doc/bgpd.texi

@c -*-texinfo-*-
@c This is part of the Quagga Manual.
@c @value{COPYRIGHT_STR}
@c Portions: 
@c   Copyright @copyright{} 2015 Hewlett Packard Enterprise Development LP
@c See file quagga.texi for copying conditions.
@node BGP
@chapter BGP

@acronym{BGP} stands for a Border Gateway Protocol.  The lastest BGP version
is 4.  It is referred as BGP-4.  BGP-4 is one of the Exterior Gateway
Protocols and de-fact standard of Inter Domain routing protocol.
BGP-4 is described in @cite{RFC1771, A Border Gateway Protocol
4 (BGP-4)}.

Many extensions have been added to @cite{RFC1771}.  @cite{RFC2858,
Multiprotocol Extensions for BGP-4} provides multiprotocol support to
BGP-4.

@menu
* Starting BGP::                
* BGP router::                  
* BGP MED::
* BGP network::                 
* BGP Peer::                    
* BGP Peer Group::              
* BGP Address Family::          
* Autonomous System::           
* BGP Communities Attribute::   
* BGP Extended Communities Attribute::  
* Displaying BGP routes::       
* Capability Negotiation::      
* Route Reflector::             
* Route Server::                
* How to set up a 6-Bone connection::  
* Dump BGP packets and table::  
* BGP Configuration Examples::
@end menu

@node Starting BGP
@section Starting BGP

Default configuration file of @command{bgpd} is @file{bgpd.conf}.
@command{bgpd} searches the current directory first then
@value{INSTALL_PREFIX_ETC}/bgpd.conf.  All of bgpd's command must be
configured in @file{bgpd.conf}.

@command{bgpd} specific invocation options are described below.  Common
options may also be specified (@pxref{Common Invocation Options}).

@table @samp
@item -p @var{PORT}
@itemx --bgp_port=@var{PORT}
Set the bgp protocol's port number.

@item -r
@itemx --retain
When program terminates, retain BGP routes added by zebra.

@item -l
@itemx --listenon
Specify a specific IP address for bgpd to listen on, rather than its 
default of INADDR_ANY / IN6ADDR_ANY. This can be useful to constrain bgpd
to an internal address, or to run multiple bgpd processes on one host.

@end table

@node BGP router
@section BGP router

  First of all you must configure BGP router with @command{router bgp}
command.  To configure BGP router, you need AS number.  AS number is an
identification of autonomous system.  BGP protocol uses the AS number
for detecting whether the BGP connection is internal one or external one.

@deffn Command {router bgp @var{asn}} {}
Enable a BGP protocol process with the specified @var{asn}.  After
this statement you can input any @code{BGP Commands}.  You can not
create different BGP process under different @var{asn} without
specifying @code{multiple-instance} (@pxref{Multiple instance}).
@end deffn

@deffn Command {no router bgp @var{asn}} {}
Destroy a BGP protocol process with the specified @var{asn}.
@end deffn

@deffn {BGP} {bgp router-id @var{A.B.C.D}} {}
This command specifies the router-ID.  If @command{bgpd} connects to @command{zebra} it gets
interface and address information.  In that case default router ID value
is selected as the largest IP Address of the interfaces.  When
@code{router zebra} is not enabled @command{bgpd} can't get interface information
so @code{router-id} is set to 0.0.0.0.  So please set router-id by hand.
@end deffn

@menu
* BGP distance::                
* BGP decision process::        
* BGP route flap dampening::      
@end menu

@node BGP distance
@subsection BGP distance

@deffn {BGP} {distance bgp <1-255> <1-255> <1-255>} {}
This command change distance value of BGP.  Each argument is distance
value for external routes, internal routes and local routes.
@end deffn

@deffn {BGP} {distance <1-255> @var{A.B.C.D/M}} {}
@deffnx {BGP} {distance <1-255> @var{A.B.C.D/M} @var{word}} {}
This command set distance value to 
@end deffn

@node BGP decision process
@subsection BGP decision process

The decision process Quagga BGP uses to select routes is as follows:

@table @asis
@item 1. Weight check
prefer higher local weight routes to lower routes.
  
@item 2. Local preference check
prefer higher local preference routes to lower.

@item 3. Local route check
Prefer local routes (statics, aggregates, redistributed) to received routes.

@item 4. AS path length check
Prefer shortest hop-count AS_PATHs. 

@item 5. Origin check
Prefer the lowest origin type route.  That is, prefer IGP origin routes to
EGP, to Incomplete routes. 

@item 6. MED check
Where routes with a MED were received from the same AS,
prefer the route with the lowest MED. @xref{BGP MED}.

@item 7. External check
Prefer the route received from an external, eBGP peer
over routes received from other types of peers.

@item 8. IGP cost check
Prefer the route with the lower IGP cost.

@item 9. Multi-path check
If multi-pathing is enabled, then check whether
the routes not yet distinguished in preference may be considered equal. If
@ref{bgp bestpath as-path multipath-relax} is set, all such routes are
considered equal, otherwise routes received via iBGP with identical AS_PATHs
or routes received from eBGP neighbours in the same AS are considered equal.

@item 10 Already-selected external check

Where both routes were received from eBGP peers, then prefer the route which
is already selected.  Note that this check is not applied if @ref{bgp
bestpath compare-routerid} is configured.  This check can prevent some cases
of oscillation.

@item 11. Router-ID check
Prefer the route with the lowest @w{router-ID}.  If the
route has an @w{ORIGINATOR_ID} attribute, through iBGP reflection, then that
router ID is used, otherwise the @w{router-ID} of the peer the route was
received from is used.

@item 12. Cluster-List length check
The route with the shortest cluster-list
length is used.  The cluster-list reflects the iBGP reflection path the
route has taken.

@item 13. Peer address
Prefer the route received from the peer with the higher
transport layer address, as a last-resort tie-breaker.

@end table

@deffn {BGP} {bgp bestpath as-path confed} {}
This command specifies that the length of confederation path sets and
sequences should should be taken into account during the BGP best path
decision process.
@end deffn

@deffn {BGP} {bgp bestpath as-path multipath-relax} {}
@anchor{bgp bestpath as-path multipath-relax}
This command specifies that BGP decision process should consider paths
of equal AS_PATH length candidates for multipath computation. Without
the knob, the entire AS_PATH must match for multipath computation.
@end deffn

@deffn {BGP} {bgp bestpath compare-routerid} {}
@anchor{bgp bestpath compare-routerid}

Ensure that when comparing routes where both are equal on most metrics,
including local-pref, AS_PATH length, IGP cost, MED, that the tie is broken
based on router-ID.

If this option is enabled, then the already-selected check, where
already selected eBGP routes are preferred, is skipped.

If a route has an @w{ORIGINATOR_ID} attribute because it has been reflected,
that @w{ORIGINATOR_ID} will be used.  Otherwise, the router-ID of the peer the
route was received from will be used.

The advantage of this is that the route-selection (at this point) will be
more deterministic.  The disadvantage is that a few or even one lowest-ID
router may attract all trafic to otherwise-equal paths because of this
check.  It may increase the possibility of MED or IGP oscillation, unless
other measures were taken to avoid these.  The exact behaviour will be
sensitive to the iBGP and reflection topology.

@end deffn


@node BGP route flap dampening
@subsection BGP route flap dampening

@deffn {BGP} {bgp dampening @var{<1-45>} @var{<1-20000>} @var{<1-20000>} @var{<1-255>}} {}
This command enables BGP route-flap dampening and specifies dampening parameters.

@table @asis
@item @asis{half-life}
Half-life time for the penalty
@item @asis{reuse-threshold}
Value to start reusing a route
@item @asis{suppress-threshold}
Value to start suppressing a route
@item @asis{max-suppress}
Maximum duration to suppress a stable route
@end table

The route-flap damping algorithm is compatible with @cite{RFC2439}. The use of this command
is not recommended nowadays, see @uref{http://www.ripe.net/ripe/docs/ripe-378,,RIPE-378}.
@end deffn

@node BGP MED
@section BGP MED

The BGP MED (Multi_Exit_Discriminator) attribute has properties which can
cause subtle convergence problems in BGP.  These properties and problems
have proven to be hard to understand, at least historically, and may still
not be widely understood.  The following attempts to collect together and
present what is known about MED, to help operators and Quagga users in
designing and configuring their networks.

The BGP @acronym{MED, Multi_Exit_Discriminator} attribute is intended to
allow one AS to indicate its preferences for its ingress points to another
AS.  The MED attribute will not be propagated on to another AS by the
receiving AS - it is `non-transitive' in the BGP sense.

