The iBGP Multipath Load Sharing feature functions similarly in a Multiprotocol Label Switching (MPLS)
Virtual Private Network (VPN) with a service provider backbone.
For multiple paths to the same destination to be considered as multipaths, the following criteria must be met:
•
All attributes must be the same. The attributes include weight, local preference, autonomous system
path (entire attribute and not just length), origin code, Multi Exit Discriminator (MED), and Interior
Gateway Protocol (iGP) distance.
•
The next hop router for each multipath must be different.
Even if the criteria are met and multiple paths are considered multipaths, the BGP speaking router will still
designate one of the multipaths as the best path and advertise this best path to its neighbors.
After a change in multipath, IGP metrics are not considered while evaluating eiBGP multipath candidates
and a sub-optimal path can be used.
Note
BGP Selective Multipath
Traditional BGP multipath feature allows a router receiving parallel paths to the same destination to install
the multiple paths in the routing table. By default, this multipath feature is applied to all configured peers.
BGP selective multipath allows application of the multipath feature only to selected peers.
The BGP router receiving multiple paths is configured with the maximum-paths ... selective option. The
iBGP/eBGP neighbors sharing multiple paths are configured with the multipath option, while being added
as neighbors on the BGP router.
The following behavior is to be noted while using BGP selective multipath:
•
BGP selective multipath does not impact best path calculations. A best path is always included in the
set of multipaths.
•
For VPN prefixes, the PE paths are always eligible to be multipaths.
For information on the maximum-paths and multipath commands, see the Cisco ASR 9000 Series Aggregation
Services Router Routing Command Reference.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
54 OL-30423-03
Implementing BGP
BGP Selective Multipath
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