PIM source-specific multicast (SSM) uses a subset of PIM sparse mode and IGMP version 3 (IGMPv3) to allow a client to receive multicast traffic directly from the source. PIM SSM uses the PIM sparse-mode functionality to create an SPT between the receiver 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 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 on the receiver's LAN. If PIM sparse mode is not explicitly configured on both the source and group member interfaces, multicast packets are not forwarded. Source lists, supported in IGMPv3, are used in PIM SSM. As sources become active and start sending multicast packets, interested receivers in the SSM group receive the multicast packets.
In a PIM SSM-configured network, a host subscribes to an SSM channel (by means of IGMPv3), announcing a desire to join group G and source S (see Figure 25). The directly connected PIM sparse-mode router, the receiver's DR, sends an (S,G) join message to its RPF neighbor for the source. Notice in Figure 25 that the RP is not contacted in this process by the receiver, as would be the case in normal PIM sparse-mode operations.
Figure 25: Receiver Announces Desire to Join Group G and Source S
The (S,G) join message initiates the source tree, then builds it out hop by hop until it reaches the source. In Figure 26, the source tree is built across the network to Router 3, the last-hop router connected to the source.
Figure 26: Router 3 (Last-Hop Router) Joins the Source Tree
Using the source tree, multicast traffic is delivered to the subscribing host (see Figure 27).
Figure 27: The (S,G) State Is Built Between the Source and the Receiver
To configure additional SSM groups, include the ssm-groups statement at the [edit routing-options multicast] hierarchy level.
For more information about PIM SSM, see Example: Configuring PIM SSM on a Network.
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