[c-nsp] bgp update wait-install - RIB/FIB inconsistency

[Marcin Kurek]

Hi all,

During a recent MW I ran into a fishy situation.

Long story short - ASR9001 running XR 6.4.2 SP4 was rebooted, after it 
went back online I noticed slow route installation in FIB. It took 
around 20 minutes to install > 700k prefixes.

Apparently, some customers experienced problems at that time, so I'm 
suspecting some RIB/FIB inconsistency here.

I've been researching this topic for a while and now I'm even more confused.

Can someone help me answer below questions, please?

Assuming our router has established its e/iBGP sessions and finished 
receiving updates from its neighbors, will it advertise its best paths 
even though FIB programming is still in progress?

Common sense answer would be "no", because if it starts advertising 
prefixes that are not installed in FIB, it also starts attracting and 
possibly blackholing traffic.

But I discovered the command "update wait-install" as part of BGP RIB 
feedback mechanism introduced in XR 4.3. When it's enabled, routes that 
have not been installed in FIB are not advertised. Looks like that it's 
turned off by default though.

If so, the answer to my 1st question should be "yes", although to me, it 
doesn't make much sense.

I've been trying to find best practices / recommendations for that 
command, but without luck.

Can anyone shed some light on that, please?

Thanks,

marcin