3 Replies Latest reply: Mar 12, 2018 5:42 PM by Jeffrey RSS

    Multicast Curiosity


      Hey CLN, I am wondering how multicast works, I get that it is not part of the CCNP but as the routing protocols communicate multicast I feel like a should atleast have a vague idea and got some info through google but I don't know if it's correct or not. So say I have two routers using EIGRP in VLAN2 and they are communicating via IP and multicast mac 01:00:5e:00:00:0a. How does the switch process this destination traffic? I read that a switch will forward it out all active interfaces except the one it received the traffic on in the VLAN like it is broadcast? Is multicast handled like broadcast traffic? By the way running 3750s. If you have the knowledge it is greatly appreciated.

        • 1. Re: Multicast Curiosity
          Steven Davidson

          For the most part it is.  There are multicast flooding optimizations which can be enabled (if the switch supports it) which will constrain IP multicast traffic to only ports which have joined a given multicast group.  There are exceptions to what will be constrained by the switch.  I do not know if this applies to EIGRP (or any other multicast group using

          • 2. Re: Multicast Curiosity

            Steven Davidson wrote:


            I do not know if this applies to EIGRP (or any other multicast group using

            AFAIK, CGMP and IGMP  snooping does not work for Local Network Control Block Frames are flooded to all ports within VLAN as broadcast traffic would do.

            Quick search suggests this discussion Regarding IGMP suppression

            • 3. Re: Multicast Curiosity

              Wikipedia actually explained it pretty well. To sum it up I would say it is because the least significant bit of the first byte. That is how the switch decided to broadcast both broadcast and multicast traffic. And that is also why you see the addresses starting with 01 the last bit in the first byte will always be a one.