Advertising Prefixes with Duplicate AS Numbers

When a BGP speaker receives a route that has the speaker’s AS number in its AS path, the speaker declares that route to be a loop and discards it. However, in some circumstances, as in the implementation of a hub-and-spoke VPN topology, this is not the desired behavior. You want the BGP speaker (hub) to accept such routes. You can use the neighbor allowas-in command to specify the number of times that a route’s AS path can contain the BGP speaker’s AS number.

The behavior is different within the VPNv4 address family than it is in other address families. For other address families, you must configure the feature on all the peers. In contrast, IBGP peers within the VPNv4 address family always accept routes containing their own AS number by default. Issuing this command in the VRF for such a peer has no effect on the behavior of IBGP peers in this address family. This behavior reduces the provisioning overhead for VPNv4 IBGP peers.

However, you must configure the feature on the peer router at the hub. Consider the hub-and-spoke topology shown in Figure 98. PE 1, PE 2, and PE 3 are peers in the VPNv4 address family. Routes received from CE 1 may contain the AS number (777) local to the PE routers. You must issue the neighbor allowas-in command for VRF A on PE 1.

Figure 98: Allowing Local AS in VPNv4 Address Family

neighbor allowas-in

• Use to enable the acceptance of all routes whose AS path contains the BGP speaker’s AS number up to the specified number of times.
• If the AS path of a route contains the speaker’s AS number more than the specified number of times, the route is determined to be a loop and is discarded.
• 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

  host1(config-router)#neighbor allowas-in

• Use the no version to prevent the acceptance of these routes, resulting in the BGP speaker’s discarding the routes.
• See neighbor allowas-in.

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