EBGP load balancing was introduced with the BGP 4 Multipath Support feature. Initially, EBGP supported up to six maximum paths, IOS release 12.3T+ to 16.When the EBGP load balancing is enabled with the maximum-paths number router configuration command, the router with multiple EBGP sessions selects a single EBGP path as the best path (following the BGP best path selection algorithm). The selected path is marked as best in the BGP table. BGP might also select additional equivalent paths for multipath load sharing; all paths used for BGP load sharing are marked with multipath.
The BGP attributes Local Preference, Multi-Exit Discriminator, Origin and AS-Path of the selected multipath routes have to be identical to the best path. The AS-path of all multipath routes has to be an exact match of the AS-path of the best path. This requirement can be relaxed with the bgp bestpath as-path multipath-relax router configuration command, resulting in EBGP load balancing across multiple autonomous systems.
BGP performs equal-cost load balancing between all multipath routes, unless the bgp dmzlink-bw is configured within the BGP routing process and dmzlink-bw option is configured on EBGP neighbors.
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