bgp.tex

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xx\=xx\=xx\=xx\=xx\=\kill
protocols \{\\
\>bgp \{\\
\>\>bgp-id: 128.16.32.1\\
\>\>local-as: 45678\\
\\
\>\>peer 192.168.150.1 \{\\
\>\>\>local-ip: 128.16.64.4\\
\>\>\>as: 34567\\
\>\>\>next-hop: 128.16.64.4\\
\>\>\>holdtime: 120\\
\\
\>\>\>/* IPv4 unicast is enabled by default  */\\
\>\>\>ipv4-unicast: true\\
\\
\>\>\>/* Optionally enable other AFI/SAFI combinations */\\
\>\>\>ipv4-multicast: true\\
\>\>\>ipv6-unicast: true\\
\>\>\>ipv6-multicast: true\\
\>\>\}\\
\>\}\\
\}
\end{tabbing}
\end{alltt}
\end{minipage}
}
\vspace{0.1in}

This configuration is from a BGP router in AS 45678.  The router has a
BGP identifier of 128.16.32.1, which will normally be one of the router's
IP addresses.

This router has only one BGP peering configured, with a peer on IP
address 192.168.150.1.  This peering is an EBGP connection because the
peer is in a different AS (34567).  This router's IP address used for
this peering is 128.16.64.4, and the router is also configured to set
the next hop router field in routes it advertises to the peer to be
128.16.64.4.  Setting local-ip and next-hop to be the same is common
for EBGP connections.  The holdtime for the peering is configured to
be 120 seconds, but the precise value of the holdtime actually used
depends on negotiation with the peer.  In addition to IPv4 unicast
routing, which is enabled by default, this peering is configured to
allow the sending an receiving of IPv4 multicast routes and IPv6
unicast routes.

This router is also configured to {\it originate} routing
advertisements for two subnets.  These subnets might be directly
connected, or might be reachable via IGP routing.

The first advertisement this router originates is for subnet
128.16.16/24, reachable via both unicast and multicast.  The nexthop
specified in 128.16.64.1, and this must be reachable via other routes
in the routing table, or this advertisement will not be made.  If this
router had any IBGP peerings, then the BGP route advertised to those
peers would indicate that 128.16.16/24 was reachable via next hop
128.16.64.1. However in this case the only peering is an EBGP peering,
and the next hop in {\it all} routes sent to that peer is set to
128.16.64.4 according to the {\stt nexthop} directive for the peering.

The second advertisement is for an IPv6 route, configured to be usable
only by IPv6 unicast traffic.

\newpage
\section{Monitoring BGP}

On a router running BGP, the BGP routing state can be displayed using
the {\stt show bgp} operational-mode command.  Information is available
about the status of BGP peerings and about the routes received and
used.  In the 1.0 release, the set of commands is fairly limited, and
will be increased in future releases to provide better ways to display
subsets of this information.

As always, command completion using $<$TAB$>$ or ? will display the
available sub-commands and parameters:

\vspace{0.1in}
\noindent\framebox[\textwidth][l]{\scriptsize
\begin{minipage}{6in}
\begin{alltt}
\begin{tabbing}
xx\=xxxxxxxxxxxxxxxxxxxxx\=\kill
user@hostname> \textbf{show bgp ?}\\
Possible completions:\\
\>peers\>Show BGP peers info\\
\>routes\>Print BGP routes\\
\>|\>Pipe through a command\\
\end{tabbing}
\end{alltt}
\end{minipage}
}

The {\stt show bgp peers} command will display information about the
BGP peerings that have been configured.  It supports the optional
parameter {\stt detail} to give a lot more information:

\vspace{0.1in}
\noindent\framebox[\textwidth][l]{\scriptsize
\begin{minipage}{6in}
\begin{alltt}
\begin{tabbing}
xx\=xxxxxxxxxxxxxxxxxxxxx\=\kill
user@hostname> \textbf{show bgp peers ?}\\
Possible completions:\\
\><[Enter]>\>Execute this command\\
\>detail\>Show detailed BGP peers info\\
\>|\>Pipe through a command\\
\end{tabbing}
\end{alltt}
\end{minipage}
}

