # Secondary routing table
            table auxtable;
            kernel table 100;
            export all;
    }

## 6.6 OSPF

### Introduction

Open Shortest Path First (OSPF) is a quite complex interior gateway protocol. The current IPv4 version (OSPFv2) is defined in RFC 2328 ftp://ftp.rfc-editor.org/in-notes/rfc2328.txt and the current IPv6 version (OSPFv3) is defined in RFC 5340 ftp://ftp.rfc-editor.org/in-notes/rfc5340.txt It's a link state (a.k.a. shortest path first) protocol -- each router maintains a database describing the autonomous system's topology. Each participating router has an identical copy of the database and all routers run the same algorithm calculating a shortest path tree with themselves as a root. OSPF chooses the least cost path as the best path.

In OSPF, the autonomous system can be split to several areas in order to reduce the amount of resources consumed for exchanging the routing information and to protect the other areas from incorrect routing data. Topology of the area is hidden to the rest of the autonomous system.

Another very important feature of OSPF is that it can keep routing information from other protocols (like Static or BGP) in its link state database as external routes. Each external route can be tagged by the advertising router, making it possible to pass additional information between routers on the boundary of the autonomous system.

OSPF quickly detects topological changes in the autonomous system (such as router interface failures) and calculates new loop-free routes after a short period of convergence. Only a minimal amount of routing traffic is involved.

Each router participating in OSPF routing periodically sends Hello messages to all its interfaces. This allows neighbors to be discovered dynamically. Then the neighbors exchange theirs parts of the link state database and keep it identical by flooding updates. The flooding process is reliable and ensures that each router detects all changes.

### Configuration

In the main part of configuration, there can be multiple definitions of OSPF areas, each with a different id. These definitions includes many other switches and multiple definitions of interfaces. Definition of interface may contain many switches and constant definitions and list of neighbors on nonbroadcast networks.

    protocol ospf <name> {
            rfc1583compat <switch>;
            stub router <switch>;
            tick <num>;
            ecmp <switch> [limit <num>];
            merge external <switch>;
            area <id> {
                    stub;
                    nssa;
                    summary <switch>;
                    default nssa <switch>;
                    default cost <num>;
                    default cost2 <num>;
                    translator <switch>;
                    translator stability <num>;

                    networks {
                            <prefix>;
                            <prefix> hidden;
                    }
                    external {
                            <prefix>;
                            <prefix> hidden;
                            <prefix> tag <num>;
                    }
                    stubnet <prefix>;
                    stubnet <prefix> {
                            hidden <switch>;
                            summary <switch>;
                            cost <num>;
                    }
                    interface <interface pattern> [instance <num>] {
                            cost <num>;
                            stub <switch>;
                            hello <num>;
                            poll <num>;
                            retransmit <num>;
                            priority <num>;
                            wait <num>;
                            dead count <num>;
                            dead <num>;
                            secondary <switch>;
                            rx buffer [normal|large|<num>];
                            tx length <num>;
                            type [broadcast|bcast|pointopoint|ptp|
                                    nonbroadcast|nbma|pointomultipoint|ptmp];
                            strict nonbroadcast <switch>;
                            real broadcast <switch>;
                            ptp netmask <switch>;
                            check link <switch>;
                            bfd <switch>;
                            ecmp weight <num>;
                            ttl security [<switch>; | tx only]
                            tx class|dscp <num>;
                            tx priority <num>;
                            authentication [none|simple|cryptographic];
                            password "<text>";
                            password "<text>" {
                                    id <num>;
                                    generate from "<date>";
                                    generate to "<date>";
                                    accept from "<date>";
                                    accept to "<date>";
                            };
                            neighbors {
                                    <ip>;
                                    <ip> eligible;
                            };
                    };
                    virtual link <id> [instance <num>] {
                            hello <num>;
                            retransmit <num>;
                            wait <num>;
                            dead count <num>;
                            dead <num>;
                            authentication [none|simple|cryptographic];
                            password "<text>";
                    };
            };
    }

`rfc1583compat switch`

  This option controls compatibility of routing table calculation with RFC 1583 ftp://ftp.rfc-editor.org/in-notes/rfc1583.txt. Default value is no.

`stub router switch`

  This option configures the router to be a stub router, i.e., a router that participates in the OSPF topology but does not allow transit traffic. In OSPFv2, this is implemented by advertising maximum metric for outgoing links, as suggested by RFC 3137 ftp://ftp.rfc-editor.org/in-notes/rfc3137.txt. In OSPFv3, the stub router behavior is announced by clearing the R-bit in the router LSA. Default value is no.

