[c-nsp] Multicast state behavior differences between 7600 and ASR9k

[John Neiberger]

>> If you're using SSM, what state exists when nothing is joined? There are no
>> PIM asserts being sent to the PIM RP i.e. no (*,g).
>>
>> Clearly the 7600 has a forwarding architecture that necessitates a null-OIF
>> MRIB entry (guess: to prevent CPU punts); presumably the ASR9k doesn't have
>> the same architecture, and just drops packets without an MRIB entry.
>>
>> That doesn't mean it wouldn't be useful of course; but presumably the ASR9k
>> would need to be leaking packets to the CPU to build this state in the first
>> place, which has its own set of problems.
>
> That's an excellent point. This is SSM, but we have so many sources,
> it's very useful to see entries in the mroute table in IOS, even when
> there are no active joins. We need to be able to verify which groups a
> source is sending to. So, in IOS XR, it appears that our options are
> to either run NetFlow and look at the cache, or we can manually join
> groups during troubleshooting. I'd much rather be able to just look at
> the mrib in the same we can can look at the mroute table on the 7600.
> Maybe they'll add "show mrib route null-oil" for us.  :-)
>

Here's another thought I just had regarding this. The behavior of the
ASR (and the new behavior of IOS, I have heard) is not to create an
entry in the mrib (or mroute) table for sources unless something has
joined. This makes it very difficult to find out information about
sources with no receivers. The router must already be keeping track of
those sources, though. If an IGMP or PIM join arrives, the router will
know to start forwarding that traffic. So, it seems as if the router
has been keeping an eye on the multicast traffic on that port. Why not
have a simple command that shows it?

Maybe "show mrib route null-oil" or "show mrib connected-sources" or
something like that. I'd gladly contribute a few cases of adult
beverages to whatever IOS XR programmer wants to add that feature.  :)