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?
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) firstname.lastname@example.org México móvil: +52 1 55 8312 4915 Cisco México Paseo de la Reforma 222 Piso 19 Cuauhtémoc, Juárez Ciudad de México, 06600 México
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.
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