Preventing Routing Loops - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

JunosE 11.2.x BGP and MPLS Configuration Guide
neighbor as-override

Preventing Routing Loops

448
host1:vr1(config-router-af)#neighbor 172.25.14.12 as-override
Use to enable the use of the same AS number for all CE sites by substituting the current
router's AS number in routing tables for that of the neighboring router.
If you specify a BGP peer group by using the peer-group-name argument, all the
members of the peer group inherit the characteristic configured with this command.
You cannot override the characteristic for a specific member of the peer group.
New policy values are applied to all routes that are sent (outbound policy) or received
(inbound policy) after you issue the command.
To apply the new policy 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.
Behavior is different for outbound policies configured for peer groups for which you
have enabled Adj-RIBs-Out. If you change the outbound policy for such a peer group
and want to fill the Adj-RIBs-Out table for that peer group with the results of the new
policy, you must use the clear ip bgp peer-group command to perform a hard clear or
outbound soft clear of the peer group. You cannot merely perform a hard clear or
outbound soft clear for individual peer group members because that causes BGP to
resend only the contents of the Adj-RIBs-Out table.
Example 1
host1:vr1(config-router)#neighbor 192.168.255.255 as-override
Example 2
host1(config-router)#neighbor 192.168.1.158 as-override
Use the no version to halt the substitution of the AS numbers. Use the default version
to remove the explicit configuration from the peer or peer group and reestablish
inheritance of the feature configuration.
See neighbor as-override.
Routing loops can occur when routes learned from a peer are later advertised back to
that peer. Normally such routing loops are prevented by the AS path attribute. However,
the AS path cannot prevent routing loops in a network configuration with the following
characteristics:
BGP is running between CE and PE routers.
You use a single AS number for all customer sites, and have issued the neighbor
as-override command for the PE routers.
A CE router is dual-homed to two or more PE routers.
The site-of-origin extended community attribute enables BGP to filter out such routes
to prevent routing loops in this network. You can use the set extcommunity command
to specify a site of origin and then use the match extcommunity command and an
Copyright © 2010, Juniper Networks, Inc.