Discussion

This command also shows other information about the BGP session. The first line
shows the addresses and AS number of the peer border router and the address and
AS number of the local router:
Peer: 10.0.31.1+1778 AS 65505 Local: 10.0.31.2+179 AS 65500
The number following the remote peer’s address is the TCP port number used for
the connection. The second line shows the peer description that you configured with
the set description command.
The Options line shows the BGP options that the peers have agreed to use for the
session:
Options:
The next lines of the output show the specific values of options and other information
about the session:
Holdtime: 90 Preference: 170
Holdtime is the hold timer value, which is the maximum length of time that one peer
will wait to get a BGP message from the other side (either an update or a keepalive
message) before assuming that this session is down. RPD uses the Preference value
to select among routes learned from different sources (see Table 8-2). These two output
fields report the values configured with the hold-time and preference statements.
Because we haven’t configured these values, both fields show the default
values: 170 for the route preference and 90 seconds for the hold time (three times the
default keepalive message interval of 30 seconds).
A few lines down in the output, you see the keepalive interval. The Number of flaps
field tells you whether the BGP session has gone down and come back up:
Number of flaps: 0
Because we just established the session, the value is 0. You can use this field to track
whether the session has been interrupted. The Peer ID and Local ID fields show the
router ID of each peer, which are configured with the set routing-options router-id
command. The Active Holdtime field is the hold timer that has been negotiated
between the BGP peers and is actually being used on the session.
Peer ID: 192.168.14.1 Local ID: 192.168.16.1 Active Holdtime: 90
The next several lines show the NLRI learned from the BGP update messages sent on
this session. These two lines show the address family that is being advertised by the
peer and used by the session, which can be either unicast (as shown here) or multicast:
NLRI advertised by peer: inet-unicast
NLRI for this session: inet-unicast
The route refresh line shows that the peer supports the BGP route refresh capability,
defined in RFC 2918, which allows BGP peers to readvertise their prefixes to the
peer. Route refresh facilitates nondisruptive routing-policy changes.
426 | Chapter 13: BGP
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.