E.g., if AS X and AS Y have 2 different BGP peering points, then AS X
might set a MED of 100 on routes advertised at one and a MED of 200 at the
other.  When AS Y selects between otherwise equal routes to or via
AS X, AS Y should prefer to take the path via the lower MED peering of 100 with
AS X.  Setting the MED allows an AS to influence the routing taken to it
within another, neighbouring AS.

In this use of MED it is not really meaningful to compare the MED value on
routes where the next AS on the paths differs.  E.g., if AS Y also had a
route for some destination via AS Z in addition to the routes from AS X, and
AS Z had also set a MED, it wouldn't make sense for AS Y to compare AS Z's
MED values to those of AS X.  The MED values have been set by different
administrators, with different frames of reference.

The default behaviour of BGP therefore is to not compare MED values across
routes received from different neighbouring ASes.  In Quagga this is done by
comparing the neighbouring, left-most AS in the received AS_PATHs of the
routes and only comparing MED if those are the same.

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Unfortunately, this behaviour of MED, of sometimes being compared across
routes and sometimes not, depending on the properties of those other routes,
means MED can cause the order of preference over all the routes to be
undefined.  That is, given routes A, B, and C, if A is preferred to B, and B
is preferred to C, then a well-defined order should mean the preference is
transitive (in the sense of orders @footnote{For some set of objects to have
an order, there @emph{must} be some binary ordering relation that is defined
for @emph{every} combination of those objects, and that relation @emph{must}
be transitive.  I.e.@:, if the relation operator is @mprec{}, and if 
a @mprec{} b and b @mprec{} c then that relation must carry over
and it @emph{must} be that a @mprec{} c for the objects to have an
order.  The ordering relation may allow for equality, i.e. 
a @mprec{} b and b @mprec{} a may both be true amd imply that
a and b are equal in the order and not distinguished by it, in
which case the set has a partial order.  Otherwise, if there is an order,
all the objects have a distinct place in the order and the set has a total
order.}) and that A would be preferred to C.

However, when MED is involved this need not be the case.  With MED it is
possible that C is actually preferred over A.  So A is preferred to B, B is
preferred to C, but C is preferred to A.  This can be true even where BGP
defines a deterministic ``most preferred'' route out of the full set of
A,B,C.  With MED, for any given set of routes there may be a
deterministically preferred route, but there need not be any way to arrange
them into any order of preference.  With unmodified MED, the order of
preference of routes literally becomes undefined.

That MED can induce non-transitive preferences over routes can cause issues. 
Firstly, it may be perceived to cause routing table churn locally at
speakers; secondly, and more seriously, it may cause routing instability in
iBGP topologies, where sets of speakers continually oscillate between
different paths.

The first issue arises from how speakers often implement routing decisions. 
Though BGP defines a selection process that will deterministically select
the same route as best at any given speaker, even with MED, that process
requires evaluating all routes together.  For performance and ease of
implementation reasons, many implementations evaluate route preferences in a
pair-wise fashion instead.  Given there is no well-defined order when MED is
involved, the best route that will be chosen becomes subject to
implementation details, such as the order the routes are stored in.  That
may be (locally) non-deterministic, e.g.@: it may be the order the routes
were received in.  

This indeterminism may be considered undesirable, though it need not cause
problems.  It may mean additional routing churn is perceived, as sometimes
more updates may be produced than at other times in reaction to some event .

This first issue can be fixed with a more deterministic route selection that
ensures routes are ordered by the neighbouring AS during selection. 
@xref{bgp deterministic-med}.  This may reduce the number of updates as
routes are received, and may in some cases reduce routing churn.  Though, it
could equally deterministically produce the largest possible set of updates
in response to the most common sequence of received updates.

A deterministic order of evaluation tends to imply an additional overhead of
sorting over any set of n routes to a destination.  The implementation of
deterministic MED in Quagga scales significantly worse than most sorting
algorithms at present, with the number of paths to a given destination. 
That number is often low enough to not cause any issues, but where there are
many paths, the deterministic comparison may quickly become increasingly
expensive in terms of CPU.

Deterministic local evaluation can @emph{not} fix the second, more major,
issue of MED however.  Which is that the non-transitive preference of routes
MED can cause may lead to routing instability or oscillation across multiple
speakers in iBGP topologies.  This can occur with full-mesh iBGP, but is
particularly problematic in non-full-mesh iBGP topologies that further
reduce the routing information known to each speaker.  This has primarily
been documented with iBGP route-reflection topologies.  However, any
route-hiding technologies potentially could also exacerbate oscillation with
MED.

This second issue occurs where speakers each have only a subset of routes,
and there are cycles in the preferences between different combinations of
routes - as the undefined order of preference of MED allows - and the routes
are distributed in a way that causes the BGP speakers to 'chase' those
cycles.  This can occur even if all speakers use a deterministic order of
evaluation in route selection.

E.g., speaker 4 in AS A might receive a route from speaker 2 in AS X, and
from speaker 3 in AS Y; while speaker 5 in AS A might receive that route
from speaker 1 in AS Y.  AS Y might set a MED of 200 at speaker 1, and 100
at speaker 3. I.e, using ASN:ID:MED to label the speakers:

@example

           /---------------\
 X:2------|--A:4-------A:5--|-Y:1:200
 Y:3:100--|-/               |
           \---------------/

@end example

Assuming all other metrics are equal (AS_PATH, ORIGIN, 0 IGP costs), then
based on the RFC4271 decision process speaker 4 will choose X:2 over
Y:3:100, based on the lower ID of 2.  Speaker 4 advertises X:2 to speaker 5. 
Speaker 5 will continue to prefer Y:1:200 based on the ID, and advertise
this to speaker 4.  Speaker 4 will now have the full set of routes, and the
Y:1:200 it receives from 5 will beat X:2, but when speaker 4 compares
Y:1:200 to Y:3:100 the MED check now becomes active as the ASes match, and
now Y:3:100 is preferred.  Speaker 4 therefore now advertises Y:3:100 to 5,
which will also agrees that Y:3:100 is preferred to Y:1:200, and so
withdraws the latter route from 4.  Speaker 4 now has only X:2 and Y:3:100,
and X:2 beats Y:3:100, and so speaker 4 implicitly updates its route to
speaker 5 to X:2.  Speaker 5 sees that Y:1:200 beats X:2 based on the ID,
and advertises Y:1:200 to speaker 4, and the cycle continues.

The root cause is the lack of a clear order of preference caused by how MED
sometimes is and sometimes is not compared, leading to this cycle in the
preferences between the routes:

@example

       /---> X:2 ---beats---> Y:3:100 --\
      |                                  |
      |                                  |
       \---beats--- Y:1:200 <---beats---/

@end example

This particular type of oscillation in full-mesh iBGP topologies can  be
avoided by speakers preferring already selected, external routes rather than
choosing to update to new a route based on a post-MED metric (e.g. 
router-ID), at the cost of a non-deterministic selection process.  Quagga
implements this, as do many other implementations, so long as it is not
overridden by setting @ref{bgp bestpath compare-routerid}, and see also
@ref{BGP decision process}, .

However, more complex and insidious cycles of oscillation are possible with
iBGP route-reflection, which are not so easily avoided.  These have been
documented in various places.  See, e.g., @cite{McPherson, D.  and Gill, V. 
and Walton, D., "Border Gateway Protocol (BGP) Persistent Route Oscillation
Condition", IETF RFC3345}, and @cite{Flavel, A.  and M.  Roughan, "Stable
and flexible iBGP", ACM SIGCOMM 2009}, and @cite{Griffin, T.  and G.  Wilfong, 
"On the correctness of IBGP configuration", ACM SIGCOMM 2002} for concrete 
examples and further references.

There is as of this writing @emph{no} known way to use MED for its original
purpose; @emph{and} reduce routing information in iBGP topologies;
@emph{and} be sure to avoid the instability problems of MED due the
non-transitive routing preferences it can induce; in general on arbitrary
networks.

There may be iBGP topology specific ways to reduce the instability risks,
even while using MED, e.g.@: by constraining the reflection topology and by
tuning IGP costs between route-reflector clusters, see RFC3345 for details. 
In the near future, the Add-Path extension to BGP may also solve MED
oscillation while still allowing MED to be used as intended, by distributing
"best-paths per neighbour AS".  This would be at the cost of distributing at
least as many routes to all speakers as a full-mesh iBGP would, if not more,
while also imposing similar CPU overheads as the "Deterministic MED" feature
at each Add-Path reflector.

More generally, the instability problems that MED can introduce on more
complex, non-full-mesh, iBGP topologies may be avoided either by:

@itemize 

@item
Setting @ref{bgp always-compare-med}, however this allows MED to be compared
across values set by different neighbour ASes, which may not produce
coherent desirable results, of itself.

