Configuring Pim; Overview; Pim-Dm Overview - HP MSR1002-4 Configuration Manual

Configuring 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 RIP, OSPF, IS-IS,
or BGP. PIM uses the underlying unicast routing to generate a multicast routing table without relying
on any particular unicast routing protocol.
PIM uses the 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 a multicast routing entry and forwards the packet. If the RPF check fails, the device discards
the packet. For more information about RPF, see "Configuring multicast routing and forwarding."
Based on the implementation mechanism, PIM includes 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)
In this document, a PIM domain refers to a network that contains PIM routers.

PIM-DM overview

PIM-DM uses the push mode for multicast forwarding, and is suitable for small-sized networks with
densely distributed multicast members.
PIM-DM assumes that all downstream nodes want to receive multicast data from a source, so
multicast data is flooded to all downstream nodes on the network. Branches without downstream
receivers are pruned from the forwarding trees. When a pruned branch has new receivers, the graft
mechanism turns the pruned branch into a forwarding branch.
In PIM-DM, the multicast forwarding paths for a multicast group constitutes a forwarding tree. The
forwarding tree is rooted at the multicast source and has multicast group members as its "leaves."
Because the forwarding tree consists of the shortest paths from the multicast source to the receivers,
it is also called a "shortest path tree (SPT)."
Neighbor discovery
In a PIM domain, each PIM interface on a router periodically multicasts PIM hello messages to all
other PIM routers (identified by the address 224.0.0.13) on the local subnet. Through the exchanging
of hello messages, all PIM routers on the subnet determine their PIM neighbors, maintain PIM
neighboring relationship with other routers, and build and maintain SPTs.
SPT building
The process of building an SPT is the flood-and-prune process:
1. In a PIM-DM domain, the multicast data from the multicast source S to the multicast group G is
flooded throughout the domain. A router performs an RPF check on the multicast data. If the
RPF check succeeds, the router creates an (S, G) entry and forwards the data to all
downstream nodes on the network. In the flooding process, all the routers in the PIM-DM
domain create the (S, G) entry.
2. The nodes without downstream receivers are pruned. A router that has no downstream
receivers multicasts a prune message to all PIM routers on the subnet. When an upstream
node receives the prune message, it removes the receiving interface from the (S, G) entry. In
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