[c-nsp] IOS XR and router rib rump always-replicate

[John Neiberger]

I ran into an issue today that I hadn't seen before. I was helping someone
troubleshoot some multicast problems where everything seemed to be correct
but the joins weren't working. I was totally stumped until someone noticed
the following:

router rib
 address-family ipv4 unicast
  rump always-replicate MULTICAST_ROUTES

There is an ACL called MULTICAST_ROUTES that matches prefixes that we want
to be available as multicast sources. I've never seen this before, so I'm
wondering what exactly it does. I read the command reference and it wasn't
exactly clear.

It seems that it is filtering what unicast routes are available as
potential RPF neighbors. If a PIM join arrives and there is no valid RPF
neighbor, it seems to just get dropped. The RPF entry for prefixes not in
the ACL look like this:

RP/0/RP0/CPU0:router#   show pim rpf 1.2.3.4
Fri Mar  1 05:28:33.026 utc

Table: IPv4-Multicast-default
* 1.2.3.4/32 [4294967295/4294967295]
    via Null with rpf neighbor 0.0.0.0


Entries for prefixes that are in the access list look like you would expect
and those PIM joins succeed. So what exactly does "rump always-replicate"
do? Am I right that it's basically only allowing the prefixes in the ACL to
be used for multicast RPF?


Thanks,

John