I ran into an issue recently where I had 4 routers (2 pairs of 2 HSRP-groups) on the same VLAN which contained my multicast receiver. I get some multicast data from each set of routers coming into that VLAN.
I couldn’t figure out why my RP router kept timing out it’s S, G route for the VLAN in the OIL. It seemed to make no sense. Receivers were sending IGMP reports for that group… senders were sending the data without issue. Strangely, I’d watch the interface expiry timer go down second by second (via show ip mroute 126.96.36.199) even though I was constantly getting data from the multicast source.
After banging my head on this for a while I discovered HSRP-aware PIM settings and decided to try them out “ip pim redundancy dr-priority 50”. Sure enough, it worked like a charm. Instead of my multicast stopping and starting like it did in the past, it now works well even in various failover conditions.
Can you explain what goes on in the situation and why HSRP-aware PIM is needed? I tried to read through the Cisco documentation, but honestly most was not so clear to me.
What I observed is the router receiving the incoming multicast data onto the multiaccess VLAN (with 4 routers) kept timing out the S, G for the VLAN in the OIL which caused multicast to stop transmitting (router auto-sent prune to upstream interface). Then, the DR (a different router in the VLAN) would send a join/prune which restarted the S, G by adding the VLAN back in the OIL. This process repeated itself every 3-5 minutes. After activating HSRP-aware PIM, the expiry timer only goes down about 60 seconds or so prior to resetting itself by to 03m 30 secs (i.e. working as expected). Previously it would go all the way down to zero and require another join from the DR to restart things.
PS - today I found on a blog which helped explain a bit better what is going on.
I still think it could be useful to add this topic (HSRP-aware PIM) to your multicast collection.