Multicast Forwarders

Rishard Chapoteau

I just had a good lesson last night with Cisco TAC in multicast Forwarding that took a few hours to figure out.  The initial call I received was that multicast was going in and out on one of our networks.  I logged in and took a look at something I rarely look at on this particular system which was the ip mroute table on 7K1.  It showed all of the outgoing interfaces were zeroed out which I though was extremely weird.  It turned out to not be an actual problem, but the subject of the article.  What actually happened was a misconfiguration between 7K1 and 7K2 where 7K1 did not have ip igmp version 3 configured on the SVI, where 7k2 did.  Looking at the IGMP snooping table revealed the our systems would flip between V2 and v3 upon a receiver reboot which caused all of the multicast to temporarily take a hit while the flip happened.  We first tried to add V3 to 7K1, but that caused issues with several devices, so we removed V3 from both 7K1 and 7K2 to fix the ultimate issue and now snooping shows V2 for all multicast.

 

Again that isnt’ the main subject of this particular article.  The main thing I found out while talking to TAC was that if 2 devices are doing HSRP, regardless of who is the active group member, Multicast will choose the higher address as teh forwarder.  If I did a show ip mroute on 7K2 it properly showed all of the outgoing interfaces.  7K1 however does not.

In the above picture this is what 7K2 looks like for this specific multicast group.  It had both *,G and S,G for the same group because the multicast is not part of the SSM range.  Some devices are joining this group with the source and some without which is why both are shown with an outgoing interface.  On 7K1 we do not see this. We see S,G listed, but we do not see anything for the outgoing interface because 7K1 is doing the forwarding.  It essentially looks broken but it is not.  Because of this we also see more traffic being originated from 7K2 that is being sent towards 7K1 via the port channel even though the traffic should be about equal.