Source Specific Multicast (SSM) | NetworkLessons.com

The multicast that you are probably familiar with (PIM sparse and dense mode) using IGMPv2 are 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 is able to receive the group from a specified source. When we use SSM / IGMPv3 there are no shared trees. We only build SPTs (Shortest Path Trees) towards 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 3 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 int 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

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:

Forum Replies

Dear Sir,
You explain in a very good way, your explanation make the tough things simpler. Thanks for your contribution to the Networking Society.

Hi Rene,
I am a beginner and interested in learning IGMP and PIM protocols . Can you please point me the links to start these protocols?

Thanks,
Srini

Rene,
Can we enable SSM with Dense-mode?

Regards,
Ulrich

Hello DJAN

When configuring SSM, you can use either sparse mode or sparse-dense mode. More information on configuring SSM can be found here:

I hope this has been helpful!

Laz

Hi Vanya,

It’s not flooded. With sparse mode, we initially use an RP as a central point since we don’t know where the source is. Once we learn the source, we switch to SPT (unless you disable this).

With SSM, you have to specify the source so that’s why you don’t need the RP. Traffic is forwarded with sparse mode.

Rene

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