RP Paths and SPT Paths

The preceding figure shows two paths for packets from the source for group 239.255.162.1 and a receiver for the group. The source is attached to PIM Sparse device A and the recipient is attached to PIM Sparse device C. PIM Sparse device B in 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 device A and device C, which bypasses the RP (device B).

To optimize PIM traffic, the protocol contains a mechanism for calculating the Shortest Path Tree (SPT) between a given source and receiver. PIM Sparse devices can use the SPT as an alternative to using the RP for forwarding traffic from a source to a receiver. By default, the RUCKUS device forwards the packets they receive from a given source to a given receiver using the RP path, but forward subsequent packets from that source to that receiver through the SPT. In the preceding figure, device A forwards the packets from group 239.255.162.1’s source to the destination by sending the packet to device B, which is the RP. Device B then sends the packet to device C. Later, all packets that device A receives from the source for the receiver, device A forwards them directly to device C using the SPT path.