Source Specific Multicast (SSM)

The multicast that you are probably familiar with (PIM sparse and dense mode) using IGMPv2 is also known as ASM (Any Source Multicast). This means that the receivers really don’t care what source they receive multicast traffic from, all sources are accepted.

Using sparse mode, our receivers require the RP (Rendezvous Point) to discover new sources in the network. SSM (Source Specific Multicast) requires IGMPv3 and lets us join multicast groups from specified source addresses.

Besides just joining any group, the receiver can receive the group from a specified source. When we use SSM / IGMPv3, there are no shared trees. We only build SPTs (Shortest Path Trees) toward our sources. This also means that we don’t use any RPs …you won’t need Auto-RP or Bootstrap anymore!

Configuring source specific multicast is pretty easy. Let’s use the following topology to take a look at the configuration:

We have a very small network with only three routers. Let’s prepare them by enabling OSPF and multicast routing:

R1,R2 and R3:
ip multicast-routing
router ospf 1
network 0.0.0.0 255.255.255.255 area 0

Let’s enable PIM sparse-mode on the interfaces:

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip pim sparse-mode

R2(config)#interface fastEthernet 0/0
R2(config-if)#ip pim sparse-mode 
R2(config-if)#interface fastEthernet 0/1
R2(config-if)#ip pim sparse-mode

R3(config)#interface fastEthernet 0/0
R3(config-if)#ip pim sparse-mode

Now we can dive into the configuration of SSM. First, we’ll enable SSM on all routers. If you do not specify a group, then the 232.0.0.0/8 range will be used. If you do try a (*, G) join for any group in the SSM range, then it will be dropped.

R1,R2 and R3:
ip pim ssm default

The next step is to enable IGMPv3 on the source and receiver:

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip igmp version 3

R3(config)#interface fastEthernet 0/0
R3(config-if)#ip igmp version 3

Now we’ll configure R3 to join a multicast group address in the 232.0.0.0/8 range, and we’ll specify R1 as the source: