Added BGP documentation.
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Martin Mares
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doc/bird.sgml
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doc/bird.sgml
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@ -61,7 +61,7 @@ and Zebra <HTMLURL URL="http://www.zebra.org">), but their capabilities are very
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they are very hard to configure and maintain.
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<p>BIRD is an Internet Routing Daemon designed to avoid all of these shortcomings,
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to support all the routing technology used in today's Internet or planned to be
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to support all the routing technology used in the today's Internet or planned to be
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used in near future and to have a clean extensible architecture allowing new routing
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protocols to be incorporated easily. Among other features, BIRD supports:
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@ -454,7 +454,7 @@ if 1234 = i then printn "."; else { print "*** FAIL: if 1 else"; }
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<sect1>BGP
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<p>The Border Gateway Protocol is the routing protocol used for backbone
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level routing in today's Internet. Contrary to other protocols, its convergence
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level routing in the today's Internet. Contrary to other protocols, its convergence
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doesn't rely on all routers following the same rules for route selection,
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making it possible to implement any routing policy at any router in the
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network, the only restriction being that if a router advertises a route,
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@ -472,16 +472,165 @@ routing table it wishes to export along with complete path information
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(a list of AS'es the packet will travel through if it uses that particular
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route) in order to avoid routing loops.
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<p>In BIRD, each instance of BGP corresponds to one neighboring router.
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This allows to set routing policy and all other parameters differently
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for each neighbor.
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<p>BIRD supports all requirements of the BGP4 standard as defined in
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RFC 1771<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1771.txt">
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including several enhancements from the
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latest draft<htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-09.txt">.
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It also supports the community attributes as per
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RFC 1997<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt">,
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capability negotiation draft<htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-cap-neg-06.txt">.
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For IPv6, it uses the standard multiprotocol extensions defined in
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RFC 2283<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2283.txt">
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including changes described in the
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latest draft <htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-multiprotocol-v2-05.txt">
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and applied to IPv6 according to
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RFC 2545<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2545.txt">.
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<sect2>Route selection rules
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<p>BGP doesn't have any simple metric, so the rules for selection of an optimal
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route among multiple BGP routes with the same preference are a bit more complex
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and are implemented according to the following algorithm. First it uses the first
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rule, if there are more "best" routes, then it uses the second rule to choose
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among them and so on.
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<itemize>
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<item>Prefer route with the highest local preference attribute.
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<item>Prefer route with the shortest AS path.
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<item>Prefer IGP origin over EGP and EGP over incomplete.
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<item>Prefer the lowest value of the Multiple Exit Discriminator.
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<item>Prefer internal routes over external routes.
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<item>Prefer route with the lowest value of router ID of the
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advertising router.
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</itemize>
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<sect2>Configuration
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<p>Each instance of the BGP corresponds to one neighboring router.
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This allows to set routing policy and all other parameters differently
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for each neighbor using the following protocol parameters:
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<descrip>
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<tag>local as <m/number/</tag> Define which AS we are part of. (Note that
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contrary to other IP routers, BIRD is able to act as a router located
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in multiple AS'es simultaneously, but in such cases you need to tweak
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the BGP paths manually in the filters to get consistent behavior.)
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This parameter is mandatory.
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<tag>neighbor <m/ip/ as <m/number/</tag> Define neighboring router
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this instance will be talking to and what AS it's located in. Unless
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you use the <cf/multihop/ clause, it must be directly connected to one
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of your router's interfaces. This parameter is mandatory.
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<tag>multihop <m/number/ via <m/ip/</tag> Configure multihop BGP to a
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neighbor which is connected at most <m/number/ hops far and to which
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we should route via our direct neighbor with address <m/ip/.
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Default: switched off.
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<tag>next hop self</tag> Avoid calculation of the Next Hop attribute
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and always advertise our own source address (see below) as a next hop.
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This needs to be used only
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occasionally to circumvent misconfigurations of other routers.
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Default: disabled.
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<tag>source address <m/ip/</tag> Define local address we should use
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for next hop calculation. Default: the address of the local end
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of the interface our neighbor is connected to.
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<tag>disable after error <m/switch/</tag> When an error is encountered (either
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locally or by the other side), disable the instance automatically
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and wait for an administrator to solve the problem manually. Default: off.
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<tag>hold time <m/number/</tag> Time in seconds to wait for a keepalive
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message from the other side before considering the connection stale.
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Default: depends on agreement with the neighboring router, we prefer
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240 seconds if the other side is willing to accept it.
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<tag>startup hold time <m/number/</tag> Value of the hold timer used
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before the routers have a chance to exchange OPEN messages and agree
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on the real value. Default: 240 seconds.
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<tag>keepalive time <m/number/</tag> Delay in seconds between sending
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of two consecutive keepalive messages. Default: One third of the hold time.
