Ethernet multicast query

Posted on 2011-10-05
Last Modified: 2012-05-12
Just a quick question guys, maybe somebody can answer...

i know that the idea of multicast versus broadcast is to reduce the traffic that is sent over the LAN. If for example, my router sends out an EIGRP message to layer 3 multicast address, in theory this should be processed by other hosts on the LAN which are listening on that layer 3 address, that's fine, i get that.

i was just wondering how the corresponding layer 2 behaviour is dealt with? i assume there would be a corresponding layer 2 Ethernet address, and if so, how would the LAN switches forward the frame? i know a broadcast to ffff.ffff.ffff would forward out of all ports. does the frames with multicast destination addresses do the same, or just forward out of specific ports?

i can't seem to find the answer in my google searches.

thanks in advance.
Question by:L-Plate
    LVL 3

    Accepted Solution


    Generally switches will treat multicast as broadcast and forward the frame to all ports. Managed switches should support multicast usually via:
    Manual configuration in the switch by an administrator defining the ports much like VLAN's.
    Dymanic configuration where the switch is able to use IGMP snooping to configure port filters.
    LVL 50

    Assisted Solution

    by:Don Johnston
    The answer is both.

    By default, switches flood multicast just like broadcasts.

    However, some switches also support IGMP snooping. In which case the switch will watch IGMP join messages and will then do a selective flood where the multicast frames are only sent out ports of hosts that have issues IGMP join messages.

    Author Comment

    thank you both.

    actually i do now remember reading about IGMP snooping in some Cisco book once, and this has brought the conept back to me.

    thanks again,


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