Regarding eBGP Multipath

Question

In one of the blogs, i read:

"Unlike most routing protocols, BGP only selects a single best path for each prefix. It doesn’t do ECMP (Equal Cost Multi-Path Routing) by default but it is possible to enable this.

In order for BGP to use the second path, the following attributes have to match:

Weight

Local Preference

AS Path (both AS number and AS path length)

Origin code

MED

IGP metric

Also, the next hop address for each path must be different. This comes into play when you are multihomed to the same router."

Que: Even if all the above attributes match, will the "Router ID" (choose the one with lowest router-id) not be used as a tie-breaker ?

Answer 1

If you enable ECMP, the router ID will not be used as tie breaker, because that would defeat the purpose. If all those attributes match, routes are considered to be equal and you can utilise all links with matching attributes.

Answer 2

BGP will always choose one best route regardless of multipath or other configuration. As RFC 4271 9.1.2.2 states only one route can be passed from Adj-RIBs-In to Loc-RIB. So next tie breaker will be used (in some cases it can be oldest route).

Of course you can enable multipath and use more than one route for ECMP. You can enable Add-Path and advertise more than one route to your peers. But still, BGP will select one best path using all possible tie breakers.

And by the way Juniper, for example, will show you one route as selected, regardless of any configuration knobs.