Multicast AutoRP
In PIM Sparse mode, a router needs send Join message to the RP to receive multicast traffic. With AutoRP, the mapping agent send RP info using 224.0.1.40, however, since PIM is in sparse mode, the 224.0.1.40 multicast won't flood in entire network, other router wants to receive this traffic but isn't aware of any RP to send join message, this is chicken-and-egg problem.
There are two solutions:
1. PIM Sparse-Dense mode
a router uses Dense mode if it has no info where the RP is, so 224.0.1.40 traffic can flood entire network in dense mode, then the router switch to Sparse mode since it knows RP.
2. PIM autorp listener.
This command causes the router flood 224.0.1.40 traffic out all interfaces.
AutoRP is enabled on the Cisco router by default, so all routers have the following mroute by default:
(*, 224.0.1.40), 00:07:57/00:00:07, RP 0.0.0.0, flags: DP
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list: Null
In case we don't use AutoRP, we can implicitly disabe AutoRP to remove the above route from the multicast routing table.
R2(config)#no ip pim autorp
After timer (3 minutes) expired, this mroute is removed.
There are two solutions:
1. PIM Sparse-Dense mode
a router uses Dense mode if it has no info where the RP is, so 224.0.1.40 traffic can flood entire network in dense mode, then the router switch to Sparse mode since it knows RP.
2. PIM autorp listener.
This command causes the router flood 224.0.1.40 traffic out all interfaces.
AutoRP is enabled on the Cisco router by default, so all routers have the following mroute by default:
(*, 224.0.1.40), 00:07:57/00:00:07, RP 0.0.0.0, flags: DP
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list: Null
In case we don't use AutoRP, we can implicitly disabe AutoRP to remove the above route from the multicast routing table.
R2(config)#no ip pim autorp
After timer (3 minutes) expired, this mroute is removed.
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