BGP neighbor allowas-in

The allowas-in command prevents the looped-back information from being dropped, breaking the routing loop protection:

If the AS_PATH attribute of a BGP route contains an AS loop, the BGP route should be excluded from the Phase 2 decision function. AS loop detection is done by scanning the full AS path (as specified in the AS_PATH attribute), and checking that the autonomous system number of the local system does not appear in the AS path. It is critical that BGP speakers within an AS do not make conflicting decisions regarding route selection that would cause forwarding loops to occur.

Video Demonstration

from: BGP Commands on Cisco IOS

allowas-in

To allow an AS path with the provider edge (PE) autonomous system number (ASN) a specified number of times, use the allowas-in command in an appropriate configuration mode. To restore the system to its default condition, use the no form of this command.

allowas-in [as-occurrence-number]

no allowas-in [as-occurrence-number]

Syntax Description

as-occurrence-number

(Optional) Number of times a PE ASN is allowed. Range is 1 to 10.

Files:
Topology
R1
R2
R3