@item 
Effectively ignoring MED by setting MED to the same value (e.g.@: 0) using
@ref{routemap set metric} on all received routes, in combination with
setting @ref{bgp always-compare-med} on all speakers. This is the simplest
and most performant way to avoid MED oscillation issues, where an AS is happy
not to allow neighbours to inject this problematic metric.

@end itemize

As MED is evaluated after the AS_PATH length check, another possible use for
MED is for intra-AS steering of routes with equal AS_PATH length, as an
extension of the last case above.  As MED is evaluated before IGP metric,
this can allow cold-potato routing to be implemented to send traffic to
preferred hand-offs with neighbours, rather than the closest hand-off
according to the IGP metric.

Note that even if action is taken to address the MED non-transitivity
issues, other oscillations may still be possible.  E.g., on IGP cost if
iBGP and IGP topologies are at cross-purposes with each other - see the
Flavel and Roughan paper above for an example.  Hence the guideline that the
iBGP topology should follow the IGP topology.

@deffn {BGP} {bgp deterministic-med} {}
@anchor{bgp deterministic-med}

Carry out route-selection in way that produces deterministic answers
locally, even in the face of MED and the lack of a well-defined order of
preference it can induce on routes.  Without this option the preferred route
with MED may be determined largely by the order that routes were received
in.

Setting this option will have a performance cost that may be noticeable when
there are many routes for each destination.  Currently in Quagga it is
implemented in a way that scales poorly as the number of routes per
destination increases.

The default is that this option is not set.
@end deffn

Note that there are other sources of indeterminism in the route selection
process, specifically, the preference for older and already selected routes
from eBGP peers, @xref{BGP decision process}.

@deffn {BGP} {bgp always-compare-med} {}
@anchor{bgp always-compare-med}

Always compare the MED on routes, even when they were received from
different neighbouring ASes.  Setting this option makes the order of
preference of routes more defined, and should eliminate MED induced
oscillations.

If using this option, it may also be desirable to use @ref{routemap set
metric} to set MED to 0 on routes received from external neighbours.

This option can be used, together with @ref{routemap set metric} to use MED
as an intra-AS metric to steer equal-length AS_PATH routes to, e.g., desired
exit points.
@end deffn



@node BGP network
@section BGP network

@menu
* BGP route::                   
* Route Aggregation::           
* Redistribute to BGP::         
@end menu

@node BGP route
@subsection BGP route

@deffn {BGP} {network @var{A.B.C.D/M}} {}
This command adds the announcement network.
@example
@group
router bgp 1
 network 10.0.0.0/8
@end group
@end example
This configuration example says that network 10.0.0.0/8 will be
announced to all neighbors.  Some vendors' routers don't advertise
routes if they aren't present in their IGP routing tables; @code{bgpd}
doesn't care about IGP routes when announcing its routes.
@end deffn

@deffn {BGP} {no network @var{A.B.C.D/M}} {}
@end deffn

@node Route Aggregation
@subsection Route Aggregation

@deffn {BGP} {aggregate-address @var{A.B.C.D/M}} {}
This command specifies an aggregate address.
@end deffn

@deffn {BGP} {aggregate-address @var{A.B.C.D/M} as-set} {}
This command specifies an aggregate address.  Resulting routes include
AS set.
@end deffn

@deffn {BGP} {aggregate-address @var{A.B.C.D/M} summary-only} {}
This command specifies an aggregate address.  Aggreated routes will
not be announce.
@end deffn

@deffn {BGP} {no aggregate-address @var{A.B.C.D/M}} {}
@end deffn

@node Redistribute to BGP
@subsection Redistribute to BGP

@deffn {BGP} {redistribute kernel} {}
Redistribute kernel route to BGP process.
@end deffn

@deffn {BGP} {redistribute static} {}
Redistribute static route to BGP process.
@end deffn

@deffn {BGP} {redistribute connected} {}
Redistribute connected route to BGP process.
@end deffn

@deffn {BGP} {redistribute rip} {}
Redistribute RIP route to BGP process.
@end deffn

@deffn {BGP} {redistribute ospf} {}
Redistribute OSPF route to BGP process.
@end deffn

@node BGP Peer
@section BGP Peer

@menu
* Defining Peer::               
* BGP Peer commands::           
* Peer filtering::              
@end menu

@node Defining Peer
@subsection Defining Peer

@deffn {BGP} {neighbor @var{peer} remote-as @var{asn}} {}
Creates a new neighbor whose remote-as is @var{asn}.  @var{peer}
can be an IPv4 address or an IPv6 address.
@example
@group
router bgp 1
 neighbor 10.0.0.1 remote-as 2
@end group
@end example
In this case my router, in AS-1, is trying to peer with AS-2 at
10.0.0.1.

This command must be the first command used when configuring a neighbor.
If the remote-as is not specified, @command{bgpd} will complain like this:
@example
can't find neighbor 10.0.0.1
@end example
@end deffn

@node BGP Peer commands
@subsection BGP Peer commands

In a @code{router bgp} clause there are neighbor specific configurations
required.

@deffn {BGP} {neighbor @var{peer} shutdown} {}
@deffnx {BGP} {no neighbor @var{peer} shutdown} {}
Shutdown the peer.  We can delete the neighbor's configuration by
@code{no neighbor @var{peer} remote-as @var{as-number}} but all
configuration of the neighbor will be deleted.  When you want to
preserve the configuration, but want to drop the BGP peer, use this
syntax.
@end deffn

@deffn {BGP} {neighbor @var{peer} ebgp-multihop} {}
@deffnx {BGP} {no neighbor @var{peer} ebgp-multihop} {}
@end deffn

@deffn {BGP} {neighbor @var{peer} description ...} {}
@deffnx {BGP} {no neighbor @var{peer} description ...} {}
Set description of the peer.
@end deffn

@deffn {BGP} {neighbor @var{peer} version @var{version}} {}
Set up the neighbor's BGP version.  @var{version} can be @var{4},
@var{4+} or @var{4-}.  BGP version @var{4} is the default value used for
BGP peering.  BGP version @var{4+} means that the neighbor supports
Multiprotocol Extensions for BGP-4.  BGP version @var{4-} is similar but
the neighbor speaks the old Internet-Draft revision 00's Multiprotocol
Extensions for BGP-4.  Some routing software is still using this
version.
@end deffn

@deffn {BGP} {neighbor @var{peer} interface @var{ifname}} {}
@deffnx {BGP} {no neighbor @var{peer} interface @var{ifname}} {}
When you connect to a BGP peer over an IPv6 link-local address, you 
have to specify the @var{ifname} of the interface used for the 
connection. To specify IPv4 session addresses, see the 
@code{neighbor @var{peer} update-source} command below.

This command is deprecated and may be removed in a future release. Its
use should be avoided.
@end deffn

@deffn {BGP} {neighbor @var{peer} next-hop-self [all]} {}
@deffnx {BGP} {no neighbor @var{peer} next-hop-self [all]} {}
This command specifies an announced route's nexthop as being equivalent
to the address of the bgp router if it is learned via eBGP.
If the optional keyword @code{all} is specified the modifiation is done
also for routes learned via iBGP.
@end deffn

@deffn {BGP} {neighbor @var{peer} update-source @var{<ifname|address>}} {}
@deffnx {BGP} {no neighbor @var{peer} update-source} {}
Specify the IPv4 source address to use for the @acronym{BGP} session to this
neighbour, may be specified as either an IPv4 address directly or
as an interface name (in which case the @command{zebra} daemon MUST be running
in order for @command{bgpd} to be able to retrieve interface state).
@example
@group
router bgp 64555
 neighbor foo update-source 192.168.0.1
 neighbor bar update-source lo0
@end group
@end example
@end deffn

@deffn {BGP} {neighbor @var{peer} default-originate} {}
@deffnx {BGP} {no neighbor @var{peer} default-originate} {}
@command{bgpd}'s default is to not announce the default route (0.0.0.0/0) even it
is in routing table.  When you want to announce default routes to the
peer, use this command.
@end deffn

@deffn {BGP} {neighbor @var{peer} port @var{port}} {}
@deffnx {BGP} {neighbor @var{peer} port @var{port}} {}
@end deffn

@deffn {BGP} {neighbor @var{peer} send-community} {}
@deffnx {BGP} {neighbor @var{peer} send-community} {}
@end deffn

@deffn {BGP} {neighbor @var{peer} weight @var{weight}} {}
@deffnx {BGP} {no neighbor @var{peer} weight @var{weight}} {}
This command specifies a default @var{weight} value for the neighbor's
routes.
@end deffn

@deffn {BGP} {neighbor @var{peer} maximum-prefix @var{number}} {}
@deffnx {BGP} {no neighbor @var{peer} maximum-prefix @var{number}} {}
@end deffn

@deffn {BGP} {neighbor @var{peer} local-as @var{as-number}} {}
@deffnx {BGP} {neighbor @var{peer} local-as @var{as-number} no-prepend} {}
@deffnx {BGP} {neighbor @var{peer} local-as @var{as-number} no-prepend replace-as} {}
@deffnx {BGP} {no neighbor @var{peer} local-as} {}
Specify an alternate AS for this BGP process when interacting with the
specified peer.  With no modifiers, the specified local-as is prepended to
the received AS_PATH when receiving routing updates from the peer, and
prepended to the outgoing AS_PATH (after the process local AS) when
transmitting local routes to the peer.

