Multicast in hsrp scenario

Picture designed for monospace type. e.g. courier.

---------- -------- | sw1 |------| pc1 Tx | ---------- -------- / \\ / \\ / \\ / \\ / \\ / \\ / \\ E0 / \\ E0 ---------- X ---------- | rA |-------------| rB | ---------- ---------- | | | -------- | pc2 Rx | --------

Multicast query.

We have 2 routers A, B with routed links between them. A switch (with a single VLAN) connects routers A, B.

rA port E0 and rB port E0 and all the switch ports are on the same broadcast domain, all other links are point to point routed links. All router ports are configured for PIM sparse mode.

We are running (say) OSPF and all links are the same cost.

pc2 registers for some multicast group with IGMP and pc1 sends frames to that group.

The issue that I am trying to understand is that as far as I can see the multicast may or may not work depending on the PIM designated router selected. This will be either rA-E0 or rB-E0.

In the event that rB-E0 is the designated router the multicast frames will be forwarded by rB and not by rA. When these frames reach port X they will fail the RPF test and be dropped.

Is this an invalid configuration for multicast operation?

Guess I must have the wrong end of some stick or other.


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Multicast traffic doesn't use HSRP. IP Multicast is just a way to do layer

2 multicast over a layer 3 network. A default gateway is only used to figure out the layer 2 address of the next hop gateway. If you look at a unicast packet the destination IP is the destination IP address of the packet and the layer 2 address is the MAC address on the next layer 2 hop, which is the MAC address of the router. A multicast packet has the destination IP of the IP multicast group that the packet belongs to, which is computed from the layer 2 multicast address, which NEVER changes. So in your scenario, even if rB is the active HSRP router, rA is going to be forwarding the multicast traffic.


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