Enterasys Networks X-PeditionTM manual Phase 2 Topology Shortest Path Tree Between Sender and RP

Describing the XSR’s PIM-SM v2 Features

interconnects with a router which is already on the shortest path tree from S to the same multicast group, the Join message can end on that router to get a short-cut path.

After the path is established, both the native packet along the SPT tree and Register encapsulated packet will be received by RP. When RP gets two copies of the same packet - one native and one encapsulated packet - it drops the encapsulated packet and sends a RegisterStop message back to the DR of S to let it know that it can now stop encapsulating packets in Register packets. After the DR of S receives the RegisterStop message, it starts a RegisterStop timer and stops encapsulating the packets until the RegisterStop timer expires.

At the end of phase two, packets flows natively from the sender to RP along the SPT tree, and then are passed by RP to all the receivers along the shared distribution tree., as illustrated in Figure 7-3.

Figure 7-3 Phase 2 Topology: Shortest Path Tree Between Sender and RP

RP RP

		Native Packet
Native Packet		Register Stop		Native Packet	Native Packet
	Register		Sender		Sender
	Packet
Native Packet				Native Packet
Native				Native
Packet				Packet
Receiver	Receiver			Receiver	Receiver

Phase 3: Building Shortest Path Tree Between Sender & Receiver

At the start of phase 3, the path from the sender to RP and on to the receiver is still not optimal for sender or receiver since some receivers may want to optimize the path between themselves and specific senders. In PIM-SM, the DR for the receiver has the flexibility to initiate a process to join the shortest path tree from the sender to the multicast group. To do so, the DR for the receiver desirous of optimizing the multicast path from sender S will send a source-specific Join message towards S. The path passed by the Join message will be pinned especially for traffic from S to the specific multicast group. After the SPT tree is set up between S and the DR of the receiver, traffic from S to that multicast group will flow both on the SPT tree and the shared RP tree. When the receiver can receive the same packet from both trees, the DR or upstream router for the receiver begins to drop the packet from the shared RP tree and sends a Prune message to tell the RP tree not to forward the traffic from the sender S for the specific multicast group.

After phase three, traffic from the sender will flow natively along the shortest path tree to the receiver, as shown in Figure 7-4.

XSR User’s Guide 7-9