Juniper JUNOSE 11.2.X BGP AND MPLS User Manual

Checking AS Path Length
You can use the bgp maxas-limit command to prevent the forwarding of routes having
AS paths longer than a specified limit.
bgp maxas-limit
• Use to require BGP to check the AS path in all received update messages.
• If a received AS path is longer than the specified limit:
• The route is stored in the BGP routing table and therefore is displayed by the show
ip bgp commands.
• The route is not a candidate for being selected as a best path, is not stored in the
forwarding information base, and is not propagated to external or internal peers.
• Changes in the limit do not affect routes previously received. Clearing the BGP sessions
(clear ip bgp) forces a resend of all routes; the new limits are then applied on receipt
of the routes.
• Example
host1(config-router)#bgp maxas-limit 42
• Causes BGP to check the AS path of all routes received after you issue the command.
To apply the new behavior to routes that are already present in the BGP routing table,
you must use the clear ip bgp command to perform a soft clear or hard clear of the
current BGP session.
• Use the no version to halt checking of received AS path lengths.
• See bgp maxas-limit
If you use the fields as-path option with the show ip bgp command, the display indicates
routes whose AS path exceeds the limit. The following display illustrates the result of
setting the AS path length limit to 5:
host1:3# show ip bgp fields intro best peer loc-pref as-path
Local router ID 13.13.13.3, local AS 200
10 paths, 5 distinct prefixes (520 bytes used)
6 paths selected for route table installation
14 path attribute entries (1943 bytes used)
Status codes: > best
Prefix Peer LocPrf AS-path
10.23.40.1/32 192.168.13.1 200 100 211 32 15 67 44 (too long)
> 10.23.40.1/32 172.123.23.2 100 100 211
> 10.23.40.2/32 192.168.13.1 200 100 211 32 15 67
10.23.40.2/32 172.123.23.2 100 100 211 32
> 10.23.40.3/32 192.168.13.1 100 211 32 15
10.23.40.3/32 172.123.23.2 100 211 32 15
10.23.40.4/32 192.168.13.1 100 100 211 32
> 10.23.40.4/32 172.123.23.2 200 100 211 32 15 67
> 10.23.40.5/32 192.168.13.1 100 100 211
10.23.40.5/32 172.123.23.2 200 100 211 32 15 67 44 (too long)
Copyright © 2010, Juniper Networks, Inc.36
JunosE 11.2.x BGP and MPLS Configuration Guide

Juniper JUNOSE 11.2.X BGP AND MPLS Specifications

General

Brand	Juniper
Model	JUNOSE 11.2.X BGP AND MPLS
Category	Software
Language	English