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

[adamv0025 at netconsultings.com]

Hi Marcin,

Well yes you’re right, hence the path in form of a “update wait-install” feature,

Though the best remedy to this slow FIB programming problem is to maintain the FIB entries no matter what, 

Opt1 Having two sessions on a single box in case one fails the other will keep the FIB entries in place -just NH changes would follow -which in todays hierarchical FIBs is a single pointer change operation.

Opt2 “advertise best external” feature so in case you have a box with just a single eBGP session it learns prefixes via iBGP from some other border node with the “advertise best external” feature enabled. 

 

adam  

 

From: Marcin Kurek <md.kurek at gmail.com> 
Sent: Friday, February 28, 2020 1:19 PM
To: adamv0025 at netconsultings.com; cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] bgp update wait-install - RIB/FIB inconsistency

 

Hi Adam,

Thanks for the reply. I'm still in doubt though. 

If we look here for example:

 <https://xrdocs.io/ncs5500/tutorials/ncs5500-fib-programming-speed/> https://xrdocs.io/ncs5500/tutorials/ncs5500-fib-programming-speed/

we see that BGP programming speed is always faster than FIB programming speed.

If best path is advertised regardless of the FIB state by default, this would always mean a potential for longer/shorter blackhole...

Thanks,
Marcin

W dniu 26.02.2020 o 14:13, adamv0025 at netconsultings.com <mailto:adamv0025 at netconsultings.com>  pisze:

Hi Marcin,
There was a thread on the topic of slow FIB  download/upload on this forum
some time back.
 
Yes BGP will advertise the best path regardless of the FIB state by default.
I'd recommend enabling it. (though subject to testing on your code version
as always)
 
adam

-----Original Message-----
From: cisco-nsp  <mailto:cisco-nsp-bounces at puck.nether.net> <cisco-nsp-bounces at puck.nether.net> On Behalf Of Marcin
Kurek
Sent: Friday, February 21, 2020 10:40 AM
To: cisco-nsp at puck.nether.net <mailto:cisco-nsp at puck.nether.net> 
Subject: [c-nsp] bgp update wait-install - RIB/FIB inconsistency
 
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
 
 
 
 
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