Sunday, January 18, 2009

IE Vol 1 Auto-RP Complete

Again, nothing fancy here. First the interfaces are configured for sparse-dense mode. Once configured this way, the multicast flow works, since it falls back to dense mode when an RP is not known for a flow.

So the trick is to get that RP learned dynamically. There are two dynamic RP protocols: Auto-RP and bootstrap RP. This exercise uses auto-rp, but they are both very similar.

Two things are required for the dynamic registration to happen. First, at least one router must advertise one of its interfaces as a candidate rp, using ip pim send-rp-announce. This advertisement happens via multicast group 224.0.1.39. Since there is no rp for this group, it uses dense mode and will get flooded/pruned.

Next, a router must be configured to listen for 224.0.1.39 messages. This is done via the ip pim send-rp-discovery command, and this router is called a mapping agent. It can be on the same or a different router. Once added, the mapping agent acts as a receiver on 224.0.1.39. Therefore, the routers must graft this receiver on to be a listener for this group. This means any RPF checks must be successful on the 224.0.1.39 group to ensure messages from the candidate rp(s) are being received by the mapping agent.

The mapping agent decides on a candidate rp and advertises its selection out via dense mode to group 224.0.1.40. Through this group, all routers learn the RP for the group, and can the flow can use sparse mode.

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