3Com 4800G Series, Configuration Manual

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632 C
HAPTER
47: PIM C
ONFIGURATION
Figure 188 Assert mechanism
As shown in Figure 188, 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 local interface, receive a duplicate packet
forwarded by the other. Upon detecting this condition, both routers send an assert
message to all PIM routers (224.0.0.13) 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 unicast route to the source. By comparing these parameters,
either Router A or Router B becomes the unique forwarder of the subsequent (S,
G) packets on the multi-access subnet. The comparison process is as follows:
1 The router with a higher unicast route preference to the source wins;
2 If both routers have the same unicast route preference to the source, the router
with a smaller metric to the source wins;
3 If there is a tie in route metric to the source, the router with a higher IP address of
the local interface wins.
Introduction to PIM-SM PIM-DM uses the “flood and prune” principle to build SPTs for 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.
PIM-SM is a type of sparse mode multicast protocol. It uses the “pull mode” for
multicast forwarding, and 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 to receive multicast data. In the PIM-SM
mode, routers must specifically request a particular multicast stream before the
data is forwarded to them. The core task for PIM-SM to implement multicast
forwarding is to build and maintain rendezvous point trees (RPTs). An RPT is
rooted at a router in the PIM domain as the common node, or rendezvous
point (RP), through which the multicast data travels along the RPT and reaches
the receivers.
Ethernet
Router A Router B
Router C
Receiver
Multicast packets
Assert message