By itself, {\stt show bgp peers} provides a short list of the peerings
that are configured, irrespective of whether the peering is
in established state or not:

\vspace{0.1in}
\noindent\framebox[\textwidth][l]{\scriptsize
\begin{minipage}{6in}
\begin{alltt}
\begin{tabbing}
xx\=xxxxxxxxxxxxxxxx\=\kill
user@hostname> \textbf{show bgp peers} \\
Peer 1: local 192.150.187.112/179 remote 69.110.224.158/179\\
Peer 2: local 192.150.187.112/179 remote 192.150.187.2/179\\
Peer 3: local 192.150.187.112/179 remote 192.150.187.78/179\\
Peer 4: local 192.150.187.112/179 remote 192.150.187.79/179\\
Peer 5: local 192.150.187.112/179 remote 192.150.187.109/179\\
\end{tabbing}
\end{alltt}
\end{minipage}
}

\newpage
The command {\stt show bgp peers detail} will give a large amount of
information about all the peerings:  

\vspace{0.1in}
\noindent\framebox[\textwidth][l]{\scriptsize
\begin{minipage}{6in}
\begin{alltt}
\begin{tabbing}
xx\=xx\=xx\=xx\=xx\=\kill
user@hostname> \textbf{show bgp peers detail}\\
Peer 1: local 192.150.187.112/179 remote 69.110.224.158/179\\
\>Peer ID: none\\
\>Peer State: ACTIVE\\
\>Admin State: START\\
\>Negotiated BGP Version: n/a\\
\>Peer AS Number: 65014\\
\>Updates Received: 0,  Updates Sent: 0\\
\>Messages Received: 0,  Messages Sent: 0\\
\>Time since last received update: n/a\\
\>Number of transitions to ESTABLISHED: 0\\
\>Time since last in ESTABLISHED state: n/a\\
\>Retry Interval: 120 seconds\\
\>Hold Time: n/a,  Keep Alive Time: n/a\\
\>Configured Hold Time: 120 seconds,  Configured Keep Alive Time: 40 seconds\\
\>Minimum AS Origination Interval: 0 seconds\\
\>Minimum Route Advertisement Interval: 0 seconds\\
\\
Peer 2: local 192.150.187.112/179 remote 192.150.187.2/179\\
\>Peer ID: 192.150.187.2\\
\>Peer State: ESTABLISHED\\
\>Admin State: START\\
\>Negotiated BGP Version: 4\\
\>Peer AS Number: 64999\\
\>Updates Received: 52786,  Updates Sent: 28\\
\>Messages Received: 52949,  Messages Sent: 189\\
\>Time since last received update: 2 seconds\\
\>Number of transitions to ESTABLISHED: 17\\
\>Time since last entering ESTABLISHED state: 6478 seconds\\
\>Retry Interval: 120 seconds\\
\>Hold Time: 120 seconds,  Keep Alive Time: 40 seconds\\
\>Configured Hold Time: 120 seconds,  Configured Keep Alive Time: 40 seconds\\
\>Minimum AS Origination Interval: 0 seconds\\
\>Minimum Route Advertisement Interval: 0 seconds\\
\\
\end{tabbing}
\end{alltt}
\end{minipage}
} 

\vspace{0.1in}
The most important piece of information is typically whether or not
the peering is in ESTABLISHED state, indicating that the peering is up
and capable of exchanging routes.  ACTIVE state means that the peering
is configured to be up on this router, but for some reason the peering
is not currently up.  Typically this is because the remote peer is
unreachable, or because no BGP instance is running on the remote peer.

