PIM-DM Interface Grafting

Nov 24,2010 by alperen

PIM-DM Interface Grafting


	Interfaces that have been pruned from the oilist for a router inter face can be added back into the source-based tree for a multicast source using PIM-DM graft messages (see Figure 6-17). PIM-DM graft messages are the only messages that are acknowledged. The graft messages are acknowledged using the packet format shown in Figure 6-18.


	Figure 6-17: PIM Graft Packet format


	Figure 6-18: PIM Graft-Ack Packet format


	The network in Figure 6-19 will be used as an example of PIM-DM grafting. Router A is forwarding multicast traffic to router B (step 1). Since router B has no downstream PIM-DM neighbors or multicast receivers, router B sends a prune message to router A (step 2). The oilist for the S1 interface on router A is now null and a prune timer has been set using the timer value in the prune message. If a multicast receiver attached to the ethernet on router B wishes to receive traffic, an IGMP join message is sent to router B (step 3). Router B can either wait for the prune timer on router A to expire, which will cause router A to add interface S1 to the oilist for the source, or router B can send a graft message to router A (step 4). The serial interface on router A is in the prune state for the source and has a prune lifetime timer running. Router B has (S,G) and (*,G) entries for the source but these entries are in the prune state. So router B will send a graft message to router A and A will acknowledge will graft acknowledgment message (step 5).


	Figure 6-19: PIM-DM interface pruning and grafting message flow.

One very important characteristic of dense mode protocols is the prune/broadcast cycle. In Figure 6-19, if router B never had any attached receivers or downstream PIM-DM neighbors, then multicast traffic would never need to be forwarded to router B. Initially, router B will prune itself from any source-based delivery trees. Since prunes have a limited lifetime, router B would again be sent multicast traffic from router A. Router B would again send a prune to A, which would timeout, and cause A to forward to B. This triggers a prune, and so it goes. If you are certain that multicast traffic does not need to go to a particular router, then don’t enable PIM-DM on the interfaces.