If the no-prepend attribute is specified, then the supplied local-as is not
prepended to the received AS_PATH.

If the replace-as attribute is specified, then only the supplied local-as is
prepended to the AS_PATH when transmitting local-route updates to this peer.

Note that replace-as can only be specified if no-prepend is.

This command is only allowed for eBGP peers.
@end deffn

@deffn {BGP} {neighbor @var{peer} ttl-security hops @var{number}} {}
@deffnx {BGP} {no neighbor @var{peer} ttl-security hops @var{number}} {}
This command enforces Generalized TTL Security Mechanism (GTSM), as
specified in RFC 5082. With this command, only neighbors that are the
specified number of hops away will be allowed to become neighbors. This
command is mututally exclusive with @command{ebgp-multihop}.
@end deffn

@node Peer filtering
@subsection Peer filtering

@deffn {BGP} {neighbor @var{peer} distribute-list @var{name} [in|out]} {}
This command specifies a distribute-list for the peer.  @var{direct} is
@samp{in} or @samp{out}.
@end deffn

@deffn {BGP command} {neighbor @var{peer} prefix-list @var{name} [in|out]} {}
@end deffn

@deffn {BGP command} {neighbor @var{peer} filter-list @var{name} [in|out]} {}
@end deffn

@deffn {BGP} {neighbor @var{peer} route-map @var{name} [in|out]} {}
Apply a route-map on the neighbor.  @var{direct} must be @code{in} or
@code{out}.
@end deffn

@c -----------------------------------------------------------------------
@node BGP Peer Group
@section BGP Peer Group

@deffn {BGP} {neighbor @var{word} peer-group} {}
This command defines a new peer group.
@end deffn

@deffn {BGP} {neighbor @var{peer} peer-group @var{word}} {}
This command bind specific peer to peer group @var{word}.
@end deffn

@node BGP Address Family
@section BGP Address Family

Multiprotocol BGP enables BGP to carry routing information for multiple
Network Layer protocols. BGP supports multiple Address Family
Identifier (AFI), namely IPv4 and IPv6. Support is also provided for
multiple sets of per-AFI information via Subsequent Address Family
Identifiers (SAFI).  In addition to unicast information, VPN information
@cite{RFC4364} and @cite{RFC4659}, and Encapsulation information
@cite{RFC5512} is supported.

@deffn {Command} {show ip bgp vpnv4 all} {}
@deffnx {Command} {show ipv6 bgp vpn all} {}
Print active IPV4 or IPV6 routes advertised via the VPN SAFI.
@end deffn

@deffn {Command} {show ip bgp encap all} {}
@deffnx {Command} {show ipv6 bgp encap all} {}
Print active IPV4 or IPV6 routes advertised via the Encapsulation SAFI.
@end deffn

@deffn {Command} {show bgp ipv4 encap summary} {}
@deffnx {Command} {show bgp ipv4 vpn summary} {}
@deffnx {Command} {show bgp ipv6 encap summary} {}
@deffnx {Command} {show bgp ipv6 vpn summary} {}
Print a summary of neighbor connections for the specified AFI/SAFI combination.
@end deffn

@c -----------------------------------------------------------------------
@node Autonomous System
@section Autonomous System

The @acronym{AS,Autonomous System} number is one of the essential
element of BGP.  BGP is a distance vector routing protocol, and the
AS-Path framework provides distance vector metric and loop detection to
BGP. @cite{RFC1930, Guidelines for creation, selection, and
registration of an Autonomous System (AS)} provides some background on
the concepts of an AS.

The AS number is a two octet value, ranging in value from 1 to 65535.
The AS numbers 64512 through 65535 are defined as private AS numbers. 
Private AS numbers must not to be advertised in the global Internet.

@menu
* AS Path Regular Expression::  
* Display BGP Routes by AS Path::  
* AS Path Access List::         
* Using AS Path in Route Map::  
* Private AS Numbers::          
@end menu

@node AS Path Regular Expression
@subsection AS Path Regular Expression

AS path regular expression can be used for displaying BGP routes and
AS path access list.  AS path regular expression is based on
@code{POSIX 1003.2} regular expressions.  Following description is
just a subset of @code{POSIX} regular expression.  User can use full
@code{POSIX} regular expression.  Adding to that special character '_'
is added for AS path regular expression.

@table @code
@item .
Matches any single character.
@item *
Matches 0 or more occurrences of pattern.
@item +
Matches 1 or more occurrences of pattern.
@item ?
Match 0 or 1 occurrences of pattern.
@item ^
Matches the beginning of the line.
@item $
Matches the end of the line.
@item _
Character @code{_} has special meanings in AS path regular expression.
It matches to space and comma , and AS set delimiter @{ and @} and AS
confederation delimiter @code{(} and @code{)}.  And it also matches to
the beginning of the line and the end of the line.  So @code{_} can be
used for AS value boundaries match.  @code{show ip bgp regexp _7675_}
matches to all of BGP routes which as AS number include @var{7675}.
@end table

@node Display BGP Routes by AS Path
@subsection Display BGP Routes by AS Path

To show BGP routes which has specific AS path information @code{show
ip bgp} command can be used.  

@deffn Command {show ip bgp regexp @var{line}} {}
This commands display BGP routes that matches AS path regular
expression @var{line}.
@end deffn

@node AS Path Access List
@subsection AS Path Access List

AS path access list is user defined AS path.

@deffn {Command} {ip as-path access-list @var{word} @{permit|deny@} @var{line}} {}
This command defines a new AS path access list.
@end deffn

@deffn {Command} {no ip as-path access-list @var{word}} {}
@deffnx {Command} {no ip as-path access-list @var{word} @{permit|deny@} @var{line}} {}
@end deffn

@node Using AS Path in Route Map
@subsection Using AS Path in Route Map

@deffn {Route Map} {match as-path @var{word}} {}
@end deffn

@deffn {Route Map} {set as-path prepend @var{as-path}} {}
Prepend the given string of AS numbers to the AS_PATH.
@end deffn

@deffn {Route Map} {set as-path prepend last-as @var{num}} {}
Prepend the existing last AS number (the leftmost ASN) to the AS_PATH.
@end deffn

@node Private AS Numbers
@subsection Private AS Numbers

@c -----------------------------------------------------------------------
@node BGP Communities Attribute
@section BGP Communities Attribute

BGP communities attribute is widely used for implementing policy
routing.  Network operators can manipulate BGP communities attribute
based on their network policy.  BGP communities attribute is defined
in @cite{RFC1997, BGP Communities Attribute} and
@cite{RFC1998, An Application of the BGP Community Attribute
in Multi-home Routing}.  It is an optional transitive attribute,
therefore local policy can travel through different autonomous system.

Communities attribute is a set of communities values.  Each
communities value is 4 octet long.  The following format is used to
define communities value.

@table @code
@item AS:VAL
This format represents 4 octet communities value.  @code{AS} is high
order 2 octet in digit format.  @code{VAL} is low order 2 octet in
digit format.  This format is useful to define AS oriented policy
value.  For example, @code{7675:80} can be used when AS 7675 wants to
pass local policy value 80 to neighboring peer.
@item internet
@code{internet} represents well-known communities value 0.
@item no-export
@code{no-export} represents well-known communities value @code{NO_EXPORT}@*
@r{(0xFFFFFF01)}.  All routes carry this value must not be advertised
to outside a BGP confederation boundary.  If neighboring BGP peer is
part of BGP confederation, the peer is considered as inside a BGP
confederation boundary, so the route will be announced to the peer.
@item no-advertise
@code{no-advertise} represents well-known communities value
@code{NO_ADVERTISE}@*@r{(0xFFFFFF02)}.  All routes carry this value
must not be advertise to other BGP peers.
@item local-AS
@code{local-AS} represents well-known communities value
@code{NO_EXPORT_SUBCONFED} @r{(0xFFFFFF03)}.  All routes carry this
value must not be advertised to external BGP peers.  Even if the
neighboring router is part of confederation, it is considered as
external BGP peer, so the route will not be announced to the peer.
@end table

  When BGP communities attribute is received, duplicated communities
value in the communities attribute is ignored and each communities
values are sorted in numerical order.
  
