BGP ECMP

The BGP (Border Gateway Protocol) ECMP (Equal Cost Multi Paths) feature supports load sharing by creating a multipath to a destination. This multipath contains multiple routes that are determined to have an equal cost because the following parameters are the same for each route:

Weight
Local preference (for IBGP multipaths)
AS path (entire attribute, not just the length)
Origin code
Multi Exit Discriminator (MED)
IGP distance to the next hop
Source session (EBGP or IBGP)

Note

ECMP does not install an additional path if the next hop is the same as that of the best path. All paths within a multipath must have a unique next hop value.

BGP ECMP does not affect the best path selection. For example, the router continues to designate one of the paths as the best path and advertise this best path to its neighbors. EBGP paths are preferred to IBGP paths.

The BGP ECMP feature allows you to define the maximum number of equal cost paths (up to 64) in a multipath. A multipath for an IBGP destination is called an IBGP multipath, and the multipath for an EBGP destination is called an EBGP multipath.

If there are more equal cost paths for a destination than the configured maximum, the BGP identifier for the advertising BGP speaker is used to establish a path priority. The lower BGP identifier values have priority over the higher values. For example, if the configuration supports 4 paths in a multipath, only the four paths with the lowest BGP identifier values become part of the multipath.

Published March 2020prev | next

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