R21xx-HP FlexFabric 11900 IP Multicast Configuration Guide

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Figure 25 Assert mechanism
As shown in Figure 25, after Router A and Router B receive an (S, G) packet from the upstream node, they
both forward the packet to the local subnet. As a result, the downstream node Router C receives two
identical multicast packets, and 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
preference and metric of the unicast 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 preference to the multicast source wins.
2. If both routers have the same preference to the source, the router with a smaller metric to the
multicast source wins.
3. If a tie exists in route metric to the multicast source, 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 and 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.
The basic implementation of PIM-SM is as follows:
• PIM-SM assumes that no hosts need multicast data. In the PIM-SM mode, a host must express its
interest in the multicast data for a multicast group before the data is forwarded to it. PIM-SM
implements multicast forwarding by building and maintaining rendezvous point trees (RPTs). An
RPT is rooted at a router that has been configured as the rendezvous point (RP) for a multicast group,
and the multicast data to the group is forwarded by the RP to the receivers along the RPT.