Pim-Sm Overview - HP FlexNetwork 7500 Series Configuration Manual

Figure 35 Assert mechanism
As shown in
they both forward the packet to the local subnet. As a result, the downstream node Router C
receives two identical multicast packets. In addition, both Router A and Router B, on their
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:
The router with a higher metric preference to the multicast source wins.
1.
If both routers have the same metric preference, the router with a smaller metric wins.
2.
If both routers have the same metric, the router with a higher IP address on the downstream
3.
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.
Therefore, 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.
PIM-SM assumes that no hosts need multicast data. A multicast receiver must express its interest in
the multicast data for a multicast group before the data is forwarded to it. A rendezvous point (RP) is
the core of a PIM-SM domain. Relying on the RP, SPTs and rendezvous point trees (RPTs) are
established and maintained to implement multicast data forwarding. An SPT is rooted at the
multicast source and has the RPs as its leaves. An RPT is rooted at the RP and has the receiver
hosts as its leaves.
Neighbor discovery
PIM-SM uses the same neighbor discovery mechanism as PIM-DM does. For more information, see
"Neighbor discovery."
DR election
A designated router (DR) is required on both the source-side network and receiver-side network. A
source-side DR acts on behalf of the multicast source to send register messages to the RP. The
receiver-side DR acts on behalf of the multicast receivers to send join messages to the RP.
Figure 35, after Router A and Router B receive an (S, G) packet from the upstream node,
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