BGP Multipath Load balancing

[YHam]

Hello Everyone,

I wanted to ask whether BGP multipath feature works for a route learned from eBGP and iBGP neighbors.

Please see the topology below. In this topology, Router C learns 10.0.0.0/8 from A via eBGP and from D via iBGP. Can both paths (via eBGP ad iBGP) on Router C be installed in routing table (CEF) using Multipath (maximum prefix 2) feature so the traffic to net 10.0.0.0/8 can be load shared?
From my understanding, when the best path selection runs, eBGP learned route will take preference over iBGP and therefore multipath feature will not kick in. All the best path selection criteria have to tie.

[Joseph W. Doherty]

"Can both paths (via eBGP ad iBGP) on Router C be installed in routing table (CEF) using Multipath (maximum prefix 2) feature so the traffic to net 10.0.0.0/8 can be load shared?"

No.

"From my understanding, when the best path selection runs, eBGP learned route will take preference over iBGP and therefore multipath feature will not kick in."

Correct.

Consider if it worked the way to support your first question, what if RtrD was configured the same? It too might see two "equal" paths (i.e. via Rtrs B and C).

"All the best path selection criteria have to tie."

Yea, but just having multipath >1 isn't always enough. For some situations you also need to add the "secret/hidden" command, "bgp multipath as-path relax". See https://ipwithease.com/bgp-multipath-as-path-relax for more info.

[e.ciollaro]

Hi, as far as I know the only way is to configure an address-family under BGP and use the command

maximum-paths eibgp <value>

In this way both eBGP and iBGP path are considered

Attached an example (GNS3 with a 4 router architecture)  where you can see the effect of the command

Bye

enrico.

Please rate if useful