Configuring Source-Specific Multicast Groups
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 a list of IP addresses with the [edit routing-options multicast ssm-groups] statement.
To deploy SSM, configure PIM sparse mode on all routing device 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 1). 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 1 that the RP is not contacted in this process by the receiver, as would be the case in normal PIM sparse-mode operations.
Figure 1: Receiver Sends Messages 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 2, the source tree is built across the network to Router 3, the last-hop router connected to the source.
Figure 2: Router 3 (Last-Hop Router) Joins the Source Tree
Using the source tree, multicast traffic is delivered to the subscribing host (see Figure 3).
Figure 3: (S,G) State Is Built Between the Source and the Receiver
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 routing device 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:
For a list of the hierarchy levels at which you can include this statement, see the statement summary section for this statement.
You can configure the Junos OS to accept any-source multicast (ASM) join messages (*,G) for group addresses that are within the default or configured range of source-specific multicast (SSM) groups. This allows you to support a mix of any-source and source-specific multicast groups simultaneously.
To configure the Junos OS to accept any-source multicast join messages for groups within the SSM address range, include the asm-override-ssm statement:
For a list of the hierarchy levels at which you can include 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 Junos Logical Systems Feature Guide.
