BGP Multipath

The second option available is to use BGP multipath. This should be considered as an alternative to using eBGP multihop and is an option used by several ISPs also. The configuration could be something like the following:

Router A: router bgp 65534 neighbor 1.2.3.1 remote-as 100 neighbor 1.2.3.5 remote-as 100 neighbor 1.2.3.9 remote-as 100 maximum-paths 3

Router B: router bgp 100 neighbor 1.2.3.2 remote-as 65534 neighbor 1.2.3.6 remote-as 65534 neighbor 1.2.3.10 remote-as 65534 maximum-paths 3

Addresses 1.2.3.1, 1.2.3.5, and 1.2.3.9 are the addresses on Router A for the point-to-point link between A and B. Router B has the corresponding /30 addresses. The only extra requirement over a standard BGP configuration is the maximum-paths 3 directive— this tells the router to install three parallel paths into the FIB.

The downside to the multipath command is that multiple BGP sessions are required. In the rare circumstances in which the full routing table is exchanged between peers, this can require a considerable amount of memory on the router to store many copies of the routing table rather than the one copy that would have been required for the eBGP multihop option.

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