@menu
* BGP Community Lists::         
* Numbered BGP Community Lists::  
* BGP Community in Route Map::  
* Display BGP Routes by Community::  
* Using BGP Communities Attribute::  
@end menu

@node BGP Community Lists
@subsection BGP Community Lists

  BGP community list is a user defined BGP communites attribute list.
BGP community list can be used for matching or manipulating BGP
communities attribute in updates.

There are two types of community list.  One is standard community
list and another is expanded community list.  Standard community list
defines communities attribute.  Expanded community list defines
communities attribute string with regular expression.  Standard
community list is compiled into binary format when user define it.
Standard community list will be directly compared to BGP communities
attribute in BGP updates.  Therefore the comparison is faster than
expanded community list.

@deffn Command {ip community-list standard @var{name} @{permit|deny@} @var{community}} {}
This command defines a new standard community list.  @var{community}
is communities value.  The @var{community} is compiled into community
structure.  We can define multiple community list under same name.  In
that case match will happen user defined order.  Once the
community list matches to communities attribute in BGP updates it
return permit or deny by the community list definition.  When there is
no matched entry, deny will be returned.  When @var{community} is
empty it matches to any routes.
@end deffn

@deffn Command {ip community-list expanded @var{name} @{permit|deny@} @var{line}} {}
This command defines a new expanded community list.  @var{line} is a
string expression of communities attribute.  @var{line} can include
regular expression to match communities attribute in BGP updates.
@end deffn

@deffn Command {no ip community-list @var{name}} {}
@deffnx Command {no ip community-list standard @var{name}} {}
@deffnx Command {no ip community-list expanded @var{name}} {}
These commands delete community lists specified by @var{name}.  All of
community lists shares a single name space.  So community lists can be
removed simpley specifying community lists name.
@end deffn

@deffn {Command} {show ip community-list} {}
@deffnx {Command} {show ip community-list @var{name}} {}
This command display current community list information.  When
@var{name} is specified the specified community list's information is
shown.

@example
# show ip community-list 
Named Community standard list CLIST
    permit 7675:80 7675:100 no-export
    deny internet
Named Community expanded list EXPAND
    permit :

# show ip community-list CLIST
Named Community standard list CLIST
    permit 7675:80 7675:100 no-export
    deny internet
@end example
@end deffn

@node Numbered BGP Community Lists
@subsection Numbered BGP Community Lists

When number is used for BGP community list name, the number has
special meanings.  Community list number in the range from 1 and 99 is
standard community list.  Community list number in the range from 100
to 199 is expanded community list.  These community lists are called
as numbered community lists.  On the other hand normal community lists
is called as named community lists.

@deffn Command {ip community-list <1-99> @{permit|deny@} @var{community}} {}
This command defines a new community list.  <1-99> is standard
community list number.  Community list name within this range defines
standard community list.  When @var{community} is empty it matches to
any routes.
@end deffn

@deffn Command {ip community-list <100-199> @{permit|deny@} @var{community}} {}
This command defines a new community list.  <100-199> is expanded
community list number.  Community list name within this range defines
expanded community list.
@end deffn

@deffn Command {ip community-list @var{name} @{permit|deny@} @var{community}} {}
When community list type is not specifed, the community list type is
automatically detected.  If @var{community} can be compiled into
communities attribute, the community list is defined as a standard
community list.  Otherwise it is defined as an expanded community
list.  This feature is left for backward compability.  Use of this
feature is not recommended.
@end deffn

@node BGP Community in Route Map
@subsection BGP Community in Route Map

In Route Map (@pxref{Route Map}), we can match or set BGP
communities attribute.  Using this feature network operator can
implement their network policy based on BGP communities attribute.

Following commands can be used in Route Map.

@deffn {Route Map} {match community @var{word}} {}
@deffnx {Route Map} {match community @var{word} exact-match} {}
This command perform match to BGP updates using community list
@var{word}.  When the one of BGP communities value match to the one of
communities value in community list, it is match.  When
@code{exact-match} keyword is spcified, match happen only when BGP
updates have completely same communities value specified in the
community list.
@end deffn

@deffn {Route Map} {set community none} {}
@deffnx {Route Map} {set community @var{community}} {}
@deffnx {Route Map} {set community @var{community} additive} {}
This command manipulate communities value in BGP updates.  When
@code{none} is specified as communities value, it removes entire
communities attribute from BGP updates.  When @var{community} is not
@code{none}, specified communities value is set to BGP updates.  If
BGP updates already has BGP communities value, the existing BGP
communities value is replaced with specified @var{community} value.
When @code{additive} keyword is specified, @var{community} is appended
to the existing communities value.
@end deffn

@deffn {Route Map} {set comm-list @var{word} delete} {}
This command remove communities value from BGP communities attribute.
The @var{word} is community list name.  When BGP route's communities
value matches to the community list @var{word}, the communities value
is removed.  When all of communities value is removed eventually, the
BGP update's communities attribute is completely removed.
@end deffn

@node Display BGP Routes by Community
@subsection Display BGP Routes by Community

To show BGP routes which has specific BGP communities attribute,
@code{show ip bgp} command can be used.  The @var{community} value and
community list can be used for @code{show ip bgp} command.

@deffn Command {show ip bgp community} {}
@deffnx Command {show ip bgp community @var{community}} {}
@deffnx Command {show ip bgp community @var{community} exact-match} {}
@code{show ip bgp community} displays BGP routes which has communities
attribute.  When @var{community} is specified, BGP routes that matches
@var{community} value is displayed.  For this command, @code{internet}
keyword can't be used for @var{community} value.  When
@code{exact-match} is specified, it display only routes that have an
exact match.
@end deffn

@deffn Command {show ip bgp community-list @var{word}} {}
@deffnx Command {show ip bgp community-list @var{word} exact-match} {}
This commands display BGP routes that matches community list
@var{word}.  When @code{exact-match} is specified, display only routes
that have an exact match.
@end deffn

@node Using BGP Communities Attribute
@subsection Using BGP Communities Attribute

Following configuration is the most typical usage of BGP communities
attribute.  AS 7675 provides upstream Internet connection to AS 100.
When following configuration exists in AS 7675, AS 100 networks
operator can set local preference in AS 7675 network by setting BGP
communities attribute to the updates.

@example
router bgp 7675
 neighbor 192.168.0.1 remote-as 100
 neighbor 192.168.0.1 route-map RMAP in
!
ip community-list 70 permit 7675:70
ip community-list 70 deny
ip community-list 80 permit 7675:80
ip community-list 80 deny
ip community-list 90 permit 7675:90
ip community-list 90 deny
!
route-map RMAP permit 10
 match community 70
 set local-preference 70
!
route-map RMAP permit 20
 match community 80
 set local-preference 80
!
route-map RMAP permit 30
 match community 90
 set local-preference 90
@end example

Following configuration announce 10.0.0.0/8 from AS 100 to AS 7675.
The route has communities value 7675:80 so when above configuration
exists in AS 7675, announced route's local preference will be set to
value 80.

@example
router bgp 100
 network 10.0.0.0/8
 neighbor 192.168.0.2 remote-as 7675
 neighbor 192.168.0.2 route-map RMAP out
!
ip prefix-list PLIST permit 10.0.0.0/8
!
route-map RMAP permit 10
 match ip address prefix-list PLIST
 set community 7675:80
@end example

Following configuration is an example of BGP route filtering using
communities attribute.  This configuration only permit BGP routes
which has BGP communities value 0:80 or 0:90.  Network operator can
put special internal communities value at BGP border router, then
limit the BGP routes announcement into the internal network.

@example
router bgp 7675
 neighbor 192.168.0.1 remote-as 100
 neighbor 192.168.0.1 route-map RMAP in
!
ip community-list 1 permit 0:80 0:90
!
route-map RMAP permit in
 match community 1
@end example

Following exmaple filter BGP routes which has communities value 1:1.
When there is no match community-list returns deny.  To avoid
filtering all of routes, we need to define permit any at last.

@example
router bgp 7675
 neighbor 192.168.0.1 remote-as 100
 neighbor 192.168.0.1 route-map RMAP in
!
ip community-list standard FILTER deny 1:1
ip community-list standard FILTER permit
!
route-map RMAP permit 10
 match community FILTER
@end example

Communities value keyword @code{internet} has special meanings in
standard community lists.  In below example @code{internet} act as
match any.  It matches all of BGP routes even if the route does not
have communities attribute at all.  So community list @code{INTERNET}
is same as above example's @code{FILTER}.

@example
ip community-list standard INTERNET deny 1:1
ip community-list standard INTERNET permit internet
@end example

Following configuration is an example of communities value deletion.
With this configuration communities value 100:1 and 100:2 is removed
from BGP updates.  For communities value deletion, only @code{permit}
community-list is used.  @code{deny} community-list is ignored.

