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Source-Specific Multicast Groups Overview

SSM is typically supported with a subset of IGMPv3 and PIM sparse mode known as PIM SSM. Using SSM, a client can receive multicast traffic directly from the source. PIM SSM uses the PIM sparse-mode functionality to create an SPT between the client and the source, but builds the SPT without the help of an RP.

By default, the SSM group multicast address is limited to the IP address range from 232.0.0.0 through 232.255.255.255. However, you can extend SSM operations into another Class D range by including the address statement at the [edit routing-options multicast ssm-groups] hierarchy level.

An SSM-configured network has distinct advantages over a traditionally configured PIM sparse-mode network. There is no need for shared trees or RP mapping (no RP is required), or for RP-to-RP source discovery through the Multicast Source Discovery Protocol (MSDP).

Deploying SSM is easy. You need only configure PIM sparse mode on all router interfaces and issue the necessary SSM commands, including specifying IGMPv3 or MLDv2 on the receiver's LAN. If PIM sparse mode is not explicitly configured on both the source and group members interfaces, multicast packets are not forwarded. Source lists, supported in IGMPv3 and MLDv2, are used in PIM SSM. Only sources that are specified send traffic to the SSM group.

In a PIM SSM-configured network, a host subscribes to an SSM channel (by means of IGMPv3 or MLDv2) to join group G and source S (see Figure 11). The directly connected PIM sparse-mode router, the receiver's designated router (DR), sends an (S,G) join message to its reverse-path forwarding (RPF) neighbor for the source. Notice in Figure 11 that the RP is not contacted in this process by the receiver, as would be the case in normal PIM sparse-mode operations.

Figure 11: Receiver Announces Desire to Join Group G and Source S

Image g017105.gif

The (S,G) join message initiates the source tree, then builds it out hop by hop until it reaches the source. In Figure 12, the source tree is built across the network to Router 3, the last-hop router connected to the source.

Figure 12: Router 3 (Last-Hop Router) Joins the Source Tree

Image g017106.gif

Using the source tree, multicast traffic is delivered to the subscribing host (see Figure 13).

Figure 13: The (S,G) State Is Built Between the Source and the Receiver

Image g017107.gif

SSM can operate in include mode as previously described or in exclude mode. In exclude mode the receiver specifies a list of sources that it does not want to receive the multicast group traffic from. The router forwards traffic to the receiver from any source except the sources specified in the exclusion list. The receiver accepts traffic from any sources except the sources specified in the exclusion list.

To configure additional SSM groups, include the ssm-groups statement:




multicast {



ssm-groups {

address; 
}


}

For a list of the hierarchy levels at which you can configure this statement, see the statement summary section for this statement.

For an overview of logical systems and a detailed example of logical system configuration, see the logical systems chapter of the JUNOS Feature Guide.

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