RP paths and SPT paths

In a typical IPv6 PIM Sparse domain, there may be two or more paths from a designated router (DR) for a multicast source to an IPv6 PIM group receiver.

  • Path through the RP—This is the path the device uses the first time it receives traffic for an IPv6 PIM group. However, the path through the RP may not be the shortest path from the device to the receiver.
  • Shortest Path—Each IPv6 PIM Sparse router that is a DR for an IPv6 receiver calculates a short path tree (SPT) towards the source of the IPv6 multicast traffic. The first time the device configured as an IPv6 PIM router receives a packet for an IPv6 group, it sends the packet to the RP for that group, which in turn will forward it to all the intended DRs that have registered with the RP. The first time the device is a recipient, it receives a packet for an IPv6 group and evaluates the shortest path to the source and initiates a switchover to the SPT. Once the device starts receiving data on the SPT, the device proceeds to prune itself from the RPT.

To optimize PIM traffic, the protocol contains a mechanism for calculating the Shortest Path Tree (SPT) between a given source and a receiver. PIM Sparse routers can use the SPT as an alternative to using the RP for forwarding traffic from a source to a receiver. By default, the device forwards the packets it receives from a given source to a given receiver using the RP path, but subsequent packets from that source to that receiver through the SPT. You can change the number of packets the device receives using the RP before switching to using the SPT.

The following examples demonstrate the use of SPT:

Figure 1 shows two paths for packets from the source for group fec0:1111::1 and a receiver for the group. The source is attached to an IPv6 PIM Sparse router A and the recipient is attached to an IPv6 PIM Sparse router C. IPv6 PIM Sparse router B is the RP for this multicast group. As a result, the default path for packets from the source to the receiver is through the RP. However, the path through the RP sometimes is not the shortest path. In this case, the shortest path between the source and the receiver is over the direct link between router A and router C, which bypasses the RP (router B).

In Figure 1, router A forwards the packets from group fec0:1111::1 source to the destination by sending the packet to router B, which is the RP. Router B then sends the packet to router C. Later packets that router A receives from the source for the receiver, router A forwards them directly to router C using the SPT path.