130 CHAPTER 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-DM PIM-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