I've been studying deployment of PIM in campus environment, and it is mentioned that the HSRP active and the PIM designated router (PIM-DR) should be synchronized on the same distribution switch. Why is that? what happens if unicast is being sent through the HSRP active distribution switch, and multicast is being sent through the HSRP standby distribution switch
This makes sense, as the DR is the responsible for the segment, and does it make sense to have the "standby" node being active in the multicast and standby in the unicast routing decisions? For sure not.
Let's say you are tracking a WAN interface and the actual standby was the primary, but its WAN interface came offline, and now it is the standby. In this case, you would have an extra hop for the multicast traffic (assuming that standby is the DR), because it would have to be sent to the active unit, which has the active WAN link, for instance.
It is also mentioned later that if a first hop load balancing protocol (e.g GLBP) is used, then the reverse is recommended. i.e. the HSRP active shoul be the non-DR router.
This is true because in some way, you can achieve a better load-balancing method, by sending the multicast traffic to one of the AVFs, and the tasks of the AVG being distributed to another router that does not care about the multicast in the local segment.
Please rate this if satisfied your concern.
Thank you.
