Source Specific Multicast (SSM)

Hello Chad

If you have a topology on which you run multicast, and you want to introduce SSM in parallel with Any Source Multicast (ASM), then it is possible to introduce VRFs in order to segregate the multicast operations. When implementing multicast in an environment where VRFs are used, each multicast instance remains contained within the VRF. Specifically keep in mind that:

  • When you enable a multicast routing protocol like PIM within a VRF, that instance of PIM operates independently of PIM instances in other VRFs or in the global routing table. PIM neighbors, the RP, and multicast distribution trees are all specific to the VRF.
    *Similarly, IGMP operates independently within each VRF. IGMP reports from hosts are processed by the IGMP instance in the VRF to which the hosts belong.
    *By default, multicast traffic is not shared across VRFs. A source in one VRF cannot send multicast traffic to receivers in another VRF. If sharing of multicast traffic across VRFs is required, Multicast VPN (MVPN) or similar solutions would need to be implemented.

So it is possible to introduce VRFs to segregate SSM from ASM. However, the question is, do you really want to do this?

Introducing VRFs will introduce a level of complexity to your network that may be counterproductive. VRFs are applied to interfaces, so unicast traffic will be affected by VRF configurations. You must ensure that routing for unicast traffic is also taken care of.

I would avoid using VRFs in this situation because it is possible to allow both SSM and ASM multicast to operate simultaneously over the same infrastructure. Both ASM and SSM can coexist in the same network, with some groups being serviced by ASM and others by SSM, depending on the needs of the receivers. The network can distinguish between ASM and SSM groups based on the group addresses. A specific range of group addresses can be set aside for SSM, and this can be configured using an access list. For more info on this, take a look at the NetworkLessons note on how to configure ASM and SSM on the same network.

I hope this has been helpful!