`tick num`

  The routing table calculation and clean-up of areas' databases is not performed when a single link state change arrives. To lower the CPU utilization, it's processed later at periodical intervals of num seconds. The default value is 1.

`ecmp switch [limit number]`

  This option specifies whether OSPF is allowed to generate ECMP (equal-cost multipath) routes. Such routes are used when there are several directions to the destination, each with the same (computed) cost. This option also allows to specify a limit on maximal number of nexthops in one route. By default, ECMP is disabled. If enabled, default value of the limit is 16.

`merge external switch`

  This option specifies whether OSPF should merge external routes from different routers/LSAs for the same destination. When enabled together with `ecmp`, equal-cost external routes will be combined to multipath routes in the same way as regular routes. When disabled, external routes from different LSAs are treated as separate even if they represents the same destination. Default value is no.

`area id`

  This defines an OSPF area with given area ID (an integer or an IPv4 address, similarly to a router ID). The most important area is the backbone (ID 0) to which every other area must be connected.

`stub`

  This option configures the area to be a stub area. External routes are not flooded into stub areas. Also summary LSAs can be limited in stub areas (see option `summary`). By default, the area is not a stub area.

`nssa`

  This option configures the area to be a NSSA (Not-So-Stubby Area). NSSA is a variant of a stub area which allows a limited way of external route propagation. Global external routes are not propagated into a NSSA, but an external route can be imported into NSSA as a (area-wide) NSSA-LSA (and possibly translated and/or aggregated on area boundary). By default, the area is not NSSA.

`summary switch`

  This option controls propagation of summary LSAs into stub or NSSA areas. If enabled, summary LSAs are propagated as usual, otherwise just the default summary route (0.0.0.0/0) is propagated (this is sometimes called totally stubby area). If a stub area has more area boundary routers, propagating summary LSAs could lead to more efficient routing at the cost of larger link state database. Default value is no.

`default nssa switch`

  When `summary` option is enabled, default summary route is no longer propagated to the NSSA. In that case, this option allows to originate default route as NSSA-LSA to the NSSA. Default value is no.

`default cost num`

  This option controls the cost of a default route propagated to stub and NSSA areas. Default value is 1000.

`default cost2 num`

  When a default route is originated as NSSA-LSA, its cost can use either type 1 or type 2 metric. This option allows to specify the cost of a default route in type 2 metric. By default, type 1 metric (option `default cost`) is used.

`translator switch`

  This option controls translation of NSSA-LSAs into external LSAs. By default, one translator per NSSA is automatically elected from area boundary routers. If enabled, this area boundary router would unconditionally translate all NSSA-LSAs regardless of translator election. Default value is no.

`translator stability num`

  This option controls the translator stability interval (in seconds). When the new translator is elected, the old one keeps translating until the interval is over. Default value is 40.

`networks { set }`

  Definition of area IP ranges. This is used in summary LSA origination. Hidden networks are not propagated into other areas.

`external { set }`

  Definition of external area IP ranges for NSSAs. This is used for NSSA-LSA translation. Hidden networks are not translated into external LSAs. Networks can have configured route tag.

`stubnet prefix { options }`

  Stub networks are networks that are not transit networks between OSPF routers. They are also propagated through an OSPF area as a part of a link state database. By default, BIRD generates a stub network record for each primary network address on each OSPF interface that does not have any OSPF neighbors, and also for each non-primary network address on each OSPF interface. This option allows to alter a set of stub networks propagated by this router.

  Each instance of this option adds a stub network with given network prefix to the set of propagated stub network, unless option `hidden` is used. It also suppresses default stub networks for given network prefix. When option `summary` is used, also default stub networks that are subnetworks of given stub network are suppressed. This might be used, for example, to aggregate generated stub networks.

`interface pattern [instance num]`

  Defines that the specified interfaces belong to the area being defined. See interface common option for detailed description. In OSPFv2, extended interface clauses are used, because OSPFv2 handles each network prefix as a separate virtual interface. In OSPFv3, you can specify instance ID for that interface description, so it is possible to have several instances of that interface with different options or even in different areas.

`virtual link id [instance num]`

  Virtual link to router with the router id. Virtual link acts as a point-to-point interface belonging to backbone. The actual area is used as transport area. This item cannot be in the backbone. In OSPFv3, you could also use several virtual links to one destination with different instance IDs.

`cost num`

  Specifies output cost (metric) of an interface. Default value is 10.

`stub switch`

  If set to interface it does not listen to any packet and does not send any hello. Default value is no.

`hello num`

  Specifies interval in seconds between sending of Hello messages. Beware, all routers on the same network need to have the same hello interval. Default value is 10.

