Source Specific Multicast (SSM)

Hello Sean

Keep in mind that just because the SSM multicast range is being used, it does not mean that SSM is actually employed. Cisco devices can apply SSM to any range, simply by using the config found in this particular lesson. The SSM configuration is necessarily configured on a Cisco IOS router.

In order to get SSM to function correctly, the receivers (hosts) must support IGMPv3. The multicast source (and thus the application you have that attempts to utilize multicast SSM) cannot enable it. It must be enabled on the network by an IOS router, and on the hosts themselves.

It is likely that your network is not employing SSM at all, and is running all multicast applications using any source multicast. As you investigate further, let us know how you get along, and if we can be of further help.

I hope this has been helpful!

Laz

Thanks Laz,
In SSM multicast I’m wondering about IGMPV3. Where exactly it is needed ? As you say “In order to get SSM to function correctly, the receivers (hosts) must support IGMPv3” and thats fine but what about the SVI’s on the subnets on the switches that the hosts are connected to.
Do these need to also run IGMPV3 ?
Lets say for example in a strict Layer 2 multicast network (Not multicast routing).
Lets say in this layer 2 multicast network I have 10 switches daisy chained in a row numbered from left to right switch 1 to switch 10.
Lets say on switch one (the furthest left) I have my multicast source and switch 5 in the middle is the Querier and switch 10 (the furthest right) I have my receiver lets say a windows server.
If the windows server (receiver) is running IGMPV3 and configured for SSM does the SVI on that subnet on the switch 10 have to be running IGMPV3 also or can it run IGMPV2 and same for all the switches in between source and receiver ? Also with the Querier, Does the querier have to be running IGMPV3 for SSM to work or can it to be running IGMPV2 ?
I’m trying to understand the relationship between the IGMP version on the SVI’s and IGMP version on the receivers (hosts) in a real world scenario as a lot of the examples here use routers and switches to simulate end devices.
Is IGMP version 3 needed on all the SVI’s in the subnet or just the querier or none and just the host or its required on everything ?
Thanks Laz.

Hello Sean

IGMPv3 includes a field within the IGMP message for the source address. This information is needed to enable SSM. Which devices need this information? Well, the multicast receiver uses that field to specify from which source it is expecting multicast traffic. The last hop router must also support IGMPv3 so it can determine if a host should or should not receive multicast traffic from a particular source.

Now, do your layer 2 switches need to support IGMPv3 to achieve IGMP snooping? No they don’t. IGMP snooping simply keeps a record of multicast IP group and host MAC address so that multicast traffic is not flooded to all hosts, but goes only to those hosts that have requested it. Because IGMPv3 is backward compatible with v2 and v1, any switches receiving v3 messages will be able to interpret them as v2 or v1 if needed. However, enabling IGMPv3 snooping on switches will add one more parameter to the mapping of host MAC addresses to multicast groups. In other words, it will use source-based filtering as well. More on this can be found in the following Cisco documentation:

Now you asked about SVIs. SVIs are essentially the last hop router because they are Layer 3 interfaces. Also, the example you described was a strict layer 2 multicast network, but if you have SVIs on each switch, then there is Layer 3 as well, so the example given is not quite clear. However, I hope your question has been answered… If not please clarify, and we can continue the conversation.

I hope this has been helpful!

Laz