[c-nsp] fast multicast convergence

[Oliver Boehmer (oboehmer)]

> Consider this PIM-SM scenario
> 
> Designated Router has two paths towards the Source and RPF has chosen one
> 
> The m-cast traffic will flow via the RPF interface down to the interested
> receivers as long as the DR will be sending the periodic Join messages up
> the Source-Tree
> 
> Now when the RPF interface goes down and after the IGP converges
> 
> -will the DR send triggered Join message via the new RPF interface towards
> the Source immediately after it learns the alternate path towards the Source
> please?
> 
> -or the DR will simply wait till the next scheduled Join period please?

it will send a join after a new RPF intf. has been identified for a given source. In earlier IOS releases, there was a backoff after the first change in the RIB, RPF process started 500 msec after first change in the RIB and went through all S,G/*,G to update RPF intf. Now this is event-driven.
 
> I'm asking because the PIM-SM convergence is based on how fast can be the
> Join message propagated from the DR to Source over the backup path -creating
> the SPT state in all routers it passes through
> 
> In MoFRR this is preprogramed by DR sending Join also via the Backup path
> (in XR it also enables triggered Joins)

true.
 
> I don't see any specific knob to control the pace at which the periodic join
> msgs are sent

you can configure the RPF backoff mentioned earlier (ip multicast rpf backoff).

> Or would also this be affected by the query-interval please?

IIRC, there is/was some interdependency with pim query-interval and link-up to avoid a blackhole between IGP coming up and PIM establishing a neighbourship.. possibly something to check in your specific environment..

	oli