`poll num`

  Specifies interval in seconds between sending of Hello messages for some neighbors on NBMA network. Default value is 20.

`retransmit num`

  Specifies interval in seconds between retransmissions of unacknowledged updates. Default value is 5.

`priority num`

  On every multiple access network (e.g., the Ethernet) Designed Router and Backup Designed router are elected. These routers have some special functions in the flooding process. Higher priority increases preferences in this election. Routers with priority 0 are not eligible. Default value is 1.

`wait num`

  After start, router waits for the specified number of seconds between starting election and building adjacency. Default value is 40.

`dead count num`

  When the router does not receive any messages from a neighbor in dead count*hello seconds, it will consider the neighbor down.

`dead num`

  When the router does not receive any messages from a neighbor in dead seconds, it will consider the neighbor down. If both directives `dead count` and `dead` are used, `dead` has precendence.

`secondary switch`

  On BSD systems, older versions of BIRD supported OSPFv2 only for the primary IP address of an interface, other IP ranges on the interface were handled as stub networks. Since v1.4.1, regular operation on secondary IP addresses is supported, but disabled by default for compatibility. This option allows to enable it. The option is a transitional measure, will be removed in the next major release as the behavior will be changed. On Linux systems, the option is irrelevant, as operation on non-primary addresses is already the regular behavior.

`rx buffer num`

  This option allows to specify the size of buffers used for packet processing. The buffer size should be bigger than maximal size of any packets. By default, buffers are dynamically resized as needed, but a fixed value could be specified. Value `large` means maximal allowed packet size - 65535.

`tx length num`

  Transmitted OSPF messages that contain large amount of information are segmented to separate OSPF packets to avoid IP fragmentation. This option specifies the soft ceiling for the length of generated OSPF packets. Default value is the MTU of the network interface. Note that larger OSPF packets may still be generated if underlying OSPF messages cannot be splitted (e.g. when one large LSA is propagated).

`type broadcast|bcast`

  BIRD detects a type of a connected network automatically, but sometimes it's convenient to force use of a different type manually. On broadcast networks (like ethernet), flooding and Hello messages are sent using multicasts (a single packet for all the neighbors). A designated router is elected and it is responsible for synchronizing the link-state databases and originating network LSAs. This network type cannot be used on physically NBMA networks and on unnumbered networks (networks without proper IP prefix).

`type pointopoint|ptp`

  Point-to-point networks connect just 2 routers together. No election is performed and no network LSA is originated, which makes it simpler and faster to establish. This network type is useful not only for physically PtP ifaces (like PPP or tunnels), but also for broadcast networks used as PtP links. This network type cannot be used on physically NBMA networks.

`type nonbroadcast|nbma`

  On NBMA networks, the packets are sent to each neighbor separately because of lack of multicast capabilities. Like on broadcast networks, a designated router is elected, which plays a central role in propagation of LSAs. This network type cannot be used on unnumbered networks.

`type pointomultipoint|ptmp`

  This is another network type designed to handle NBMA networks. In this case the NBMA network is treated as a collection of PtP links. This is useful if not every pair of routers on the NBMA network has direct communication, or if the NBMA network is used as an (possibly unnumbered) PtP link.

`strict nonbroadcast switch`

  If set, don't send hello to any undefined neighbor. This switch is ignored on other than NBMA or PtMP networks. Default value is no.

`real broadcast switch`

  In `type broadcast` or `type ptp` network configuration, OSPF packets are sent as IP multicast packets. This option changes the behavior to using old-fashioned IP broadcast packets. This may be useful as a workaround if IP multicast for some reason does not work or does not work reliably. This is a non-standard option and probably is not interoperable with other OSPF implementations. Default value is no.

`ptp netmask switch`

  In `type ptp` network configurations, OSPFv2 implementations should ignore received netmask field in hello packets and should send hello packets with zero netmask field on unnumbered PtP links. But some OSPFv2 implementations perform netmask checking even for PtP links. This option specifies whether real netmask will be used in hello packets on `type ptp` interfaces. You should ignore this option unless you meet some compatibility problems related to this issue. Default value is no for unnumbered PtP links, yes otherwise.

`check link switch`

  If set, a hardware link state (reported by OS) is taken into consideration. When a link disappears (e.g. an ethernet cable is unplugged), neighbors are immediately considered unreachable and only the address of the iface (instead of whole network prefix) is propagated. It is possible that some hardware drivers or platforms do not implement this feature. Default value is no.

`bfd switch`

  OSPF could use BFD protocol as an advisory mechanism for neighbor liveness and failure detection. If enabled, BIRD setups a BFD session for each OSPF neighbor and tracks its liveness by it. This has an advantage of an order of magnitude lower detection times in case of failure. Note that BFD protocol also has to be configured, see BFD section for details. Default value is no.

