Does Junos prefer route with higher localpref even when the next hop of this route is unreachable?

Question

I had a prefix 2001:db3::/64 learned from two different iBGP peers. One of those BGP peers became unreachable, but the BGP session had not yet timed out:

root@e2> show route 2001:db3::5

inet6.0: 19 destinations, 22 routes (11 active, 0 holddown, 9 hidden)
+ = Active Route, - = Last Active, * = Both

2001:db3::/64      *[BGP/170] 00:00:01, localpref 210, from 2a03:c0::6
                      AS path: I, validation-state: unverified
                      to Discard
                    [BGP/170] 00:38:27, localpref 200, from 2a03:c0::4
                      AS path: I, validation-state: unverified
                    >  to fe80::206:a00:204:fff0 via ge-0/0/0.2

root@e2>

Am I correct in understanding that if the localpref had been equal, then the second route would have become active?

Answer 1

This totally depends on what you mean by 'became unreachable', the way this happens matters a lot:

• if the interface connecting the peer to your router went down, the next-hop will be updated immediately, no need to wait for the BGP timers.

• if a protocol like BFD is used to detect if the peer is reachable, the BGP peer will go down once that is detected (usually within a second), and next-hops will be updated.

However, if there is some device in between (like a switch), and no BFD is used, your router is unable to detect if the BGP neighbor is unreachable, except for waiting until the BGP timers expire and the session gets torn down.

What happens if local preferences are equal, is that another step in the BGP path selection algorithm would be used to determine which of those two routes would be the preferred route, unless you have configured your routers to have more than one path installed for those routes.