F3726, F3211, F3174, R5135, R3816-HP Firewalls and UTM Devices Appendix Protocol Reference-6PW100

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Figure 68 Assert mechanism
As shown in Figure 68, the assert mechanism is as follows:
1. After Router A and Router B receive an (S, G) IPv6 multicast packet from the upstream node, both
of them 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 IPv6 multicast
packet that the other has forwarded.
2. 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 multicast source address (S), the multicast group address (G), and
the preference and metric of the IPv6 unicast route/IPv6 MBGP route/IPv6 multicast static route to
the source.
3. The routers compare these parameters, and either Router A or Router B becomes the unique
forwarder of the subsequent (S, G) IPv6 multicast packets on the shared-media subnet. The
comparison process is as follows:
a. The router with a higher preference to the source wins.
b. If both routers have the same preference to the source, the router with a smaller metric to the
source wins.
c. If a tie exists in the route metric to the 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 principle to build SPTs for IPv6 multicast data distribution.
Although an SPT has the shortest path, it is built with a low efficiency. Therefore the PIM-DM mode is not
suitable for large-sized and medium-sized networks.
IPv6 PIM-SM is a type of sparse-mode IPv6 multicast protocol. It uses the pull mode for IPv6 multicast
forwarding and 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 to receive IPv6 multicast data. In the IPv6 PIM-SM mode,
routers must specifically request a particular IPv6 multicast stream before the data is forwarded to
them. The core task for IPv6 PIM-SM to implement IPv6 multicast forwarding will build and maintain