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I'm trying to understand how integrated SSM multicast is to IPv6. Is it built in and sent solely by ICMPv6 messages or do you have to run something extra like PIM-SM? The reason I as is I've seen some places that say multicast just uses ICMPv6 and other that talk about running Protocol Independent Multicast software in addition to IPv6.

So, for IPv6 Layer 3 SSM, does the IPv6 protocol have all it needs, or does it have to use another protocol like PIM-SM too?

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2 Answers 2

Basically, there are two types of communication needed in an IP multicast (either IPv4 or IPv6):

  • host-multicast router
  • multicast router-multicast router

Both types have different needs.

Host-multicast router communication is handled by ICMP or MLD, which both serves just to the purpose of multicast router having an actual database of which local host wants which multicast packets.

The other type of communication - multicast router-multicast router - is much more complex and covers entirely different problems. Multicast routers need to send multicast messages to other routers who want them and to recieve messages for the groups their clients want. Given the fact no-one has to subscribe to the IP multicast group to be able to send packets to it, this is pretty hard thing to do, not even mentioning how dynamic the enviroment is and on the top of it, multicast group addresses have no hierarchic inner structure to help you route (they can't by principle). This is where PIM and similar protocols come in.

So, in short, just as Timothy said:
With just one multicast router, you will only need ICMP or MLD. With two or more routers, you will also need routing software using PIM or similar protocol on all multicast routers.

This is principal thing applicable for both IPv6 and IPv4.

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You will need a multicast routing protocol like PIM-SM etc to send and multicast traffic over 2 or more routers. With a single router, link-local or between an edge router and its upstream, ICMPv6 is suffiecent

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