Pim-Sm Overview - HP MSR2003 Configuration Manual

1. The node that needs to receive the multicast data sends a graft message to its upstream node,
telling it to rejoin the SPT.
2. After receiving this graft message, the upstream node adds the receiving interface to the outgoing
interface list of the (S, G) entry. It also sends a graft-ack message to the graft sender.
3. If the graft sender receives a graft-ack message, the graft process finishes. Otherwise, the graft
sender continues to send graft messages at a configurable interval until it receives an
acknowledgment from its upstream node.
Assert
On a subnet with more than one multicast router, the assert mechanism shuts off duplicate multicast flows
to the network. It does this by electing a unique multicast forwarder for the subnet.
Figure 30 Assert mechanism
As shown in
both forward the packet to the local subnet. As a result, the downstream node Router C receives two
identical multicast packets. Both Router A and Router B, on their own downstream interfaces, receive a
duplicate packet forwarded by the other. After detecting this condition, both routers send an assert
message to all PIM routers (224.0.0.13) on the local subnet through the interface that received the packet.
The assert message contains the multicast source address (S), the multicast group address (G), and the
metric preference and metric of the unicast route/MBGP route/static multicast route to the multicast
source. By comparing these parameters, either Router A or Router B becomes the unique forwarder of the
subsequent (S, G) packets on the shared-media LAN. The comparison process is as follows:
1. The router with a higher metric preference to the multicast source wins.
2. If both routers have the same metric preference, the router with a smaller metric to the multicast
source wins.
3. If both routers have the same metric, the router with a higher IP address on the downstream
interface wins.

PIM-SM overview

PIM-DM uses the flood-and-prune cycles to build SPTs for multicast data forwarding. Although an SPT has
the shortest paths from the multicast source to the receivers, it is built with a low efficiency. PIM-DM is not
suitable for large and medium-sized networks.
PIM-SM uses the pull mode for multicast forwarding, and it is suitable for large- and medium-sized
networks with sparsely and widely distributed multicast group members.
Figure 30, after Router A and Router B receive an (S, G) packet from the upstream node, they
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