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Multicast - PIM SSM and PIM SM on the same router

Hi all,

Do you know if it is possible to use PIM SM and PIM SSM on the same router ? I would like to use ACL to do that but I don't know if SM and SSM are compatible with each other.






it is possible. We use it in the Cat6500 platform, I guess in other platforms will be similar.

SSM is a derivation of SM, and it's not a matter of compatibility, but how the router/switch identifies the incoming multicast traffic. From the source point of view, the first hop router will decide if the incoming multicast will be SM or SSM based on the ACLs you apply to the commands "ip pim rp" and "ip pim ssm".

- If you enable PIM sparse mode in the incoming interface, and you DO NOT enable the command "ip pim ssm", the router will identify the traffic as SM, as long as it matches the ACL in the "ip pim rp" command, or if you don't use an ACL (all multicast traffic will be treated as SM). The router will try to register this group in the RP

- If you enable the command "ip pim ssm default", the range will be treated as SSM.You can use an ACL and the modifier "range" in this command to change the default SSM range.

- You should not overlap ranges in SSM and SM, so design your multicast IP addressing in a way that you dedicate a range/s for SSM and different range/s for SM.

Strictly speaking, and according to Cisco documentation, you only need SSM capable software in the last-hop router, but you need to prevent the SSM group to register in the RP, and one of the ways to do that is enabling "ip pim ssm-range" I have not tested the behaviour of the multicast traffic intended to be SSM by just not allowing in the ACL in the command "ip pim rp" and not enabling "ip pim ssm".

Now for the receiver side, you need to:

-Enable IGMP v3 in the interfaces facing the SSM receivers. IGMP v3 is backwards compatible and IGMP v2 messages will be processed correctly on those v3 enabled interfaces in case you have a mix of v3 and v2 capable receivers and the v2 ones will subscribe to SM groups.

-If the receiver doesn't support IGMPv3 you need to use SSM Mapping. In that case, enable IGMPv2 (not v3) and map the groups with the sources either statically or using DNS ( )

Anyway, if you give us some detail on the topology and platform you are using in your network, we can plan more in detail the best way for you to enable SSM.



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