Chapter 6 Pim-Dm Configuration - Huawei Quidway S3500 Series Operation Manual

Operation Manual - Multicast
Quidway S3500 Series Ethernet Switches

Chapter 6 PIM-DM Configuration

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 & prune and
graft.
I. 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.
II. Flood&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 the 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, 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. The pruning process is initiated by leaf routers first.
This process is called "flood & prune" process. In addition, 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 will first judge the correctness of the path. If the interface that the packet
arrives is the one 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 as a
redundancy packet without the multicast forwarding. The unicast routing information as
path judgment can come from any unicast routing protocol independent of any
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Chapter 6 PIM-DM Configuration