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New Member

eiBGP Load Sharing and Routing Loops

When using maximum-paths eiBGP functionality in an MPLS VPN context, what is the mechanism that

prevents routing loops between PE's?

For example, suppose two PE's have eiBGP configured under the address family associated with a particular

VPN. The two PE's are connected to two CE's (within the same VPN) that advertise the same prefix to both

PE's. As such, each PE would have two entries for that prefix in the VRF routing table associated with

the particular VPN - one route learned via eBGP from the adjacent CE and one learned via iBGP from the other PE. If a packet is received by one or the other of the PE's destined for that prefix, what prevents the packet from looping between the two PE's? Or, can this configuration not be supported with eiBGP?

Thanks for your help.

4 REPLIES
Silver

Re: eiBGP Load Sharing and Routing Loops

I'm not sure there is a routing mechanism to detect routing loops in the scenario given. Note that normally, the maximum path eibgp would be configured on any one (just one) of the routers

Cisco Employee

Re: eiBGP Load Sharing and Routing Loops

It doesn't really matter if you do it on all PEs. If you have a case where two PEs point at each other (FIB wise) for a given path an you originate traffic from one of them, the packets flow from one PE to the other, the LFIB is looked up on the egress PE and the packets are forwarded to the CE router. The problem would be if the FIB was looked up, which is not the case on the egress PE.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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New Member

Re: eiBGP Load Sharing and Routing Loops

I don't think you should do that in this scenario. What's more you should you SOO to control the loop.

Re: eiBGP Load Sharing and Routing Loops

Harold Ritter is right. Just to further clarify his statement: a packet sent from one PE to another PE containing IP packets from a VPN customer will be labeled. So the LFIB will be looked up. The label used on the egress PE is the VPN label assigned by the egress PE with the meaning "send IP packet to CE".

There could be a problem with aggregate labels, which basically mean that the egress PE has to lookup the VRF IP routing table. Aggregate labels can occur when routing summarization is used.

So to sum everything up: there is no problem with "maximum-path eibgp" except for some very specific cases.

regards

Martin

P.S.: SOO is a completely different animal ;-)

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