[c-nsp] ASR9k: RIB/FIB convergence

[Thomas Schmid]

Hi,

Am 14.08.2018 um 11:01 schrieb adamv0025 at netconsultings.com:
>> Thomas Schmid
>> Sent: Friday, August 10, 2018 10:35 AM
>>
>> it turns out you run into funny situations with that 'update wait-install'
>> command enabled:
>>
>> RTA: 'update wait-install' configured. Learns route a.b.c.0/24 from direct peer
>> ISPA.
>>
>> RTB: learns a.b.c.0/24 from eBGP peer ISPB with longer AS-path.
>>
>> RTA, RTB in BGP full mesh.
>>
>> If I prepend the route for a.b.c.0/24 on RTA, the local BGP table is updated,
>> but the announcement with the longer as-path *never* makes it to RTB,
>> probably because the CEF entry locally is not updated and does not change.
>> So RTA is holding back the BGP announcement of the longer route to his
>> neighbors.
>>
>> So RTB never sees the longer as-path for the prefix and therefore *never*
>> announces the shorter route via  back to RTA. Therefore the routing never
>> changes in the network.
>>
>> In addition: 5.3.3 has bug CSCuv02045 "Mutex in ipv4_rib/ipv6_rib when
>> update-wait-install is enabled" ...
>>
> Sorry for the late response,
> 
> Well you've got to be careful here,
> You haven’t stated that, during initial conditions, A believed that path via ISPA has been selected as the overall best path indeed. 
> Cause if A believed that route via B from ISPB is the overall best path ,then A would not advertise its own route to B (unless A is configured with "advertise best-external" which I recommend in order to speed up convergence -please note though it increases FIB usage). 
> 
> If, during initial conditions,  A did select path via ISPA as the overall best path then I agree it’s a bug and should be reported.
> (would be interesting to see from debug why the route is not advertised to B)
> Also I believe that update-wait-install routine should not be used in this scenario (should be used only when a path is changed to best path). 
> 

according to TAC, the behavior observed is intended behavior. No change in the CEF table -> no BGP update announcement. This leads to a classical deadlock situation. 'advertise best-external' might indeed help, but as you said, FIB usage goes up a lot when you do this for e.g. upstream connections.

> Seems like CSCuv02045 has been fixed only very recently ~6.2.2+
> Isn't there a SMU available for older releases?
> 

no SMU for 5.3.3

Cheers,

   Thomas