H3C Technologies H3C S10500 Series Switches - Manual (Page 141)

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PIM configuration
NOTE:
•
The term router in this document refers to both routers and Layer 3 switches.
• The term interface in the PIM features refers to Layer 3 interfaces, including VLAN interfaces and
route-mode (or Layer 3) Ethernet ports. You can set an Ethernet port to operate in route mode by using
the port link-mode route command (see Layer 2—LAN Switching Configuration Guide).
PIM overview
Protocol Independent Multicast (PIM) provides IP multicast forwarding by leveraging unicast static routes
or unicast routing tables generated by any unicast routing protocol, such as routing information protocol
(RIP), open shortest path first (OSPF), intermediate system to intermediate system (IS-IS), or border
gateway protocol (BGP). Independent of the unicast routing protocols running on the device, multicast
routing can be implemented as long as the corresponding multicast routing entries are created through
unicast routes. PIM uses the reverse path forwarding (RPF) mechanism to implement multicast forwarding.
When a multicast packet arrives on an interface of the device, it undergoes an RPF check. If the RPF check
succeeds, the device creates the corresponding routing entry and forwards the packet. If the RPF check
fails, the device discards the packet. For more information about RPF, see the chapter “Multicast routing
and forwarding configuration.”
Based on the implementation mechanism, PIM falls into the following categories:
• Protocol Independent Multicast–Dense Mode (PIM-DM)
• Protocol Independent Multicast–Sparse Mode (PIM-SM)
• Bidirectional Protocol Independent Multicast (BIDIR-PIM)
• Protocol Independent Multicast Source-Specific Multicast (PIM-SSM)
PIM-DM overview
PIM-DM is a type of dense mode multicast protocol. It uses the push mode for multicast forwarding, and
is suitable for small-sized networks with densely distributed multicast members.
The basic implementation of PIM-DM is as follows:
• PIM-DM assumes that at least one multicast group member exists on each subnet of a network.
Therefore, multicast data is flooded to all nodes on the network. Then, branches without multicast
forwarding are pruned from the forwarding tree, leaving only those branches that contain receivers.
This flood-and-prune process takes place periodically. Pruned branches resume multicast
forwarding when the pruned state times out. Data is then flooded again down these branches, and
then the branches are pruned again.
• When a new receiver on a previously pruned branch joins a multicast group, to reduce the join
latency, PIM-DM uses a graft mechanism to resume data forwarding to that branch.
Generally speaking, the multicast forwarding path is a source tree. That is, it is a forwarding tree with the
multicast source as its “root” and multicast group members as its “leaves.” Because the source tree is the
shortest path from the multicast source to the receivers, it is also called a shortest path tree (SPT).