[c-nsp] ASR9k: RIB/FIB convergence

[adamv0025 at netconsultings.com]

> Thomas Schmid
> Sent: Friday, August 03, 2018 10:08 AM
> To: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] ASR9k: RIB/FIB convergence
> 
> Hi,
> 
> Am 02.08.2018 um 13:11 schrieb Thomas Schmid:
> > Hi Adam,
> >
> > Am 02.08.2018 um 12:23 schrieb adamv0025 at netconsultings.com:
> >> First thing first,
> >> To mitigate the damage due to RIB-FIB inconsistencies you could use the:
> >> "BGP-RIB Feedback Mechanism for Update Generation"
> >> "To configure BGP to wait for feedback from RIB indicating that the routes
> that BGP installed in RIB are installed in FIB, before BGP sends out updates to
> neighbors, use the "update wait-install" command in router address-family
> IPv4 or router address-family VPNv4 configuration mode."
> >>
> >
> > good point. Haven't heard of this from Cisco yet. Will discuss this with the
> TAC.
> >
> 
> giving it a second thought: this may help in some cases, in others not. E.g.
> BGP link to upstream dies -> FIB is still pointing to upstream -> router is still
> announcing himself as exit point until all FIB entries are updated -> traffic is
> dropped.
> 
> It may help if e.g. as-path length is changing. The routing will then be
> suboptimal for a while, but the rate with which the updates are announced
> to the neighboring routers should be such that they can sync their FIB in
> time.
> 
> Waiting for feedback from TAC if this command is indeed checking the LC FIB
> or if it's just looking at the RP.
> 
Hmm, 
Very good point, but if this feature should be of any use then it should be associated only with "ADD'' operation, possibly with "UPDATE" operation, but certainly not with "DELETE" operation.
(not mentioning it should get the state back from the LC's NP HW-FIB or at least LC's CPU SW-FIB)

adam

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