@example
router bgp 7675
 neighbor 192.168.0.1 remote-as 100
 neighbor 192.168.0.1 route-map RMAP in
!
ip community-list standard DEL permit 100:1 100:2
!
route-map RMAP permit 10
 set comm-list DEL delete
@end example

@c -----------------------------------------------------------------------
@node BGP Extended Communities Attribute
@section BGP Extended Communities Attribute

BGP extended communities attribute is introduced with MPLS VPN/BGP
technology.  MPLS VPN/BGP expands capability of network infrastructure
to provide VPN functionality.  At the same time it requires a new
framework for policy routing.  With BGP Extended Communities Attribute
we can use Route Target or Site of Origin for implementing network
policy for MPLS VPN/BGP.

BGP Extended Communities Attribute is similar to BGP Communities
Attribute.  It is an optional transitive attribute.  BGP Extended
Communities Attribute can carry multiple Extended Community value.
Each Extended Community value is eight octet length.

BGP Extended Communities Attribute provides an extended range
compared with BGP Communities Attribute.  Adding to that there is a
type field in each value to provides community space structure.

There are two format to define Extended Community value.  One is AS
based format the other is IP address based format.

@table @code
@item AS:VAL
This is a format to define AS based Extended Community value.
@code{AS} part is 2 octets Global Administrator subfield in Extended
Community value.  @code{VAL} part is 4 octets Local Administrator
subfield.  @code{7675:100} represents AS 7675 policy value 100.
@item IP-Address:VAL
This is a format to define IP address based Extended Community value.
@code{IP-Address} part is 4 octets Global Administrator subfield.
@code{VAL} part is 2 octets Local Administrator subfield.
@code{10.0.0.1:100} represents 
@end table

@menu
* BGP Extended Community Lists::  
* BGP Extended Communities in Route Map::  
@end menu

@node BGP Extended Community Lists
@subsection BGP Extended Community Lists

Expanded Community Lists is a user defined BGP Expanded Community
Lists.

@deffn Command {ip extcommunity-list standard @var{name} @{permit|deny@} @var{extcommunity}} {}
This command defines a new standard extcommunity-list.
@var{extcommunity} is extended communities value.  The
@var{extcommunity} is compiled into extended community structure.  We
can define multiple extcommunity-list under same name.  In that case
match will happen user defined order.  Once the extcommunity-list
matches to extended communities attribute in BGP updates it return
permit or deny based upon the extcommunity-list definition.  When
there is no matched entry, deny will be returned.  When
@var{extcommunity} is empty it matches to any routes.
@end deffn

@deffn Command {ip extcommunity-list expanded @var{name} @{permit|deny@} @var{line}} {}
This command defines a new expanded extcommunity-list.  @var{line} is
a string expression of extended communities attribute.  @var{line} can
include regular expression to match extended communities attribute in
BGP updates.
@end deffn

@deffn Command {no ip extcommunity-list @var{name}} {}
@deffnx Command {no ip extcommunity-list standard @var{name}} {}
@deffnx Command {no ip extcommunity-list expanded @var{name}} {}
These commands delete extended community lists specified by
@var{name}.  All of extended community lists shares a single name
space.  So extended community lists can be removed simpley specifying
the name.
@end deffn

@deffn {Command} {show ip extcommunity-list} {}
@deffnx {Command} {show ip extcommunity-list @var{name}} {}
This command display current extcommunity-list information.  When
@var{name} is specified the community list's information is shown.

@example
# show ip extcommunity-list 
@end example
@end deffn

@node BGP Extended Communities in Route Map
@subsection BGP Extended Communities in Route Map

@deffn {Route Map} {match extcommunity @var{word}} {}
@end deffn

@deffn {Route Map} {set extcommunity rt @var{extcommunity}} {}
This command set Route Target value.
@end deffn

@deffn {Route Map} {set extcommunity soo @var{extcommunity}} {}
This command set Site of Origin value.
@end deffn

@c -----------------------------------------------------------------------
@node Displaying BGP routes
@section Displaying BGP Routes

@menu
* Show IP BGP::                 
* More Show IP BGP::            
@end menu

@node Show IP BGP
@subsection Show IP BGP

@deffn {Command} {show ip bgp} {}
@deffnx {Command} {show ip bgp @var{A.B.C.D}} {}
@deffnx {Command} {show ip bgp @var{X:X::X:X}} {}
This command displays BGP routes.  When no route is specified it
display all of IPv4 BGP routes.
@end deffn

@example
BGP table version is 0, local router ID is 10.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 1.1.1.1/32       0.0.0.0                  0         32768 i

Total number of prefixes 1
@end example

@node More Show IP BGP
@subsection More Show IP BGP

@deffn {Command} {show ip bgp regexp @var{line}} {}
This command display BGP routes using AS path regular expression (@pxref{Display BGP Routes by AS Path}).
@end deffn

@deffn Command {show ip bgp community @var{community}} {}
@deffnx Command {show ip bgp community @var{community} exact-match} {}
This command display BGP routes using @var{community} (@pxref{Display
BGP Routes by Community}).
@end deffn

@deffn Command {show ip bgp community-list @var{word}} {}
@deffnx Command {show ip bgp community-list @var{word} exact-match} {}
This command display BGP routes using community list (@pxref{Display
BGP Routes by Community}).
@end deffn

@deffn {Command} {show ip bgp summary} {}
@end deffn

@deffn {Command} {show ip bgp neighbor [@var{peer}]} {}
@end deffn

@deffn {Command} {clear ip bgp @var{peer}} {}
Clear peers which have addresses of X.X.X.X
@end deffn

@deffn {Command} {clear ip bgp @var{peer} soft in} {}
Clear peer using soft reconfiguration.
@end deffn

@deffn {Command} {show ip bgp dampened-paths} {}
Display paths suppressed due to dampening
@end deffn

@deffn {Command} {show ip bgp flap-statistics} {}
Display flap statistics of routes
@end deffn

@deffn {Command} {show debug} {}
@end deffn

@deffn {Command} {debug event} {}
@end deffn

@deffn {Command} {debug update} {}
@end deffn

@deffn {Command} {debug keepalive} {}
@end deffn

@deffn {Command} {no debug event} {}
@end deffn

@deffn {Command} {no debug update} {}
@end deffn

@deffn {Command} {no debug keepalive} {}
@end deffn

@node Capability Negotiation
@section Capability Negotiation

When adding IPv6 routing information exchange feature to BGP.  There
were some proposals.  @acronym{IETF,Internet Engineering Task Force}
@acronym{IDR, Inter Domain Routing} @acronym{WG, Working group} adopted
a proposal called Multiprotocol Extension for BGP.  The specification
is described in @cite{RFC2283}.  The protocol does not define new protocols. 
It defines new attributes to existing BGP.  When it is used exchanging
IPv6 routing information it is called BGP-4+.  When it is used for
exchanging multicast routing information it is called MBGP.

@command{bgpd} supports Multiprotocol Extension for BGP.  So if remote
peer supports the protocol, @command{bgpd} can exchange IPv6 and/or
multicast routing information.

Traditional BGP did not have the feature to detect remote peer's
capabilities, e.g. whether it can handle prefix types other than IPv4
unicast routes.  This was a big problem using Multiprotocol Extension
for BGP to operational network.  @cite{RFC2842, Capabilities
Advertisement with BGP-4} adopted a feature called Capability
Negotiation. @command{bgpd} use this Capability Negotiation to detect
the remote peer's capabilities.  If the peer is only configured as IPv4
unicast neighbor, @command{bgpd} does not send these Capability
Negotiation packets (at least not unless other optional BGP features
require capability negotation).

By default, Quagga will bring up peering with minimal common capability
for the both sides.  For example, local router has unicast and
multicast capabilitie and remote router has unicast capability.  In
this case, the local router will establish the connection with unicast
only capability. When there are no common capabilities, Quagga sends
Unsupported Capability error and then resets the connection.

If you want to completely match capabilities with remote peer.  Please
use @command{strict-capability-match} command.
  
@deffn {BGP} {neighbor @var{peer} strict-capability-match} {}
@deffnx {BGP} {no neighbor @var{peer} strict-capability-match} {}
Strictly compares remote capabilities and local capabilities.  If capabilities
are different, send Unsupported Capability error then reset connection.
@end deffn

You may want to disable sending Capability Negotiation OPEN message
optional parameter to the peer when remote peer does not implement
Capability Negotiation.  Please use @command{dont-capability-negotiate}
command to disable the feature.

