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# Overview

This implementation of PIM supports PIM sparse mode and PIM source-specific multicast (PIM SSM) for IPv6 multicast.

SSM is an extension to the Any Source Multicast (ASM) service model and facilitates the deployment of broadcast (one-to-many) applications, such as Internet TV and radio where large receiver audiences require traffic from a few well-known sources.

Figure 21 represents how PIM builds an (S,G) entry in an SRT. When multiple routers are connected to a multiaccess network, one router is assigned the role of the designated router. The designated router receives data from the source on interface 1/0 and multicasts the data to its downstream neighbors on interfaces 1/1, 2/0, and 2/1. In the designated router routing table, the entry for this operation lists the source as the IP address of the source and the group as the IP address of the multicast group.

Neighbors exchange hello messages periodically to determine the designated router. The router with the highest network layer address becomes the designated router. If the designated router subsequently receives a hello message from a neighbor with a higher network layer address, that neighbor becomes the designated router.

Figure 21: Source-Rooted Tree

## PIM Sparse Mode

In addition to the features PIM sparse mode supports for IPv4, this IPv6 implementation of PIM sparse mode also supports remote neighbors.

For a description of PIM sparse mode, see Configuring PIM for IPv4 Multicast .

### Joining Groups

A host's designated router (DR) sends join messages to the RP when that host wants to join a group. When a host wants to leave a group, it communicates with its designated router through MLD. When the designated router no longer has any hosts that belong to a particular group, it sends a prune message to the RP.

### Timers

PIM sparse mode uses timers to maintain the networking trees.

| Note: PIM sparse mode routers poll their neighbors and hosts for various pieces of information at set intervals.

If a PIM sparse mode router does not receive information from a neighbor or host within a specific time, known as the hold time, it removes the associated information from its routing tables.

You can configure how often an interface sends hello messages (hello interval) and how often routers send RP announce messages (RP announce interval). The hold-time associated with hello messages is 3.5 times the hello interval, and the holdtime associated with RP announce messages is 2.5 times the RP announce interval.

All other timers are fixed and take the default values recommended in:

RFC 2934—Protocol Independent Multicast MIB for IPv4 (October 2000)

## PIM Sparse Mode Bootstrap Router

PIM sparse mode routers need the address of the rendezvous point (RP) for each group for which they have (*,G) state. They obtain this address either through a bootstrap mechanism or through static configuration. Two bootstrap mechanisms exist—bootstrap router (BSR) or auto-RP. Auto-RP is not used in IPv6 implementations.

When implemented, BSR operates as follows:

1. One router in each PIM domain is elected the BSR.
2. All the routers in the domain that are configured to be RP candidates periodically unicast their candidacy to the BSR.
3. The BSR picks an RP set from the available candidates and periodically announces this set in a bootstrap message.
4. Bootstrap messages are flooded hop by hop throughout the domain until all routers in the domain learn the RP Set.

## PIM Source-Specific Multicast

PIM source-specific multicast (SSM) is an extension of the PIM protocol. Using SSM, a client can receive multicast traffic directly from the source. PIM SSM uses PIM sparse mode functionality to create an SPT between the client and the source, but builds the SPT without using an RP.

By default, the SSM group multicast address is limited to the IPv6 address range FF3x::/96 where x represents any valid scope. You can use the ipv6 pim ssm range command to change the SSM group address range.

Advantages that an SSM-configured network has over a traditionally configured PIM sparse mode network include the following:

- No need for shared trees or RP mapping (no RP is required).
- No need for RP-to-RP source discovery through Multicast Source Discovery Protocol (MSDP).
- Simplified administrative deployment; you need only configure PIM sparse mode on all router interfaces and issue the necessary SSM commands (including specifying MLDv2 on the receiver local area network).
- Support for source lists; you can use source lists, supported in MLDv2, where only specified sources send traffic to the SSM group.

In a PIM SSM–configured network, the E Series router subscribes to an SSM channel (by means of MLDv2), announcing a desire to join group G and source S. The directly connected PIM sparse mode router, the designated router of the receiver, sends an (S,G) join message to its RPF neighbor for the source. For PIM SSM, the RP is not contacted in this process by the receiver (as happens in normal PIM sparse mode operations).

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