`ttl security [switch | tx only]`

  TTL security is a feature that protects routing protocols from remote spoofed packets by using TTL 255 instead of TTL 1 for protocol packets destined to neighbors. Because TTL is decremented when packets are forwarded, it is non-trivial to spoof packets with TTL 255 from remote locations. Note that this option would interfere with OSPF virtual links.

  If this option is enabled, the router will send OSPF packets with TTL 255 and drop received packets with TTL less than 255. If this option si set to `tx only`, TTL 255 is used for sent packets, but is not checked for received packets. Default value is no.

`tx class|dscp|priority num`

  These options specify the ToS/DiffServ/Traffic class/Priority of the outgoing OSPF packets. See tx class common option for detailed description.

`ecmp weight num`

  When ECMP (multipath) routes are allowed, this value specifies a relative weight used for nexthops going through the iface. Allowed values are 1-256. Default value is 1.

`authentication none`

  No passwords are sent in OSPF packets. This is the default value.

`authentication simple`

  Every packet carries 8 bytes of password. Received packets lacking this password are ignored. This authentication mechanism is very weak.

`authentication cryptographic`

  16-byte long MD5 digest is appended to every packet. For the digest generation 16-byte long passwords are used. Those passwords are not sent via network, so this mechanism is quite secure. Packets can still be read by an attacker.

`password "text"`

  An 8-byte or 16-byte password used for authentication. See password common option for detailed description.

`neighbors { set }`

  A set of neighbors to which Hello messages on NBMA or PtMP networks are to be sent. For NBMA networks, some of them could be marked as eligible. In OSPFv3, link-local addresses should be used, using global ones is possible, but it is nonstandard and might be problematic. And definitely, link-local and global addresses should not be mixed.

### Attributes

OSPF defines four route attributes. Each internal route has a `metric`.

Metric is ranging from 1 to infinity (65535). External routes use `metric type 1` or `metric type 2`. A `metric of type 1` is comparable with internal `metric`, a `metric of type 2` is always longer than any `metric of type 1` or any `internal metric`. `Internal metric` or `metric of type 1` is stored in attribute `ospf_metric1`, `metric type 2` is stored in attribute `ospf_metric2`. If you specify both metrics only metric1 is used.

Each external route can also carry attribute `ospf_tag` which is a 32-bit integer which is used when exporting routes to other protocols; otherwise, it doesn't affect routing inside the OSPF domain at all. The fourth attribute `ospf_router_id` is a router ID of the router advertising that route / network. This attribute is read-only. Default is `ospf_metric2 = 10000` and `ospf_tag = 0`.

### Example

    protocol ospf MyOSPF {
            rfc1583compat yes;
            tick 2;
            export filter {
                    if source = RTS_BGP then {
                            ospf_metric1 = 100;
                            accept;
                    }
                    reject;
            };
            area 0.0.0.0 {
                    interface "eth*" {
                            cost 11;
                            hello 15;
                            priority 100;
                            retransmit 7;
                            authentication simple;
                            password "aaa";
                    };
                    interface "ppp*" {
                            cost 100;
                            authentication cryptographic;
                            password "abc" {
                                    id 1;
                                    generate to "22-04-2003 11:00:06";
                                    accept from "17-01-2001 12:01:05";
                            };
                            password "def" {
                                    id 2;
                                    generate to "22-07-2005 17:03:21";
                                    accept from "22-02-2001 11:34:06";
                            };
                    };
                    interface "arc0" {
                            cost 10;
                            stub yes;
                    };
                    interface "arc1";
            };
            area 120 {
                    stub yes;
                    networks {
                            172.16.1.0/24;
                            172.16.2.0/24 hidden;
                    }
                    interface "-arc0" , "arc*" {
                            type nonbroadcast;
                            authentication none;
                            strict nonbroadcast yes;
                            wait 120;
                            poll 40;
                            dead count 8;
                            neighbors {
                                    192.168.120.1 eligible;
                                    192.168.120.2;
                                    192.168.120.10;
                            };
                    };
            };
    }

## 6.7 Pipe

### Introduction

The Pipe protocol serves as a link between two routing tables, allowing routes to be passed from a table declared as primary (i.e., the one the pipe is connected to using the `table` configuration keyword) to the secondary one (declared using `peer table`) and vice versa, depending on what's allowed by the filters. Export filters control export of routes from the primary table to the secondary one, import filters control the opposite direction.

