Anycast was described in detail in a previous lesson. If you haven’t seen it or you want a refresher it can be found here.

While Anycast can be described as “one-to-nearest communication”, Anycast-RP can be described as “one-to-nearest RP communication”.

  • Benefits of Anycast-RP include:
    Fault Tolerance - When the path to an RP fails, the IGP/BGP will remove that route and replace it with the next nearest RP.
  • Load Sharing and Availability – With proper planning, different hosts can have different nearest RPs. The goal is to have multiple routers assume the RP role simultaneously. RPs also have the ability to connect to multiple sources.

How it Works

The same IP address is configured on the loopback interface of multiple RP routers. This address is advertised from each router via IGP/BGP. Although multiple devices use and advertise the same address, the path to the source(s) and receiver(s) will always be routed from the receiver to the nearest RP because the unicast routing table will only list the optimal path. All unicast rules apply. For example, the longest prefix to a destination is preferred.

Anycast-RP uses PIM Registers to communicate. In IPv6, here are no other protocols needed (i.e. IPv4's MSDP).

Note that the diagrams below are a logical topology (not a physical topology).

First, RP2 will build a Shortest Path Tree to the Sender A:

Anycast-RP (Rendezvous Point)
Anycast-RP (Rendezvous Point)

  • Anycast-RPs in the same multicast group are configured as PIM Anycast-RP Neighbors so they can synchronize with each other. RP1 and RP2 become PIM Anycast-RP Neighbors.
  • The Sender will build a Source Tree with its nearest RP by using the PIM (S,G) Register message. Sender A registers with RP1.
  • This RP will then send a dataless PIM Register message to its PIM Anycast-RP Neighbor(s) to notify them it has an active Sender. RP2 learns the address of Sender A.
  • The PIM Anycast-RP Neighbor(s) reply to the PIM Register message with a PIM Register Stop message to confirm that is has learned the address of the active Sender. RP2 sends a PIM Register Stop message to RP1 and builds a Shortest Path Tree to the Sender A.

Second, when a sender registers with RP2 the same process will occur in the other direction:

Anycast-RP Redundancy
Anycast-RP Redundancy

  • Sender B registers with RP2.
  • RP1 learns the address of Sender B.
  • RP1 builds a Shortest Path Tree to the Sender B.

Lastly, if an RP goes down there is failover:

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