Example: Configuring Source-Specific Multicast Groups with Any-Source Override

Before you begin, configure the router interfaces.

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 and 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 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.

Topology

This example works with the simple RPF topology shown in Figure 4.

Figure 4: Simple RPF Topology

• Procedure
• Results

set protocols ospf area 0.0.0.0 interface fxp0.0 disable 
set protocols ospf area 0.0.0.0 interface all 
set protocols pim rp local address 10.255.72.46 
set protocols pim rp local group-ranges 239.0.0.0/24 
set protocols pim interface fe-1/0/0.0 mode sparse 
set protocols pim interface lo0.0 mode sparse 
set routing-options multicast ssm-groups 232.0.0.0/8 
set routing-options multicast ssm-groups 239.0.0.0/8 
set routing-options multicast asm-override-ssm

user@host# show protocols
ospf {
    area 0.0.0.0 {
        interface fxp0.0 {
            disable;
        }
        interface all;
    }
}
pim {
    rp {
        local {
            address 10.255.72.46;
            group-ranges {
                239.0.0.0/24;
            }
        }
    }
    interface fe-1/0/0.0 {
        mode sparse;
    }
    interface lo0.0 {
        mode sparse;
    }
}

user@host# show routing-options
multicast {
    ssm-groups [ 232.0.0.0/8 239.0.0.0/8 ];
    asm-override-ssm;
}

To verify the configuration, run the following commands:

• show igmp group

• show igmp statistics

• show pim join