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
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 ownAS number in the AS Path of the received BGP packet, the packet is dropped. The
receiving Router assumesthat the packet was originated from its own AS and has reached the
same place from where it originatedinitially.
The feature could be a disaster if customers are using same AS number along the various sites
and disallowscustomer sites having identical AS numbers to be linked by another AS number. In
such a scenario, routingupdates 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 theAS number of the sending BGP router. The command is neighbor ip-address asoverride and can only beexecuted under the VPNv4 address-family