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Multicast Auto-RP made simple(er)

One of the main issues with sparse-mode multicast is to elect a rendezvous-point(RP) and spread information about where it is. Im not going into any details about why we need an RP in this post, but i will examine Cisco’s proprietary approach to this problem – Auto RP.

Since everything with multicast seems backwards at first, i will try to exlain this in the order i find making most sense. Starting with the general approach. Auto-RP is basically multicast for multicast RP information. There is one function for electing a RP for each group and one function is for distributing this group-to-rp-mappings. Both these things are done with multicast, routers with auto-rp enabled will start to listen and speak to two different multicast groups.

Multicast groups used for auto-rp

224.0.1.39 – Auto RP announcements. This is where every Candidate-RP(basically a router candidating for the RP role) sends information about itself and that it want to become RP.

224.0.1.40 – Auto RP discovery. This group is a well known group, every cisco router running pim will automatically listen to this group because this is where the Mapping Agent (MA) will send all the RP mappings, telling each router which RP to use for any group.

Talking sparse-mode multicast without RP ?

Before the routers get the RP mapping, or even before the candidate-routers can send its info to the mapping agents, multicast for 224.0.1.39 and 224.0.1.40 needs to be fixed. Since the routers not yet has any RP for any group yet, and therefore not these groups either, we need to solve this somehow. That can be solved in either of these ways:

A) Configure sparse-dense-mode instead of sparse-mode. This features configures the network more as a hybrid between sparse and dense mode. It basically runs as sparse mode, but with the fallback feature of being able to flood multicast (dense mode) if RP is missing.

B) Use the command “auto-rp listener”, what this feature does is that it instructs the routers to run dense-mode for the groups 224.0.1.39 and 224.0.1.40.

C) Configure each and every router with static RP mappings for the groups 224.0.1.39 and 224.0.1.40.

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