R21xx-HP FlexFabric 11900 IP Multicast Configuration Guide

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Figure 46 Assert mechanism
As shown in Figure 46, 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 IPv6 PIM routers on the local subnet through the interface that received the packet. The
assert message contains the IPv6 multicast source address (S), the IPv6 multicast group address (G), and
the preference and metric of the IPv6 unicast route to the IPv6 multicast source. By comparing these
parameters, either Router A or Router B becomes the unique forwarder of the subsequent (S, G) packets
on the subnet. The comparison process is as follows:
1. The router with a higher preference to the IPv6 multicast source wins.
2. If both routers have the same preference to the IPv6 multicast source, the router with a smaller
metric to the IPv6 multicast source wins.
3. If a tie exists in route metric to the IPv6 multicast source, the router with a higher IPv6 link-local
address on the downstream interface wins.
IPv6 PIM-SM overview
IPv6 PIM-DM uses the flood-and-prune cycles to build SPTs for IPv6 multicast data forwarding. Although
an SPT has the shortest paths from the IPv6 multicast source to the receivers, it is built with a low efficiency
and is not suitable for large- and medium-sized networks.
IPv6 PIM-SM uses the pull mode for IPv6 multicast forwarding, and it is suitable for large-sized and
medium-sized networks with sparsely and widely distributed IPv6 multicast group members.
The basic implementation of IPv6 PIM-SM is as follows:
• IPv6 PIM-SM assumes that no hosts need IPv6 multicast data. In the IPv6 PIM-SM mode, a host must
express its interest in the IPv6 multicast data for an IPv6 multicast group before the data is
forwarded to it. IPv6 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