Introduction To Ipv6 Pim-Sm - 3Com MSR 50 Series Configuration Manual

1238 C 74: IP 6 PIM C
HAPTER V
Introduction to IPv6
PIM-SM
ONFIGURATION
Assert
If multiple multicast routers exist on a multi-access subnet, duplicate IPv6 multicast
packets may flow to the same subnet. To shutoff duplicate flows, the assert
mechanism is used for election of a single IPv6 multicast forwarder on a
multi-access network.
Figure 359 Assert mechanism
Router A
Ethernet
Assert message
IPv6 multicast packets
As shown in Figure 359, after Router A and Router B receive an (S, G) IPv6
multicast 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 local interface,
receive a duplicate IPv6 multicast packet forwarded by the other. Upon detecting
this condition, both routers send an assert message to all IPv6 PIM routers through
the interface on which the packet was received. The assert message contains the
following information: the multicast source address (S), the multicast group
address (G), and the preference and metric of the IPv6 unicast route to the source.
By comparing these parameters, either Router A or Router B becomes the unique
forwarder of the subsequent (S, G) IPv6 multicast packets on the multi-access
subnet. The comparison process is as follows:
1 The router with a higher IPv6 unicast route preference to the source wins;
2 If both routers have the same IPv6 unicast route preference to the source, the
router with a smaller metric to the source wins;
3 If there is a tie in the route metric to the source, the router with a higher IP address
of the local interface wins.
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- 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- and medium-sized
networks with sparsely and widely distributed IPv6 multicast group members.
The basic implementation of IPv6 PIM-SM is as follows:
Router B
Receiver
Router C