The Pipe protocol may work in the transparent mode mode or in the opaque mode. In the transparent mode, the Pipe protocol retransmits all routes from one table to the other table, retaining their original source and attributes. If import and export filters are set to accept, then both tables would have the same content. The transparent mode is the default mode.

In the opaque mode, the Pipe protocol retransmits optimal route from one table to the other table in a similar way like other protocols send and receive routes. Retransmitted route will have the source set to the Pipe protocol, which may limit access to protocol specific route attributes. This mode is mainly for compatibility, it is not suggested for new configs. The mode can be changed by `mode` option.

The primary use of multiple routing tables and the Pipe protocol is for policy routing, where handling of a single packet doesn't depend only on its destination address, but also on its source address, source interface, protocol type and other similar parameters. In many systems (Linux being a good example), the kernel allows to enforce routing policies by defining routing rules which choose one of several routing tables to be used for a packet according to its parameters. Setting of these rules is outside the scope of BIRD's work (on Linux, you can use the `ip` command), but you can create several routing tables in BIRD, connect them to the kernel ones, use filters to control which routes appear in which tables and also you can employ the Pipe protocol for exporting a selected subset of one table to another one.

### Configuration

`peer table table`

  Defines secondary routing table to connect to. The primary one is selected by the `table` keyword.

`mode opaque|transparent`

  Specifies the mode for the pipe to work in. Default is transparent.

### Attributes

The Pipe protocol doesn't define any route attributes.

### Example

Let's consider a router which serves as a boundary router of two different autonomous systems, each of them connected to a subset of interfaces of the router, having its own exterior connectivity and wishing to use the other AS as a backup connectivity in case of outage of its own exterior line.

Probably the simplest solution to this situation is to use two routing tables (we'll call them `as1` and `as2`) and set up kernel routing rules, so that packets having arrived from interfaces belonging to the first AS will be routed according to `as1` and similarly for the second AS. Thus we have split our router to two logical routers, each one acting on its own routing table, having its own routing protocols on its own interfaces. In order to use the other AS's routes for backup purposes, we can pass the routes between the tables through a Pipe protocol while decreasing their preferences and correcting their BGP paths to reflect the AS boundary crossing.

    table as1;                              # Define the tables
    table as2;

    protocol kernel kern1 {                 # Synchronize them with the kernel
            table as1;
            kernel table 1;
    }

    protocol kernel kern2 {
            table as2;
            kernel table 2;
    }

    protocol bgp bgp1 {                     # The outside connections
            table as1;
            local as 1;
            neighbor 192.168.0.1 as 1001;
            export all;
            import all;
    }

    protocol bgp bgp2 {
            table as2;
            local as 2;
            neighbor 10.0.0.1 as 1002;
            export all;
            import all;
    }

    protocol pipe {                         # The Pipe
            table as1;
            peer table as2;
            export filter {
                    if net ~ [ 1.0.0.0/8+] then {   # Only AS1 networks
                            if preference>10 then preference = preference-10;
                            if source=RTS_BGP then bgp_path.prepend(1);
                            accept;
                    }
                    reject;
            };
            import filter {
                    if net ~ [ 2.0.0.0/8+] then {   # Only AS2 networks
                            if preference>10 then preference = preference-10;
                            if source=RTS_BGP then bgp_path.prepend(2);
                            accept;
                    }
                    reject;
            };
    }

## 6.8 RAdv

### Introduction

The RAdv protocol is an implementation of Router Advertisements, which are used in the IPv6 stateless autoconfiguration. IPv6 routers send (in irregular time intervals or as an answer to a request) advertisement packets to connected networks. These packets contain basic information about a local network (e.g. a list of network prefixes), which allows network hosts to autoconfigure network addresses and choose a default route. BIRD implements router behavior as defined in RFC 4861 ftp://ftp.rfc-editor.org/in-notes/rfc4861.txt and also the DNS extensions from RFC 6106 ftp://ftp.rfc-editor.org/in-notes/rfc6106.txt.

### Configuration

There are several classes of definitions in RAdv configuration -- interface definitions, prefix definitions and DNS definitions:

`interface pattern [, ...] { options }`

  Interface definitions specify a set of interfaces on which the protocol is activated and contain interface specific options. See interface common options for detailed description.

`prefix prefix { options }`

  Prefix definitions allow to modify a list of advertised prefixes. By default, the advertised prefixes are the same as the network prefixes assigned to the interface. For each network prefix, the matching prefix definition is found and its options are used. If no matching prefix definition is found, the prefix is used with default options.

  Prefix definitions can be either global or interface-specific. The second ones are part of interface options. The prefix definition matching is done in the first-match style, when interface-specific definitions are processed before global definitions. As expected, the prefix definition is matching if the network prefix is a subnet of the prefix in prefix definition.

