Multicast vs. Broadcast frames

Discussion in 'Cisco' started by Yaron, Nov 21, 2005.

  1. Yaron

    Yaron Guest

    I guess this one sounds a little foolish, but still: I would like to
    understand the way devices treat multicast and broadcast frames on a

    Let's assume a segment with 2 routers and 2 PCs.
    Now let's assume that RIPv1 is enabled. Since it broadcasts its routing
    table, all the rest of the devices on the segment will receive these
    In the case of RIPv2, multicast is being used.
    Since the hub (or even a switch that floods multicast) will transmit the
    information to the rest of the segment, all the devices, PCs and
    routers, with receive these fames too.
    So in both cases, all the devices receive these frames.
    What's the difference, what am I missing here?

    Thanks, Yaron
    Yaron, Nov 21, 2005
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  2. Yaron

    stephen Guest

    RIP2 can run "compatibility mode" where it uses broadcasts rather than
    adevice with a reasonable interface chip can program the chip with the set
    of multicast MAC addresses it is interested in, and ignore the rest in

    All broadcasts traditionally need the CPU to examine each packet to decide
    whether to ignore it.

    some layer 2 switches can use multicast control to limit where the packets
    go within a subnet, reducing the average bandwidth overhead per port.

    doesnt matter for a few packet / sec for RIPv2, but once you start sending
    multiple 5 Mbps MPEG2 video on multicast it gets more important......
    stephen, Nov 21, 2005
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  3. Yaron

    V. Evans Guest

    Yaron -

    On a single segment like you mention, there isn't really that much of a
    difference - other than the address that a packet is multicast to or
    broadcast to (multicast will always be in the range of - However, your PC's will typically have to have an
    application that uses multicast for them to respond / receive those
    packets. Something like the Nortel Symposium product, which allows
    certain users to view real-time data on PBX call routing.

    When you get to multiple segments or even multiple subnets, then you
    get into the real guts of multicast and how it works. Because then,
    communication between nodes and network devices allows a multicast
    server application (such as an Anti-virus app) to send feeds to only
    the hosts who need them. A multicast group can exist with a single
    multicast host on every subnet in a large vlan environment and each of
    those hosts (and only those multicast hosts) will receive the server's

    This is a quick and dirty high-level overview, but I hope it helps. The
    Cisco Press books "Routing TCP/IP" volume 1 & 2 are good books with a
    large section on multicast.
    V. Evans, Nov 21, 2005
  4. Yaron

    Yaron Guest

    Thanks for your answer, much appriciated.

    V. Evans:
    Yaron, Nov 21, 2005
  5. Yaron

    Yaron Guest

    Thank you stephen, great response.

    Yaron, Nov 21, 2005
  6. Yaron

    suny79 Guest

    CGMP or IGMP snooping on Catalyst Switches.
    suny79, Nov 21, 2005
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