@deffn {BGP} {neighbor @var{peer} dont-capability-negotiate} {}
@deffnx {BGP} {no neighbor @var{peer} dont-capability-negotiate} {}
Suppress sending Capability Negotiation as OPEN message optional
parameter to the peer.  This command only affects the peer is configured
other than IPv4 unicast configuration.
@end deffn

When remote peer does not have capability negotiation feature, remote
peer will not send any capabilities at all.  In that case, bgp
configures the peer with configured capabilities.

You may prefer locally configured capabilities more than the negotiated
capabilities even though remote peer sends capabilities.  If the peer
is configured by @command{override-capability}, @command{bgpd} ignores
received capabilities then override negotiated capabilities with
configured values.

@deffn {BGP} {neighbor @var{peer} override-capability} {}
@deffnx {BGP} {no neighbor @var{peer} override-capability} {}
Override the result of Capability Negotiation with local configuration.
Ignore remote peer's capability value.
@end deffn

@node Route Reflector
@section Route Reflector

@deffn {BGP} {bgp cluster-id @var{a.b.c.d}} {}
@end deffn

@deffn {BGP} {neighbor @var{peer} route-reflector-client} {}
@deffnx {BGP} {no neighbor @var{peer} route-reflector-client} {}
@end deffn

@node Route Server
@section Route Server

At an Internet Exchange point, many ISPs are connected to each other by
external BGP peering.  Normally these external BGP connection are done by
@samp{full mesh} method.  As with internal BGP full mesh formation,
this method has a scaling problem.

This scaling problem is well known.  Route Server is a method to resolve
the problem.  Each ISP's BGP router only peers to Route Server.  Route
Server serves as BGP information exchange to other BGP routers.  By
applying this method, numbers of BGP connections is reduced from
O(n*(n-1)/2) to O(n).

Unlike normal BGP router, Route Server must have several routing tables
for managing different routing policies for each BGP speaker.  We call the
routing tables as different @code{view}s.  @command{bgpd} can work as
normal BGP router or Route Server or both at the same time.

@menu
* Multiple instance::           
* BGP instance and view::       
* Routing policy::              
* Viewing the view::            
@end menu

@node Multiple instance
@subsection Multiple instance

To enable multiple view function of @code{bgpd}, you must turn on
multiple instance feature beforehand.

@deffn {Command} {bgp multiple-instance} {}
Enable BGP multiple instance feature.  After this feature is enabled,
you can make multiple BGP instances or multiple BGP views.
@end deffn

@deffn {Command} {no bgp multiple-instance} {}
Disable BGP multiple instance feature.  You can not disable this feature
when BGP multiple instances or views exist.
@end deffn

When you want to make configuration more Cisco like one, 

@deffn {Command} {bgp config-type cisco} {}
Cisco compatible BGP configuration output.
@end deffn

When bgp config-type cisco is specified, 

``no synchronization'' is displayed.
``no auto-summary'' is displayed.

``network'' and ``aggregate-address'' argument is displayed as
``A.B.C.D M.M.M.M''

Quagga: network 10.0.0.0/8
Cisco: network 10.0.0.0

Quagga: aggregate-address 192.168.0.0/24
Cisco: aggregate-address 192.168.0.0 255.255.255.0

Community attribute handling is also different.  If there is no
configuration is specified community attribute and extended community
attribute are sent to neighbor.  When user manually disable the
feature community attribute is not sent to the neighbor.  In case of
@command{bgp config-type cisco} is specified, community attribute is not
sent to the neighbor by default.  To send community attribute user has
to specify @command{neighbor A.B.C.D send-community} command.

@example
!
router bgp 1
 neighbor 10.0.0.1 remote-as 1
 no neighbor 10.0.0.1 send-community
!
router bgp 1
 neighbor 10.0.0.1 remote-as 1
 neighbor 10.0.0.1 send-community
!
@end example

@deffn {Command} {bgp config-type zebra} {}
Quagga style BGP configuration.  This is default.
@end deffn

@node BGP instance and view
@subsection BGP instance and view

BGP instance is a normal BGP process.  The result of route selection
goes to the kernel routing table.  You can setup different AS at the
same time when BGP multiple instance feature is enabled.

@deffn {Command} {router bgp @var{as-number}} {}
Make a new BGP instance.  You can use arbitrary word for the @var{name}.
@end deffn

@example
@group
bgp multiple-instance
!
router bgp 1
 neighbor 10.0.0.1 remote-as 2
 neighbor 10.0.0.2 remote-as 3
!
router bgp 2
 neighbor 10.0.0.3 remote-as 4
 neighbor 10.0.0.4 remote-as 5
@end group
@end example

BGP view is almost same as normal BGP process. The result of
route selection does not go to the kernel routing table.  BGP view is
only for exchanging BGP routing information.

@deffn {Command} {router bgp @var{as-number} view @var{name}} {}
Make a new BGP view.  You can use arbitrary word for the @var{name}.  This
view's route selection result does not go to the kernel routing table.
@end deffn

With this command, you can setup Route Server like below.

@example
@group
bgp multiple-instance
!
router bgp 1 view 1
 neighbor 10.0.0.1 remote-as 2
 neighbor 10.0.0.2 remote-as 3
!
router bgp 2 view 2
 neighbor 10.0.0.3 remote-as 4
 neighbor 10.0.0.4 remote-as 5
@end group
@end example

@node Routing policy
@subsection Routing policy

You can set different routing policy for a peer.  For example, you can
set different filter for a peer.

@example
@group
bgp multiple-instance
!
router bgp 1 view 1
 neighbor 10.0.0.1 remote-as 2
 neighbor 10.0.0.1 distribute-list 1 in
!
router bgp 1 view 2
 neighbor 10.0.0.1 remote-as 2
 neighbor 10.0.0.1 distribute-list 2 in
@end group
@end example

This means BGP update from a peer 10.0.0.1 goes to both BGP view 1 and view
2.  When the update is inserted into view 1, distribute-list 1 is
applied.  On the other hand, when the update is inserted into view 2,
distribute-list 2 is applied.

@node Viewing the view
@subsection Viewing the view

To display routing table of BGP view, you must specify view name.

@deffn {Command} {show ip bgp view @var{name}} {}
Display routing table of BGP view @var{name}.
@end deffn

@node How to set up a 6-Bone connection
@section How to set up a 6-Bone connection


@example
@group
zebra configuration 
=================== 
!  
! Actually there is no need to configure zebra 
!

bgpd configuration
==================
!
! This means that routes go through zebra and into the kernel.
!
router zebra
!
! MP-BGP configuration
!
router bgp 7675
 bgp router-id 10.0.0.1
 neighbor 3ffe:1cfa:0:2:2a0:c9ff:fe9e:f56 remote-as @var{as-number}
!
 address-family ipv6
 network 3ffe:506::/32
 neighbor 3ffe:1cfa:0:2:2a0:c9ff:fe9e:f56 activate
 neighbor 3ffe:1cfa:0:2:2a0:c9ff:fe9e:f56 route-map set-nexthop out
 neighbor 3ffe:1cfa:0:2:2c0:4fff:fe68:a231 remote-as @var{as-number}
 neighbor 3ffe:1cfa:0:2:2c0:4fff:fe68:a231 route-map set-nexthop out
 exit-address-family
!
ipv6 access-list all permit any
!
! Set output nexthop address.
!
route-map set-nexthop permit 10
 match ipv6 address all
 set ipv6 nexthop global 3ffe:1cfa:0:2:2c0:4fff:fe68:a225
 set ipv6 nexthop local fe80::2c0:4fff:fe68:a225
!
! logfile FILENAME is obsolete.  Please use log file FILENAME

log file bgpd.log
!
@end group
@end example

@node Dump BGP packets and table
@section Dump BGP packets and table

@deffn Command {dump bgp all @var{path} [@var{interval}]} {}
@deffnx Command {dump bgp all-et @var{path} [@var{interval}]} {}
@deffnx Command {no dump bgp all [@var{path}] [@var{interval}]} {}
Dump all BGP packet and events to @var{path} file.
If @var{interval} is set, a new file will be created for echo @var{interval} of seconds.
The path @var{path} can be set with date and time formatting (strftime).
The type ‘all-et’ enables support for Extended Timestamp Header (@pxref{Packet Binary Dump Format}).
(@pxref{Packet Binary Dump Format})
@end deffn 

@deffn Command {dump bgp updates @var{path} [@var{interval}]} {}
@deffnx Command {dump bgp updates-et @var{path} [@var{interval}]} {}
@deffnx Command {no dump bgp updates [@var{path}] [@var{interval}]} {}
Dump only BGP updates messages to @var{path} file.
If @var{interval} is set, a new file will be created for echo @var{interval} of seconds.
The path @var{path} can be set with date and time formatting (strftime).
The type ‘updates-et’ enables support for Extended Timestamp Header (@pxref{Packet Binary Dump Format}).
@end deffn

@deffn Command {dump bgp routes-mrt @var{path}} {}
@deffnx Command {dump bgp routes-mrt @var{path} @var{interval}} {}
@deffnx Command {no dump bgp route-mrt [@var{path}] [@var{interval}]} {}
Dump whole BGP routing table to @var{path}.  This is heavy process.
The path @var{path} can be set with date and time formatting (strftime).
If @var{interval} is set, a new file will be created for echo @var{interval} of seconds.
@end deffn

Note: the interval variable can also be set using hours and minutes: 04h20m00.


