Checking As Path Length - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration 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
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:
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.
Use the no version to halt the removal of private AS numbers in updates sent to
external peers. Use the default version to remove the explicit configuration from
the peer or peer group and reestablish inheritance of the feature configuration.
See neighbor remove-private-as
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
Chapter 1: Configuring BGP Routing
Configuring BGP Peer Groups
35