3Com 10014299 manual Make the following configuration in the system view, Enable Multicast Routing

512CHAPTER 35: CONFIGURING PIM-DM

operating IGMPv1 (each PIM router periodically broadcasts a hello message, and the router with higher IP address is chosen to be the DR).

■Graft Message: The host informs the router which multicast groups it wants to join by a IGMP membership report message. At this time, the port sends a graft message to the upstream router. After the upstream router receives a graft message, it adds this port to the forwarding list of the multicast group.

■Graft ACK Message: After the upstream router receives the graft message, it needs to send graft acknowledgement (ACK) message to the downstream router sending the graft message.

■Prune Message: If the router interface forwarding list is empty, or the interface forwarding list becomes empty, the prune message will be sent to the upstream router to inform it to delete the downstream router from its interface neighbor list.

■Assert Message: A shared network segment can have two upstream routers simultaneously. If both of them forward multicast packets to it, the downstream routers of this network segment will probably receive two same multicast packets. In order to avoid this condition, PIM-DM adopts the assert message mechanism. If a router receives multicast packets at the forwarding port of a shared LAN, it requires all the routers operating PIM-DM (group address is 224.0.0.13) to send an assert message. The downstream routers determine the winner by comparing the specific domains of the assert message according to the relevant series of rules. The router with little message preference wins. If the preference is the same, the router with the smaller message metric value wins. If the message metric value is the same, the router with the bigger IP address wins. The winner serves as the transmitter of the network segment, while the loser sends an output interface prune message.

PIM-DM itself does not have a routing discovery mechanism, so it has to depend on a specific unicast routing protocol. Thus the protocol implementation is quite simple.

PIM-DM Configuration PIM-DM configuration includes tasks that are described in the following sections:

■Enabling Multicast Routing

■Starting the PIM-DM Protocol

■Configuring the Time Interval for Hello Messages

Enabling Multicast Only after the multicast routing is enabled, can routers receive multicast packets.

Routing

Make the following configuration in the system view.

Table 583 Enable Multicast Routing

Operation	Command
Enable multicast routing	multicast routing-enable
Disable multicast routing	undo multicast routing-enable

By default, the system disables the multicast routing.