`rdnss { options }`

  RDNSS definitions allow to specify a list of advertised recursive DNS servers together with their options. As options are seldom necessary, there is also a short variant `rdnss address` that just specifies one DNS server. Multiple definitions are cumulative. RDNSS definitions may also be interface-specific when used inside interface options. By default, interface uses both global and interface-specific options, but that can be changed by `rdnss local` option.

`dnssl { options }`

  DNSSL definitions allow to specify a list of advertised DNS search domains together with their options. Like `rdnss` above, multiple definitions are cumulative, they can be used also as interface-specific options and there is a short variant `dnssl domain` that just specifies one DNS search domain.

`trigger prefix`

  RAdv protocol could be configured to change its behavior based on availability of routes. When this option is used, the protocol waits in suppressed state until a trigger route (for the specified network) is exported to the protocol, the protocol also returnsd to suppressed state if the trigger route disappears. Note that route export depends on specified export filter, as usual. This option could be used, e.g., for handling failover in multihoming scenarios.

  During suppressed state, router advertisements are generated, but with some fields zeroed. Exact behavior depends on which fields are zeroed, this can be configured by `sensitive` option for appropriate fields. By default, just `default lifetime` (also called `router lifetime`) is zeroed, which means hosts cannot use the router as a default router. `preferred lifetime` and `valid lifetime` could also be configured as `sensitive` for a prefix, which would cause autoconfigured IPs to be deprecated or even removed.

Interface specific options:

`max ra interval expr`

  Unsolicited router advertisements are sent in irregular time intervals. This option specifies the maximum length of these intervals, in seconds. Valid values are 4-1800. Default: 600

`min ra interval expr`

  This option specifies the minimum length of that intervals, in seconds. Must be at least 3 and at most 3/4 * `max ra interval`. Default: about 1/3 * `max ra interval`.

`min delay expr`

  The minimum delay between two consecutive router advertisements, in seconds. Default: 3

`managed switch`

  This option specifies whether hosts should use DHCPv6 for IP address configuration. Default: no

`other config switch`

  This option specifies whether hosts should use DHCPv6 to receive other configuration information. Default: no

`link mtu expr`

  This option specifies which value of MTU should be used by hosts. 0 means unspecified. Default: 0

`reachable time expr`

  This option specifies the time (in milliseconds) how long hosts should assume a neighbor is reachable (from the last confirmation). Maximum is 3600000, 0 means unspecified. Default 0.

`retrans timer expr`

  This option specifies the time (in milliseconds) how long hosts should wait before retransmitting Neighbor Solicitation messages. 0 means unspecified. Default 0.

`current hop limit expr`

  This option specifies which value of Hop Limit should be used by hosts. Valid values are 0-255, 0 means unspecified. Default: 64

`default lifetime expr [sensitive switch]`

  This option specifies the time (in seconds) how long (after the receipt of RA) hosts may use the router as a default router. 0 means do not use as a default router. For `sensitive` option, see trigger. Default: 3 * `max ra interval`, `sensitive` yes.

`default preference low|medium|high`

  This option specifies the Default Router Preference value to advertise to hosts. Default: medium.

`rdnss local switch`

  Use only local (interface-specific) RDNSS definitions for this interface. Otherwise, both global and local definitions are used. Could also be used to disable RDNSS for given interface if no local definitons are specified. Default: no.

`dnssl local switch`

  Use only local DNSSL definitions for this interface. See `rdnss local` option above. Default: no.

Prefix specific options:

`skip switch`

  This option allows to specify that given prefix should not be advertised. This is useful for making exceptions from a default policy of advertising all prefixes. Note that for withdrawing an already advertised prefix it is more useful to advertise it with zero valid lifetime. Default: no

`onlink switch`

  This option specifies whether hosts may use the advertised prefix for onlink determination. Default: yes

`autonomous switch`

  This option specifies whether hosts may use the advertised prefix for stateless autoconfiguration. Default: yes

`valid lifetime expr [sensitive switch]`

  This option specifies the time (in seconds) how long (after the receipt of RA) the prefix information is valid, i.e., autoconfigured IP addresses can be assigned and hosts with that IP addresses are considered directly reachable. 0 means the prefix is no longer valid. For `sensitive` option, see trigger. Default: 86400 (1 day), `sensitive` no.

`preferred lifetime expr [sensitive switch]`

  This option specifies the time (in seconds) how long (after the receipt of RA) IP addresses generated from the prefix using stateless autoconfiguration remain preferred. For `sensitive` option, see trigger. Default: 14400 (4 hours), `sensitive` no.

