mirror of
https://gitlab.nic.cz/labs/bird.git
synced 2024-11-17 08:38:42 +00:00
Added BGP documentation.
This commit is contained in:
Martin Mares
parent
56ab03c71f
commit
5459fac61f
161
doc/bird.sgml
161
doc/bird.sgml
@ -61,7 +61,7 @@ and Zebra <HTMLURL URL="http://www.zebra.org">), but their capabilities are very
|
||||
they are very hard to configure and maintain.
|
||||
|
||||
<p>BIRD is an Internet Routing Daemon designed to avoid all of these shortcomings,
|
||||
to support all the routing technology used in today's Internet or planned to be
|
||||
to support all the routing technology used in the today's Internet or planned to be
|
||||
used in near future and to have a clean extensible architecture allowing new routing
|
||||
protocols to be incorporated easily. Among other features, BIRD supports:
|
||||
|
||||
@ -454,7 +454,7 @@ if 1234 = i then printn "."; else { print "*** FAIL: if 1 else"; }
|
||||
<sect1>BGP
|
||||
|
||||
<p>The Border Gateway Protocol is the routing protocol used for backbone
|
||||
level routing in today's Internet. Contrary to other protocols, its convergence
|
||||
level routing in the today's Internet. Contrary to other protocols, its convergence
|
||||
doesn't 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,
|
||||
@ -472,16 +472,165 @@ routing table it wishes to export along with complete path information
|
||||
(a list of AS'es the packet will travel through if it uses that particular
|
||||
route) in order to avoid routing loops.
|
||||
|
||||
<p>In BIRD, each instance of BGP corresponds to one neighboring router.
|
||||
This allows to set routing policy and all other parameters differently
|
||||
for each neighbor.
|
||||
<p>BIRD supports all requirements of the BGP4 standard as defined in
|
||||
RFC 1771<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1771.txt">
|
||||
including several enhancements from the
|
||||
latest draft<htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-09.txt">.
|
||||
It also supports the community attributes as per
|
||||
RFC 1997<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc1997.txt">,
|
||||
capability negotiation draft<htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-cap-neg-06.txt">.
|
||||
For IPv6, it uses the standard multiprotocol extensions defined in
|
||||
RFC 2283<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2283.txt">
|
||||
including changes described in the
|
||||
latest draft <htmlurl url="ftp://ftp.rfc-editor.org/internet-drafts/draft-ietf-idr-bgp4-multiprotocol-v2-05.txt">
|
||||
and applied to IPv6 according to
|
||||
RFC 2545<htmlurl url="ftp://ftp.rfc-editor.org/in-notes/rfc2545.txt">.
|
||||
|
||||
<sect2>Route selection rules
|
||||
|
||||
<p>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 are implemented according to the following algorithm. First it uses the first
|
||||
rule, if there are more "best" routes, then it uses the second rule to choose
|
||||
among them and so on.
|
||||
|
||||
<itemize>
|
||||
<item>Prefer route with the highest local preference attribute.
|
||||
<item>Prefer route with the shortest AS path.
|
||||
<item>Prefer IGP origin over EGP and EGP over incomplete.
|
||||
<item>Prefer the lowest value of the Multiple Exit Discriminator.
|
||||
<item>Prefer internal routes over external routes.
|
||||
<item>Prefer route with the lowest value of router ID of the
|
||||
advertising router.
|
||||
</itemize>
|
||||
|
||||
<sect2>Configuration
|
||||
|
||||
<p>Each instance of the BGP corresponds to one neighboring router.
|
||||
This allows to set routing policy and all other parameters differently
|
||||
for each neighbor using the following protocol parameters:
|
||||
|
||||
<descrip>
|
||||
<tag>local as <m/number/</tag> 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.)
|
||||
This parameter is mandatory.
|
||||
<tag>neighbor <m/ip/ as <m/number/</tag> Define neighboring router
|
||||
this instance will be talking to and what AS it's located in. Unless
|
||||
you use the <cf/multihop/ clause, it must be directly connected to one
|
||||
of your router's interfaces. This parameter is mandatory.
|
||||
<tag>multihop <m/number/ via <m/ip/</tag> Configure multihop BGP to a
|
||||
neighbor which is connected at most <m/number/ hops far and to which
|
||||
we should route via our direct neighbor with address <m/ip/.
|
||||
Default: switched off.
|
||||
<tag>next hop self</tag> Avoid calculation of the Next Hop attribute
|
||||
and always advertise our own source address (see below) as a next hop.
|
||||
This needs to be used only
|
||||
occasionally to circumvent misconfigurations of other routers.
|
||||
Default: disabled.
|
||||
<tag>source address <m/ip/</tag> Define local address we should use
|
||||
for next hop calculation. Default: the address of the local end
|
||||
of the interface our neighbor is connected to.
|
||||
<tag>disable after error <m/switch/</tag> When an error is encountered (either
|
||||
locally or by the other side), disable the instance automatically
|
||||
and wait for an administrator to solve the problem manually. Default: off.
|
||||
<tag>hold time <m/number/</tag> 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.
|
||||
<tag>startup hold time <m/number/</tag> 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.
