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Context: Studying for CCNA

I'm trying to understand multicast. I understand it conceptually, but I'm trying to understand implementation. For instance, in OSPF, how is multicast group membership determined? Do multicasts never leave the OSPF area?

Same question applies for ethernet multicast addresses. It seems like most L2 ethernet operations are with unicast or broadcast. If we know that L3 is the boundary of a L2 broadcast domain, how to we determine L2 multicast membership.

Any examples appreciated!

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  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin♦ Feb 19 '18 at 19:33
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For instance, in OSPF, how is multicast group membership determined? Do multicasts never leave the OSPF area?

OSPF routers join specific multicast groups: 224.0.0.5 for all routers and 224.0.0.6 for DR/BDR routers. OSPF routers do not forward these multicasts -- they stay on the local link.

I'm not quite sure I understand your second question. Multicasts have a specific range of Ethernet L2 addresses. Switches either

  • Flood multicasts out all ports (since they never learn the address from a source)
  • Implment igmp snooping, which lets them determine which ports have multicast receivers and which don't.
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  • That makes sense for OSPF. Are there instances where multicasts are forwarded between network segments? The statement "Flood multicasts out all ports" sounds like a broadcast. The igmp snooping examples sounds good. – The_Glidd Oct 11 '17 at 16:06
  • Of course. On Cisco routers, you have to enable multicast routing and use a protocol such as PIM to create distribution trees. – Ron Trunk Oct 11 '17 at 16:19
  • Switches flood both multicast and broadcasts, but to the hosts they are very different. – Ron Trunk Oct 11 '17 at 16:20
  • Flooding is done by switches that have no specific multicast understanding (or aren't configured to care) -- they treat them as broadcast frames, which they are. Implement igmp snooping ... OR participate in multicast routing. – Ricky Beam Oct 11 '17 at 20:29
  • And "they stay on the link" because the TTL is set to 1 (one). The TTL is how systems decide how far a multicast frame should propagate. If your network has no multicast router(s/ing), obviously everything will be link scope, as there's nothing to move it elsewhere. – Ricky Beam Oct 11 '17 at 20:32

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