RDNSS specific options:

`ns address`

  This option specifies one recursive DNS server. Can be used multiple times for multiple servers. It is mandatory to have at least one `ns` option in `rdnss` definition.

`lifetime [mult] expr`

  This option specifies the time how long the RDNSS information may be used by clients after the receipt of RA. It is expressed either in seconds or (when `mult` is used) in multiples of `max ra interval`. Note that RDNSS information is also invalidated when `default lifetime` expires. 0 means these addresses are no longer valid DNS servers. Default: 3 * `max ra interval`.

DNSSL specific options:

`domain address`

  This option specifies one DNS search domain. Can be used multiple times for multiple domains. It is mandatory to have at least one `domain` option in `dnssl` definition.

`lifetime [mult] expr`

  This option specifies the time how long the DNSSL information may be used by clients after the receipt of RA. Details are the same as for RDNSS `lifetime` option above. Default: 3 * `max ra interval`.

### Example

    protocol radv {
            interface "eth2" {
                    max ra interval 5;      # Fast failover with more routers
                    managed yes;            # Using DHCPv6 on eth2
                    prefix ::/0 {
                            autonomous off; # So do not autoconfigure any IP
                    };
            };

            interface "eth*";               # No need for any other options

            prefix 2001:0DB8:1234::/48 {
                    preferred lifetime 0;   # Deprecated address range
            };

            prefix 2001:0DB8:2000::/48 {
                    autonomous off;         # Do not autoconfigure
            };

            rdnss 2001:0DB8:1234::10;       # Short form of RDNSS

            rdnss {
                    lifetime mult 10;
                    ns 2001:0DB8:1234::11;
                    ns 2001:0DB8:1234::12;
            };

            dnssl {
                    lifetime 3600;
                    domain "abc.com";
                    domain "xyz.com";
            };
    }

## 6.9 RIP

### Introduction

The RIP protocol (also sometimes called Rest In Pieces) is a simple protocol, where each router broadcasts (to all its neighbors) distances to all networks it can reach. When a router hears distance to another network, it increments it and broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some network goes unreachable, routers keep telling each other that its distance is the original distance plus 1 (actually, plus interface metric, which is usually one). After some time, the distance reaches infinity (that's 15 in RIP) and all routers know that network is unreachable. RIP tries to minimize situations where counting to infinity is necessary, because it is slow. Due to infinity being 16, you can't use RIP on networks where maximal distance is higher than 15 hosts. You can read more about RIP at http://www.ietf.org/html.charters/rip-charter.html. Both IPv4 (RFC 1723 ftp://ftp.rfc-editor.org/in-notes/rfc1723.txt) and IPv6 (RFC 2080 ftp://ftp.rfc-editor.org/in-notes/rfc2080.txt) versions of RIP are supported by BIRD, historical RIPv1 (RFC 1058 ftp://ftp.rfc-editor.org/in-notes/rfc1058.txt) is not currently supported. RIPv4 MD5 authentication (RFC 2082 ftp://ftp.rfc-editor.org/in-notes/rfc2082.txt) is supported.

RIP is a very simple protocol, and it has a lot of shortcomings. Slow convergence, big network load and inability to handle larger networks makes it pretty much obsolete. It is still usable on very small networks.

### Configuration

In addition to options common for all to other protocols, RIP supports the following ones:

`authentication none|plaintext|md5`

  Selects authentication method to be used. `none` means that packets are not authenticated at all, `plaintext` means that a plaintext password is embedded into each packet, and `md5` means that packets are authenticated using a MD5 cryptographic hash. If you set authentication to not-none, it is a good idea to add `password` section. Default: none.

`honor always|neighbor|never`

  Specifies when should requests for dumping routing table be honored. (Always, when sent from a host on a directly connected network or never.) Routing table updates are honored only from neighbors, that is not configurable. Default: never.

There are some options that can be specified per-interface:

`metric num`

  This option specifies the metric of the interface. Valid

`mode multicast|broadcast|quiet|nolisten|version1`

  This option selects the mode for RIP to use on the interface. If nothing is specified, RIP runs in multicast mode. `version1` is currently equivalent to `broadcast`, and it makes RIP talk to a broadcast address even through multicast mode is possible. `quiet` option means that RIP will not transmit any periodic messages to this interface and `nolisten` means that RIP will send to this interface butnot listen to it.

`ttl security [switch | tx only]`

  TTL security is a feature that protects routing protocols from remote spoofed packets by using TTL 255 instead of TTL 1 for protocol packets destined to neighbors. Because TTL is decremented when packets are forwarded, it is non-trivial to spoof packets with TTL 255 from remote locations.

