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Source Specific Multicast (SSM)

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:

3 cisco routers

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:

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Forum Replies

  1. 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

  2. Rene,
    Can we enable SSM with Dense-mode?

    Regards,
    Ulrich

  4. Hi Laz,

    This pdf link is no longer valid. Do you have a copy of the document elsewhere or can you summarize the purpose of the pdf?

    Thank you,

    Jay Karson

  5. Hello Sean

    If you use the command ip ssm default, as in the lesson, then the 232.0.0.0/8 range is used by default. Alternatively, you can use the ip ssm range ACL_NAME where the the ACL_NAME is the name of the access list that defines the multicast address range you want to define. This way, you can use a range different from 232.0.0.0/8. In both cases, if you try to join a group outside of the specified range (either default, or otherwise specified) it will be dropped.

    ... Continue reading in our forum

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