Wednesday, December 23, 2009

PE-CE BGP Link Protection In Case Of Dual POP

Customers always complaint about the connectivity loss in case of dual connectivity from service provider. I have seen this issue in many cases when BGP is implemented as PE-CE routing protocol. BGP default behaviour, after a PE-CE link failure nad its detection in routing, all the BGP learned routes removed via the failing link. BGP runs the best path algorithm for the failure routes and then selects the alternate path. The local labes are removed and the same message of withdrawal is sent to all the BGP neighbors. After the withdrawal message the best path algorithm runs and new prefixes are switched to the alternate paths. This is time consuming process, normally take less than 180 ms. But sometimes the applications are so critical and the customer could not bear even the loss of seconds. For more click on Read More!

For this existing problem of long convergence Cisco has introduced new solution of link protection. The MPLS VPN PE-CE Link Protection feature reduces time by sending the broken link’s traffic over a backup path instead of waiting for total network convergence. The feature accomplishes this by maintaining the local label (for 5 minutes) for the prefixes which are switching from the failing local path to the backup path. Because the label is not freed as had been the usual practice, forwarding can continue to take place, but now to the alternate (backup) path chosen by the bestpath algorithm. Thus, the local label has been applied in place of the failed BGP bestpath label; (this is sometimes called “label swapping”). Traffic thereby is locally restored, while the network propagation of the BGP withdrawal messages takes place. Eventually, the egress PE router converges and bypasses
the local repair
The given feature is enabled under vrf and address-family of particular vpn on primary PE
protection local-prefixes

People who read this post also read :


clucas said...

Really interesting post!

Thank you :)

.a7. said...

Could you please elaborate. I really did not understand how it works technically and which set of commands we need to configure and where? To what extend we can reduce convergence period?