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<tag>connect retry time <m/number/</tag> Time in seconds to wait before
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retrying a failed connect attempt. Default: 120 seconds.
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<tag>start delay time <m/number/</tag> Delay in seconds between protocol
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startup and first attempt to connect. Default: 5 seconds.
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<tag>error wait time <m/number/, <m/number/</tag> Minimum and maximum delay in seconds between protocol
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failure (either local or reported by the peer) and automatic startup.
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Doesn't apply when <cf/disable after error/ is configured. If consecutive
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errors happen, the delay is increased exponentially until it reaches the maximum. Default: 60, 300.
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<tag>error forget time <m/number/</tag> Maximum time in seconds between two protocol
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failures to treat them as a error sequence which makes the <cf/error wait time/
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increase exponentially. Default: 300 seconds.
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<tag>path metric <m/switch/</tag> Enable comparison of path lengths
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when deciding which BGP route is the best one. Default: on.
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<tag>default bgp_med <m/number/</tag> Value of the Multiple Exit
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Discriminator to be used during route selection when the MED attribute
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is missing. Default: infinite.
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<tag>default bgp_local_pref <m/number/</tag> Value of the Local Preference
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to be used during route selection when the Local Preference attribute
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is missing. Default: 0.
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</descrip>
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<sect2>Attributes
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<p>BGP defines several route attributes. Some of them (those marked with `I' in the
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table below) are available on internal BGP connections only, some of them (marked
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with `O') are optional.
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<descrip>
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<tag>path <cf/bgp_path/</tag> Sequence of AS numbers describing the AS path
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the packet will travel through when forwarded according to this route. On
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internal BGP connections it doesn't contain the number of the local AS.
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<tag>int <cf/bgp_local_pref/ [I]</tag> Local preference value used for
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selection among multiple BGP routes (see the selection rules above). It's
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used as an additional metric which is propagated through the whole local AS.
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<tag>int <cf/bgp_med/ [IO]</tag> The Multiple Exit Discriminator of the route
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which is an optional attribute which is often used within the local AS to
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reflect interior distances to various boundary routers. See the route selection
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rules above for exact semantics.
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<tag>enum <cf/bgp_origin/</tag> Origin of the route: either <cf/ORIGIN_IGP/
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if the route has originated in interior routing protocol of an AS or
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<cf/ORIGIN_EGP/ if it's been imported from the <tt>EGP</tt> protocol
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(nowadays it seems to be obsolete) or <cf/ORIGIN_INCOMPLETE/ if the origin
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is unknown.
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<tag>ip <cf/bgp_next_hop/</tag> Next hop to be used for forwarding of packets
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to this destination. On internal BGP connections, it's an address of the
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originating router if it's inside the local AS or a boundary router the
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packet will leave the AS through if it's an exterior route, so each BGP
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speaker within the AS has a chance to use the shortest interior path
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possible to this point.
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<tag>void <cf/bgp_atomic_aggr/ [O]</tag> This is an optional attribute
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which carries no value, but which sole presence indicates that the route
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has been aggregated from multiple routes by some AS on the path from
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the originator.
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<!-- we don't handle aggregators right since they are of a very obscure type
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<tag>bgp_aggregator</tag>
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-->
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<tag>clist <cf/bgp_community/ [O]</tag> List of community values associated
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with the route. Each such value is a pair (represented as a <cf/pair/ data
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type inside the filters) of 16-bit integers, the first of them containing a number of the AS which defines
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the community and the second one is a per-AS identifier. There are lots
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of uses of the community mechanism, but generally they are used to carry
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policy information like "don't export to USA peers". As each AS can define
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its own routing policy, it's also has a complete freedom about which community
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attributes it defines and what their semantics will be.
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</descrip>
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<sect2>Example
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<p><code>
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protocol bgp {
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local as 65000; # Use a private AS number
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neighbor 62.168.0.130 as 5588; # Our neighbor
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multihop 20 via 62.168.0.13; # Which is connected indirectly
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export filter { # We use non-trivial export rules
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if source = RTS_STATIC then { # Export only static routes
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bgp_community.add((65000,5678)); # Assign our community
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if bgp_path ~ / 65000 / then # Artificially increase path length
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bgp_path.prepend(65000); # by prepending local AS number twice
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accept;
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}
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reject;
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};
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import all;
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source address 62.168.0.1; # Use non-standard source address
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}
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</code>
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<sect1>Device
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<p>The Device protocol is not a real routing protocol as it doesn't generate
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@ -492,7 +641,7 @@ interfaces from the kernel.
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this protocol in the configuration since almost all other protocol
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require network interfaces to be defined in order to work.
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<p>Only configurable thing is interface scan time:
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<p>The only configurable thing is interface scan time:
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<p><descrip>
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<tag>scan time <m/number/</tag> Time in seconds between two scans
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