|
||||
<tag>keepalive time <m/number/</tag> Delay in seconds between sending
|
||||
of two consecutive keepalive messages. Default: One third of the hold time.
|
||||
<tag>connect retry time <m/number/</tag> Time in seconds to wait before
|
||||
retrying a failed connect attempt. Default: 120 seconds.
|
||||
<tag>start delay time <m/number/</tag> Delay in seconds between protocol
|
||||
startup and first attempt to connect. Default: 5 seconds.
|
||||
<tag>error wait time <m/number/, <m/number/</tag> Minimum and maximum delay in seconds between protocol
|
||||
failure (either local or reported by the peer) and automatic startup.
|
||||
Doesn't apply when <cf/disable after error/ is configured. If consecutive
|
||||
errors happen, the delay is increased exponentially until it reaches the maximum. Default: 60, 300.
|
||||
<tag>error forget time <m/number/</tag> Maximum time in seconds between two protocol
|
||||
failures to treat them as a error sequence which makes the <cf/error wait time/
|
||||
increase exponentially. Default: 300 seconds.
|
||||
<tag>path metric <m/switch/</tag> Enable comparison of path lengths
|
||||
when deciding which BGP route is the best one. Default: on.
|
||||
<tag>default bgp_med <m/number/</tag> Value of the Multiple Exit
|
||||
Discriminator to be used during route selection when the MED attribute
|
||||
is missing. Default: infinite.
|
||||
<tag>default bgp_local_pref <m/number/</tag> Value of the Local Preference
|
||||
to be used during route selection when the Local Preference attribute
|
||||
is missing. Default: 0.
|
||||
</descrip>
|
||||
|
||||
<sect2>Attributes
|
||||
|
||||
<p>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.
|
||||
|
||||
<descrip>
|
||||
<tag>path <cf/bgp_path/</tag> Sequence of AS numbers describing the AS path
|
||||
the packet will travel through when forwarded according to this route. On
|
||||
internal BGP connections it doesn't contain the number of the local AS.
|
||||
<tag>int <cf/bgp_local_pref/ [I]</tag> 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.
|
||||
<tag>int <cf/bgp_med/ [IO]</tag> The Multiple Exit Discriminator of the route
|
||||
which is an optional attribute which is often used within the local AS to
|
||||
reflect interior distances to various boundary routers. See the route selection
|
||||
rules above for exact semantics.
|
||||
<tag>enum <cf/bgp_origin/</tag> Origin of the route: either <cf/ORIGIN_IGP/
|
||||
if the route has originated in interior routing protocol of an AS or
|
||||
<cf/ORIGIN_EGP/ if it's been imported from the <tt>EGP</tt> protocol
|
||||
(nowadays it seems to be obsolete) or <cf/ORIGIN_INCOMPLETE/ if the origin
|
||||
is unknown.
|
||||
<tag>ip <cf/bgp_next_hop/</tag> 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.
|
||||
<tag>void <cf/bgp_atomic_aggr/ [O]</tag> This is an optional attribute
|
||||
which carries no value, but which sole presence indicates that the route
|
||||
has been aggregated from multiple routes by some AS on the path from
|
||||
the originator.
|
||||
<!-- we don't handle aggregators right since they are of a very obscure type
|
||||
<tag>bgp_aggregator</tag>
|
||||
-->
|
||||
<tag>clist <cf/bgp_community/ [O]</tag> List of community values associated
|
||||
with the route. Each such value is a pair (represented as a <cf/pair/ data
|
||||
type inside the filters) of 16-bit integers, the first of them containing a number of the AS which defines
|
||||
the community and the second one is 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's also has a complete freedom about which community
|
||||
attributes it defines and what their semantics will be.
|
||||
</descrip>
|
||||
|
||||
<sect2>Example
|
||||
|
||||
<p><code>
|
||||
protocol bgp {
|
||||
local as 65000; # Use a private AS number
|
||||
neighbor 62.168.0.130 as 5588; # Our neighbor
|
||||
multihop 20 via 62.168.0.13; # Which is connected indirectly
|
||||
export filter { # We use non-trivial export rules
|
||||
if source = RTS_STATIC then { # Export only static routes
|
||||
bgp_community.add((65000,5678)); # Assign our community
|
||||
if bgp_path ~ / 65000 / then # Artificially increase path length
|
||||
bgp_path.prepend(65000); # by prepending local AS number twice
|
||||
accept;
|
||||
}
|
||||
reject;
|
||||
};
|
||||
import all;
|
||||
source address 62.168.0.1; # Use non-standard source address
|
||||
}
|
||||
</code>
|
||||
|
||||
<sect1>Device
|
||||
|
||||
<p>The Device protocol is not a real routing protocol as it doesn't generate
|
||||
@ -492,7 +641,7 @@ interfaces from the kernel.
|
||||
this protocol in the configuration since almost all other protocol
|
||||
require network interfaces to be defined in order to work.
|
||||
|
||||
<p>Only configurable thing is interface scan time:
|
||||
<p>The only configurable thing is interface scan time:
|
||||
|
||||
<p><descrip>
|
||||
<tag>scan time <m/number/</tag> Time in seconds between two scans
|
||||
|
||||
Loading…
Reference in New Issue
Block a user