Figure 34: AS-Path Attributes
A routing loop exists if router London accepts the route from router Berlin. Router London
can choose not to accept the route from router Berlin because it recognizes from the
AS-path attribute (11 621 47) that the route originated in its own AS 47.
As a matter of routing policy, you can prepend additional AS numbers to the AS-path
attribute for a route with a set as-path prepend clause in an outbound route map.
Changing the AS path enables you to influence which of several routes for the same
destination prefix is selected as the best route.
Configuring a Local AS
You can change the local AS of a BGP peer or peer group within the current address
family with the neighbor local-as command. By using different local AS numbers for
different peers, you can avoid or postpone AS renumbering in the event the ASs are
merged.
neighbor local-as
• Use to assign a local AS to the given BGP peer or peer group.
• If you specify a BGP peer group by using the peerGroupName argument, all the members
of the peer group inherit the characteristic configured with this command unless it is
overridden for a specific peer.
• This command takes effect immediately and automatically bounces the BGP session.
• Use the no version for an individual peer to restore the value set for the peer group, if
present, or set globally for BGP with the router bgp command. Use the no version for
a peer group to restore the value set globally for BGP.
• See neighbor local-as.
The following example commands change the local AS number for peer 104.4.2 from
the global local AS of 100 to 32:
host1(config)#router bgp 100
host1(config-router)#address-family ipv4 unicast vrf boston
host1(config-router)#neighbor 10.4.4.2 remote-as 645
host1(config-router)#neighbor 10.4.4.2 local-as 32
Copyright © 2010, Juniper Networks, Inc.118
JunosE 11.2.x BGP and MPLS Configuration Guide
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