@node BGP Configuration Examples
@section BGP Configuration Examples

Example of a session to an upstream, advertising only one prefix to it.

@example
router bgp 64512
 bgp router-id 10.236.87.1
 network 10.236.87.0/24
 neighbor upstream peer-group
 neighbor upstream remote-as 64515
 neighbor upstream capability dynamic
 neighbor upstream prefix-list pl-allowed-adv out
 neighbor 10.1.1.1 peer-group upstream
 neighbor 10.1.1.1 description ACME ISP
!
ip prefix-list pl-allowed-adv seq 5 permit 82.195.133.0/25
ip prefix-list pl-allowed-adv seq 10 deny any

@end example

A more complex example. With upstream, peer and customer sessions.
Advertising global prefixes and NO_EXPORT prefixes and providing
actions for customer routes based on community values. Extensive use of
route-maps and the 'call' feature to support selective advertising of
prefixes. This example is intended as guidance only, it has NOT been
tested and almost certainly containts silly mistakes, if not serious
flaws.

@example
router bgp 64512
 bgp router-id 10.236.87.1
 network 10.123.456.0/24
 network 10.123.456.128/25 route-map rm-no-export
 neighbor upstream capability dynamic
 neighbor upstream route-map rm-upstream-out out
 neighbor cust capability dynamic
 neighbor cust route-map rm-cust-in in
 neighbor cust route-map rm-cust-out out
 neighbor cust send-community both
 neighbor peer capability dynamic
 neighbor peer route-map rm-peer-in in
 neighbor peer route-map rm-peer-out out
 neighbor peer send-community both
 neighbor 10.1.1.1 remote-as 64515
 neighbor 10.1.1.1 peer-group upstream
 neighbor 10.2.1.1 remote-as 64516
 neighbor 10.2.1.1 peer-group upstream
 neighbor 10.3.1.1 remote-as 64517
 neighbor 10.3.1.1 peer-group cust-default
 neighbor 10.3.1.1 description customer1
 neighbor 10.3.1.1 prefix-list pl-cust1-network in
 neighbor 10.4.1.1 remote-as 64518
 neighbor 10.4.1.1 peer-group cust
 neighbor 10.4.1.1 prefix-list pl-cust2-network in
 neighbor 10.4.1.1 description customer2
 neighbor 10.5.1.1 remote-as 64519
 neighbor 10.5.1.1 peer-group peer
 neighbor 10.5.1.1 prefix-list pl-peer1-network in
 neighbor 10.5.1.1 description peer AS 1
 neighbor 10.6.1.1 remote-as 64520
 neighbor 10.6.1.1 peer-group peer
 neighbor 10.6.1.1 prefix-list pl-peer2-network in
 neighbor 10.6.1.1 description peer AS 2
!
ip prefix-list pl-default permit 0.0.0.0/0
!
ip prefix-list pl-upstream-peers permit 10.1.1.1/32
ip prefix-list pl-upstream-peers permit 10.2.1.1/32
!
ip prefix-list pl-cust1-network permit 10.3.1.0/24
ip prefix-list pl-cust1-network permit 10.3.2.0/24
!
ip prefix-list pl-cust2-network permit 10.4.1.0/24
!
ip prefix-list pl-peer1-network permit 10.5.1.0/24
ip prefix-list pl-peer1-network permit 10.5.2.0/24
ip prefix-list pl-peer1-network permit 192.168.0.0/24
!
ip prefix-list pl-peer2-network permit 10.6.1.0/24
ip prefix-list pl-peer2-network permit 10.6.2.0/24
ip prefix-list pl-peer2-network permit 192.168.1.0/24
ip prefix-list pl-peer2-network permit 192.168.2.0/24
ip prefix-list pl-peer2-network permit 172.16.1/24
!
ip as-path access-list asp-own-as permit ^$
ip as-path access-list asp-own-as permit _64512_
!
! #################################################################
! Match communities we provide actions for, on routes receives from
! customers. Communities values of <our-ASN>:X, with X, have actions:
!
! 100 - blackhole the prefix
! 200 - set no_export
! 300 - advertise only to other customers
! 400 - advertise only to upstreams
! 500 - set no_export when advertising to upstreams
! 2X00 - set local_preference to X00
!
! blackhole the prefix of the route
ip community-list standard cm-blackhole permit 64512:100
!
! set no-export community before advertising
ip community-list standard cm-set-no-export permit 64512:200
!
! advertise only to other customers
ip community-list standard cm-cust-only permit 64512:300
!
! advertise only to upstreams
ip community-list standard cm-upstream-only permit 64512:400
!
! advertise to upstreams with no-export
ip community-list standard cm-upstream-noexport permit 64512:500
!
! set local-pref to least significant 3 digits of the community
ip community-list standard cm-prefmod-100 permit 64512:2100
ip community-list standard cm-prefmod-200 permit 64512:2200
ip community-list standard cm-prefmod-300 permit 64512:2300
ip community-list standard cm-prefmod-400 permit 64512:2400
ip community-list expanded cme-prefmod-range permit 64512:2...
!
! Informational communities
!
! 3000 - learned from upstream
! 3100 - learned from customer
! 3200 - learned from peer
!
ip community-list standard cm-learnt-upstream permit 64512:3000
ip community-list standard cm-learnt-cust permit 64512:3100
ip community-list standard cm-learnt-peer permit 64512:3200
!
! ###################################################################
! Utility route-maps
!
! These utility route-maps generally should not used to permit/deny
! routes, i.e. they do not have meaning as filters, and hence probably
! should be used with 'on-match next'. These all finish with an empty
! permit entry so as not interfere with processing in the caller.
!
route-map rm-no-export permit 10
 set community additive no-export
route-map rm-no-export permit 20
!
route-map rm-blackhole permit 10
 description blackhole, up-pref and ensure it cant escape this AS
 set ip next-hop 127.0.0.1
 set local-preference 10
 set community additive no-export
route-map rm-blackhole permit 20
!
! Set local-pref as requested
route-map rm-prefmod permit 10
 match community cm-prefmod-100
 set local-preference 100
route-map rm-prefmod permit 20
 match community cm-prefmod-200
 set local-preference 200
route-map rm-prefmod permit 30
 match community cm-prefmod-300
 set local-preference 300
route-map rm-prefmod permit 40
 match community cm-prefmod-400
 set local-preference 400
route-map rm-prefmod permit 50
!
! Community actions to take on receipt of route.
route-map rm-community-in permit 10
 description check for blackholing, no point continuing if it matches.
 match community cm-blackhole
 call rm-blackhole
route-map rm-community-in permit 20
 match community cm-set-no-export
 call rm-no-export
 on-match next
route-map rm-community-in permit 30
 match community cme-prefmod-range
 call rm-prefmod
route-map rm-community-in permit 40
!
! #####################################################################
! Community actions to take when advertising a route.
! These are filtering route-maps, 
!
! Deny customer routes to upstream with cust-only set.
route-map rm-community-filt-to-upstream deny 10
 match community cm-learnt-cust
 match community cm-cust-only
route-map rm-community-filt-to-upstream permit 20
!
! Deny customer routes to other customers with upstream-only set.
route-map rm-community-filt-to-cust deny 10
 match community cm-learnt-cust
 match community cm-upstream-only
route-map rm-community-filt-to-cust permit 20
!
! ###################################################################
! The top-level route-maps applied to sessions. Further entries could
! be added obviously..
!
! Customers
route-map rm-cust-in permit 10
 call rm-community-in
 on-match next
route-map rm-cust-in permit 20
 set community additive 64512:3100
route-map rm-cust-in permit 30
!
route-map rm-cust-out permit 10
 call rm-community-filt-to-cust
 on-match next
route-map rm-cust-out permit 20
!
! Upstream transit ASes
route-map rm-upstream-out permit 10
 description filter customer prefixes which are marked cust-only
 call rm-community-filt-to-upstream
 on-match next
route-map rm-upstream-out permit 20
 description only customer routes are provided to upstreams/peers
 match community cm-learnt-cust
!
! Peer ASes
! outbound policy is same as for upstream
route-map rm-peer-out permit 10
 call rm-upstream-out
!
route-map rm-peer-in permit 10
 set community additive 64512:3200
@end example