3Com Switch 8800 Advanced Software V5 Configuration Guide

566 CHAPTER 42: PIM CONFIGURATION

Assert

If multiple multicast routers exist on a multi-access subnet, duplicate packets may

flow to the same subnet. To shutoff duplicate flows, the assert mechanism is used

for election of a single multicast forwarder on a multi-access network.

Figure 175 Assert mechanism

As shown in Figure 175, 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 mod 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 larger 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

Ethernet

Router A Router B

Router C Receiver

Multicast packets

Assert message