3Com 10014298 manual Multicast Protocol

130CHAPTER 6: MULTICAST PROTOCOL

■Input the display igmp-snooping group command to see if the multicast group is the expected one.

■Verify that the source IP address is correct for each multicast stream.

3Multicast forwarding table set up on the bottom layer is wrong.

■Enable IGMP Snooping group in user view and then input the display igmp-snooping group command to check if MAC multicast forwarding table in the bottom layer and that created by IGMP Snooping is consistent. You may also input the display mac vlan command in all views to check if MAC multicast forwarding table under vlanid in the bottom layer and that created by IGMP Snooping is consistent.

■If they are not consistent, contact the maintenance personnel for help.

Configuring PIM-DMPIM-DM (Protocol Independent Multicast, Dense Mode) belongs to dense mode multicast routing protocols. PIM-DM is suitable for small networks. Members of multicast groups are relatively dense in such network environments.

The working procedures of PIM-DM include neighbor discovery, flood and prune, and graft.

■Neighbor discovery

The PIM-DM router needs to use Hello messages to perform neighbor discovery when it is started. All network nodes running PIM-DM keep in touch with one another with Hello messages, which are sent periodically.

■Flood and Prune

PIM-DM assumes that all hosts on the network are ready to receive multicast data. When a multicast source “S” begins to send data to a multicast group “G”, after the router receives the multicast packets, the router will perform RPF check according to the unicast routing table first. If an RPF check is passed, the router will create an (S, G) entry and then flood the data to all downstream PIM-DM nodes. If the RPF check is not passed, that is when multicast packets enter from an error interface, the packets will be discarded. After this process, an (S, G) entry will be created in the PIM-DM multicast domain.

If the downstream node has no multicast group members, it will send a Prune message to the upstream nodes to inform the upstream node not to forward data to the downstream node. Receiving the prune message, the upstream node will remove the corresponding interface from the outgoing interface list corresponding to the multicast forwarding entry (S, G). In this way, a SPT (Shortest Path Tree) rooted at Source S is built. Leaf routers initiate the pruning process.

This is called the “flood & prune” process. Nodes that are pruned provide timeout mechanism. Each router re-starts the “flood & prune” process upon pruning timeout. The consistent “flood & prune” process of PIM-DM is performed periodically.

During this process, PIM-DM uses the RPF check and the existing unicast routing table to build a multicast forwarding tree rooted at the data source. When a packet arrives, the router judges the validity of the path. If the interface is indicated by the unicast routing to the multicast source, the packet is regarded to be from the correct path, otherwise, the packet will be discarded