Home » Cisco » 642-889 » What must be enabled on the PE router?
In Layer 3 MPLS VPN implementations, if a customer is using the same AS number at both customer sites and the PE-to-CE routing protocol is BGP, what must be enabled on the PE router?
A. BGP AS override
B. BGP allowas-in
C. BGP SOO extended community
D. BGP AS path prepending
Correct Answer: A
Explanation/Reference:
Explanation:
https://supportforums.cisco.com/docs/DOC-21837
Loop prevention in BGP is done by verifying the AS number in the AS Path. If the receiving router sees its own AS number in the AS Path of the received BGP packet, the packet is dropped. The receiving Router assumes that the packet was originated from its own AS and has reached the same place from where it originated initially.
The feature could be a disaster if customers are using same AS number along the various sites and disallows customer sites having identical AS numbers to be linked by another AS number. In such a scenario, routing updates from one site will be dropped when the other site receives them. To override this feature, AS-Override function causes to replace the AS number of originating router with the AS number of the sending BGP router. The command is neighbor ip-address as- override and can only be executed under the VPNv4 address-family
