The "ip pim auto-rp listener" gets round a chicken and egg situation with pim sparse-mode.
The multicast addresses for Auto-RP are 184.108.40.206 & 220.127.116.11.
With pim sparse-mode a router uses the RP to group mappings it receives from the RP mapping agent to make requests to the RP to join a group when a locally connected host or a downstream router sends a request for this group.
But if the network is using pim sparse mode how do the 18.104.22.168 & 22.214.171.124 groups get to other routers from the RP mapping agent.
Put simply a router needs a list of the RP's before it can request a multicast stream from this RP but it doesn't know the RP's until it receives a multicast stream.
So ip pim auto-rp listener allows the RP mapping agent to flood the Auto-rp groups 126.96.36.199 & 188.8.131.52 out to all other routers. These groups are dense mode flooded even though the network is running in sparse-mode.
The alternative is to configure "ip pim sparse-dense mode" under the interface but this has an added consequence. Any group that is not mapped to an RP will be dense mode flooded throughout the network. With "ip pim auto-rp listener" only groups 184.108.40.206 & 220.127.116.11 can be dense mode flooded.
Hope that makes sense.
Jon, great explanagtion. Just to add to Jon's post, this command is configured when the routers are configured for sparse-mode and it needs to be enabled on every router including the RP itself.
Thanks dear now i got all the logic behind this.Once again thanks for your quick support. Already you solved my lot of problems regarding FWSM. Today i will rate you the best.
Jon has explained the functionality of (IP pim auto-rp listner), I just want to add point 2 ur comment:
I dont believe it has to be configured on all routers but rather, it should be only configured on the RP and the Mapping agent, the RP use it to announce 18.104.22.168 in dense-mode to the Mapping Agent, then the Mapping Agent uses it to announce 22.214.171.124 to the rest of the routers part of the Group.
It's needed in all routers with ip pim sparse-mode, otherwise - they won't receive their group mapping information:
Pls check the definition based on your link u have provided:
ip pim autorp listener
To cause IP multicast traffic for the two Auto-RP groups 126.96.36.199 and 188.8.131.52 to be Protocol Independent Multicast (PIM) dense mode flooded across interfaces operating in PIM sparse mode, use the ip pim autorp listener command in global configuration mode. To disable this feature, use the no form of this command
Right, all PIM routers running in sparse-mode need those 2 groups in order to receive group advertisement.
Without those 2 groups, they won't know where to send the multicast traffic to the RP.
You are correct, I was missing this point.
But if the rest of the router configured with (Auto-rp) or static RP mapping ,then, they should be able to know the rp without this command.
Thanks for ur clarification,
You don't need this command if:
a) You have sparse-dense mode in the interface. Those groups will be received in dense mode.
b) You have static RP configured in the device. The device will know which RP to send the multicast traffic to.
c) You aren't running Auto-RP :)