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Friday, October 29, 2010

Does ebgp-multihop can choose next-hop router?

The problem arises when users at CE1 wanted to access network from CE3, but CE3 and CE4 is advertising the same network. There are several ways we think it should resolve the issue, such as using GRE tunnel point2point. But we are reluctant to use the tunnel concept as user may affected by the mtu-sizing problem, multiple tunnels creation and such. Please take note that PE to PE communication using BGP. PE to CE using OSPF with all network advertise under area 0. PE-CE having mutual redistribution. No stub configuration created between PE-CE.

We were thinking to use ebgp-multihop, since CEs are not directly connected, we could create another BGP AS, just for CE to CE communicate. But not sure though. Is it possible to do since peering neighbors will create bgp next-hop to the peering router interfaces.

In an example, CE1 (lo0 & CE3 (lo0, both configured accordingly on the BGP/Ebgp-multihop. So if i do"show ip bgp" at CE1 router, my next-hop will show for my advertise network at CE3. By right my traffic from CE1 should go to as the next-hop. Since the traffic needs to go to PE(Could), the particular PE would then change the next-hop of my original traffic, as at PE level they may see that CE4 is having the same advertise network with better cost/metric.

Does ebgp-multihop can choose our next-hop router? Can ospf can be sagregated by multiple area topology?


Ebgp multi-hop will not work because the traffic won't be tunneled and the PE will still take part of the routing decision.

The next-hop on the ebgp multi-hop will reflect the IP address of the remote CE but a recursive lookup will be performed and guess what? the next hop to that next-hop is the PE router anyway. The PE router will determine the best path - if the destination has conflicting address - anything can happen.You can implement NAT from the conflicted source to solve this issue.

If you have access to the PE, you can have CE3 and CE4 part of a different VRF and only import the 'interested' VRF into CE1.

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