  If this option is enabled, the router will send RIP packets with TTL 255 and drop received packets with TTL less than 255. If this option si set to `tx only`, TTL 255 is used for sent packets, but is not checked for received packets. Such setting does not offer protection, but offers compatibility with neighbors regardless of whether they use ttl security.

  Note that for RIPng, TTL security is a standard behavior (required by RFC 2080), but BIRD uses `tx only` by default, for compatibility with older versions. For IPv4 RIP, default value is no.

`tx class|dscp|priority num`

  These options specify the ToS/DiffServ/Traffic class/Priority of the outgoing RIP packets. See tx class common option for detailed description.

The following options generally override behavior specified in RFC. If you use any of these options, BIRD will no longer be RFC-compliant, which means it will not be able to talk to anything other than equally configured BIRD. I have warned you.

`port number`

  Selects IP port to operate on, default 520. (This is useful when testing BIRD, if you set this to an address >1024, you will not need to run bird with UID==0).

`infinity number`

  Selects the value of infinity, default is 16. Bigger values will make protocol convergence even slower.

`period number`

  Specifies the number of seconds between periodic updates. Default is 30 seconds. A lower number will mean faster convergence but bigger network load. Do not use values lower than 12.

`timeout time number`

  Specifies how old route has to be to be considered unreachable. Default is 4*`period`.

`garbage time number`

  Specifies how old route has to be to be discarded. Default is 10*`period`.

### Attributes

RIP defines two route attributes:

`int `rip_metric``

  RIP metric of the route (ranging from 0 to `infinity`). When routes from different RIP instances are available and all of them have the same preference, BIRD prefers the route with lowest `rip_metric`. When importing a non-RIP route, the metric defaults to 5.

`int `rip_tag``

  RIP route tag: a 16-bit number which can be used to carry additional information with the route (for example, an originating AS number in case of external routes). When importing a non-RIP route, the tag defaults to 0.

### Example

    protocol rip MyRIP_test {
            debug all;
            port 1520;
            period 12;
            garbage time 60;
            interface "eth0" { metric 3; mode multicast; };
            interface "eth*" { metric 2; mode broadcast; };
            honor neighbor;
            authentication none;
            import filter { print "importing"; accept; };
            export filter { print "exporting"; accept; };
    }

## 6.10 Static

The Static protocol doesn't communicate with other routers in the network, but instead it allows you to define routes manually. This is often used for specifying how to forward packets to parts of the network which don't use dynamic routing at all and also for defining sink routes (i.e., those telling to return packets as undeliverable if they are in your IP block, you don't have any specific destination for them and you don't want to send them out through the default route to prevent routing loops).

There are five types of static routes: `classical' routes telling to forward packets to a neighboring router, multipath routes specifying several (possibly weighted) neighboring routers, device routes specifying forwarding to hosts on a directly connected network, recursive routes computing their nexthops by doing route table lookups for a given IP and special routes (sink, blackhole etc.) which specify a special action to be done instead of forwarding the packet.

When the particular destination is not available (the interface is down or the next hop of the route is not a neighbor at the moment), Static just uninstalls the route from the table it is connected to and adds it again as soon as the destination becomes adjacent again.

The Static protocol does not have many configuration options. The definition of the protocol contains mainly a list of static routes:

`route prefix via ip`

  Static route through a neighboring router.

`route prefix multipath via ip [weight num] [via ...]`

  Static multipath route. Contains several nexthops (gateways), possibly with their weights.

`route prefix via "interface"`

  Static device route through an interface to hosts on a directly connected network.

`route prefix recursive ip`

  Static recursive route, its nexthop depends on a route table lookup for given IP address.

`route prefix blackhole|unreachable|prohibit`

  Special routes specifying to silently drop the packet, return it as unreachable or return it as administratively prohibited. First two targets are also known as `drop` and `reject`.

`check link switch`

  If set, hardware link states of network interfaces are taken into consideration. When link disappears (e.g. ethernet cable is unplugged), static routes directing to that interface are removed. It is possible that some hardware drivers or platforms do not implement this feature. Default: off.

`igp table name`

  Specifies a table that is used for route table lookups of recursive routes. Default: the same table as the protocol is connected to.

Static routes have no specific attributes.

Example static config might look like this:

    protocol static {
            table testable;                  # Connect to a non-default routing table
            route 0.0.0.0/0 via 198.51.100.130; # Default route
            route 10.0.0.0/8 multipath       # Multipath route
                    via 198.51.100.10 weight 2
                    via 198.51.100.20
                    via 192.0.2.1;
            route 203.0.113.0/24 unreachable; # Sink route
            route 10.2.0.0/24 via "arc0";    # Secondary network
    }

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