Saturday, April 11, 2009

BGP As PE-CE With Backboor Link

Generally customers look for the solution where the primary link is from MPLSVPN service provider and leased line or satellite is used as backup. In these scenarios most of the time customers usually use OSPF as routing protocol in their core as well as with service provider. The prime disadvantage of using as PE-CE routing protocol is that always the leased or satellite link will prefer. To overcome this problem sham link is used which is described in draft “OSPF as the PE/CE Protocol in BGP/MPLS VPNs”. So sham link is the best option.
But what will happen if the same customer uses BGP as PE-CE routing protocol and OSPF is used by the secondary links. Which route will be preferred and why? Answer is simple always BGP route will be preferred because e-BGP learned has lower administrative distance than OSPF.
Now think a case when the link from PE-CE flaps and e-BGP goes down. The traffic will flow from the secondary link with no pros and cons. After few minutes primary link from MPLSVPN service provider comes up and the traffic should flow via MPLSVPN service provider instead of back door. This is what we think but actually this won’t happen. The traffic will never move on to the MPLSVPN service provider and it will follow the secondary path.

What the reason for this:-
Actually what happens the routes which are redistributed from OSPF to BGP are having weight of 0 and the routes which are already in routing table have weight of 32768. So when the service provider link goes down in that case the remote end route which is coming via bgp will come by OSPF and redistribute in BGP. It means that route is having weight of 32768. Now the service provider link comes up and the same route comes by service provider cloud with weight of 0. So BGP will check the weight attribute and it checks the same route is coming by two different weights, one is 32768 and another with 0. So it will always prefer the weight 32768 consequence traffic will flow from the backdoor link instead of primary link.

Solution Of Problem:-
Set the weight 37775 at CE for the routes coming from service provider cloud. So during route installation time always the service provider route will be preferred.


regards
shivlu jain

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3 comments:

Kevin said...

so.. if the backdoor is the primary link, how can you fix the issues?

Linkon Khan said...

I have read your post but i am searching leased line related post. I didn't understand clearly.
wan leased line

Linkon Khan said...

We have a leased line company we provide MPLS service.