Multicast Address 224.0.1.40 - Discover Candidates

dppagc

Q1 According to wikipedia, The Cisco multicast router AUTO-RP-DISCOVERY address is the destination address for messages from the RP mapping agent to discover candidates.

May I ask what are the "discover candidates"?
1) Is it the RP and the normal PIM join routers
OR
2) Is it just the normal PIM join routers only?

Q2 I have realized that in cisco textbooks, the mapping agent is only one. So what happens if the mapping agent router goes down?

Dieg0M

Q1. The "discover candidates" are all PIM routers in the network, so 1) would be the correct answer. If Candidate RP's didn't join this group, they wouldn't know if they are the RP or if the Mapping Agent elected another RP for the network.
Q2. If there is a single mapping agent and it goes down, no RP's would be advertised in the PIM domain and PIM enabled devices would fail the registration and join process to the RP.

dppagc

Hi DiegoM,

In that case is there any redundancy for the mapping agent? I have only seen redundancy for RP.

Dieg0M

Yes. It is actually pretty common in an Auto-RP deployment to configure more than one mapping agent in a network so if one fails, multicast will continue to operate normally. Each mapping agent functions independently from one another and send RP-discovery messages through multicast with the same Group to RP mapping information and this does not cause any problems since they transmit the same mapping information. In the same manner, all PIM enabled devices in the network create a single set of Group to RP mapping despite receiving redundant sets of information from multiple Mapping agents.