So a host sends out a multicast packet to join the multicast group (if I am reading this right) by sending a membership report to that multicast IP with a multicast mac address. My question is since both routers in an HSRP setup would get the multicast packet, they should both join the (S,G) and request the traffic, why doesn't the PC requesting the data get 2 copies of the traffic? one for each router that got the join request? I know when you statically join both routers to that multicast group you get 2 copies.
I assume it's something that HSRP does, but I am hoping for more details or verification.
Got this reply on redit (https://www.reddit.com/r/networking/comments/433x9g/multicast_in_hsrp_setup_question/)
HSRP has nothing to do with it. PIM needs to be running on the router interfaces, and when you have multiple PIM neighbors on a LAN they will elect a dedicated router (DR) to handle the PIM duties. The backup router doesn't do anything unless the DR goes away.
HSRP multicast is part of 224.0.0.0/24. Those are supposed to stay link-local so no layer 3 forwarding or PIM is ever ging to be involved.
Weirdly enough HSRP has a TTL of 2 in my captures here, where the other link-local multicass have 1 (OSPF, EIGRP).
yeah, the idea was more "a PC sends out a IGMP join to a multicast IP (239.255.6.4 for example) why don't both PIM enabled routers go ask for this data, and both send the same data to the PC?"
Wasn't there a mechanism that assigns a primary router in a subnet for IGMP?
Quote from: Reggle on February 02, 2016, 01:07:09 AM
Wasn't there a mechanism that assigns a primary router in a subnet for IGMP?
IGMP querier election. What the OP was looking for primarily though was the PIM designated router.