\newpage
The {\stt show bgp routes} command displays the routes received by BGP
from its peers.  On a router with a full BGP routing table (140000
routes as of July 2004) this command will produce a large amount of
output:

\vspace{0.1in}
\noindent\framebox[\textwidth][l]{\scriptsize
\begin{minipage}{6in}
\begin{alltt}
\begin{tabbing}
xxx\=xxxxxxxxxxxxxxxx\=xxxxxxxxxxxxxxxx\=xxxxxxxxxxxxxxx\=xx\=\kill
user@hostname> \textbf{show bgp routes}\\
Status Codes: * valid route, > best route\\
Origin Codes: i IGP, e EGP, ? incomplete\\
\\
\>Prefix\>Nexthop\>Peer\>AS Path\\
\>------\>-------\>----\>-------\\
*>\>3.0.0.0/8\>192.150.187.2\>192.150.187.2\>16694 25 2152 3356 7018 80 i\\
*>\>4.17.225.0/24\>192.150.187.2\>192.150.187.2\>16694 25 2152 11423 209 701 11853 6496 i\\
*>\>4.17.226.0/23\>192.150.187.2\>192.150.187.2\>16694 25 2152 11423 209 701 11853 6496 i\\
*>\>4.17.251.0/24\>192.150.187.2\>192.150.187.2\>16694 25 2152 11423 209 701 11853 6496 i\\
*>\>4.17.252.0/23\>192.150.187.2\>192.150.187.2\>16694 25 2152 11423 209 701 11853 6496 i\\
*>\>4.21.252.0/23\>192.150.187.2\>192.150.187.2\>16694 25 2152 11423 209 701 6389 8063 19198 i\\
*>\>4.23.180.0/24\>192.150.187.2\>192.150.187.2\>16694 25 2152 11423 209 3561 6128 30576 i\\
*>\>4.36.200.0/21\>192.150.187.2\>192.150.187.2\>16694 25 2152 174 3561 14742 11854 14135 i\\
*>\>4.78.0.0/21\>192.150.187.2\>192.150.187.2\>16694 25 2152 11423 209 3561 6347 23071 22938 i\\
*>\>4.78.32.0/21\>192.150.187.2\>192.150.187.2\>16694 25 2152 174 3491 29748 i\\
*>\>4.0.0.0/8\>192.150.187.2\>192.150.187.2\>16694 25 2152 3356 i\\
...
\end{tabbing}
\end{alltt}
\end{minipage}
}
\vspace{0.1in}

The format of the output is one route per line.  On each line:
\begin{itemize}
\item A status code is displayed, showing whether the route is valid,
  and whether it was the best BGP route this router has received.  A
  route is valid if the nexthop is reachable and it isn't filtered by
  the inbound BGP filters.
\item The network prefix for which the route applies is listed in the
  form {\stt 4.17.226.0/23}.  This indicates the base address for the
  network (address {\stt 4.17.226.0}), and the prefix length ({\stt 23} bits).
  Thus this route applies for addresses {\stt 4.17.226.0} to {\stt
  4.17.227.255} inclusive.
\item The nexthop is the IP address of the intermediate router towards
  which packet destined for the network prefix should be sent.  In
  this example all the displayed routes have the same nexthop.
\item The peer is the IP address of the BGP router which sent us this
  route.  The nexthop and the peer need not the the same (they often
  aren't with IBGP peerings for example) but in all the routes in this
  example they are the same.
\item The AS path is listed next.  This lists the AS numbers of the
  autonomous systems that the route has traversed to reach our
  router.  The AS at the left end of the path is the one nearest to
  our router and the one at the right end of the path is usually the
  AS number of the route's originator.  
\item Finally, whether the route's origin is from an IGP ({\stt i}),
  from EGP ({\stt e}, mostly obsolete), or incomplete ({\stt ?}) is
  listed.
\end{itemize}

\subsection{BGP MIB}

XORP includes SNMP support for BGP, though the BGP-4 MIB defined in
RFC 1657.

\section{BGP path selection}

BGP can receive mutiple paths to the same destination, there are a
complicated set of rules to determine which path should be used. At
this time there are no configuration options that modify the behaviour
of the the path selection process, apart from policy that explicity
drops or modifies a path.

\subsection{Reasons for ignoring a path}

BGP requires that there is an Interior Gateway Protocol (IGP) route to
the NEXT\_HOP, if no route exists the route is ignored. It is expected
in release 1.5 that a switch will be available to toggle this
requirement.

If the NEXT\_HOP in a path belongs to the router itself then the path
is ignored.

\subsection{BGP decision process}

The BGP decision process is performed exactly as defined by RFC 4271.