LAN, Switching and Routing: Multicast advice needed on Cat 6513 IOS12.2SXF5

Discussion in 'Cisco' started by Hoffa, Feb 7, 2008.

  1. Hoffa

    Hoffa Guest

    Hi folks

    I have a vlan interface configured with ip igmp snooping querier, this
    interface serves a vlan hosting alot of multicast senders and
    receivers. No multicast routing between vlans is needed at his time.
    I'm starting to get complaints from our developers that some
    applications listening on multicast channels loose their traffic from
    time to time. What I've gathered so far is that some applications on
    some servers loose their channel connectivity while the same
    application on another server still works. It seems the servers just
    drops from the multicast channels.

    Multicast is not my best area of knowledge and I have neither full
    insight into the applications having these problems. What I understand
    so far from reading the IOS manuals igmp snooping querier is a
    "passive" method of connecting a multicast sender and listener without
    reporting much back to the servers/applications. When I present this
    theory to the developers they request that I set up some sort of
    "active" multicast control on the 6513, something that checks back to
    the applications or at least is better at keeping the traffic flowing.
    A show ip igmp int vlan XX reads IGMP disabled on interface but IGMP
    snooping globally enabled. Would enabling IGMP on the vlan interface
    solve the problem?
    Have I got it all wrong or is there something I can do to improve the
    multicast reliability?

    Fredrik Hofgren
    Hoffa, Feb 7, 2008
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  2. Hoffa

    Thrill5 Guest

    IGMP is used in two different contexts on a 6500. One for layer 3 (multicast
    routing) and another context for layer 2 (multicast switching)
    IGMP is enabled by default for layer 2, and is enabled for layer 3 by
    turning on "ip multicast-routing".

    IGMP is a protocol used by multicast sources and receivers to signal that
    the are the source, or want to receive a specific multicast group. A
    multicast source, will periodically send an "IGMP source" message. To
    receive a specific multicast stream, a receiver will send an "IGMP join"

    In layer 3, the IGMP message is processed by the router to arrange for the
    multicast source to be delivered to it.

    In layer 2, IGMP messages are "snooped" passively by the switch to determine
    which ports need to receive a multicast stream If IGMP were not enabled,
    then the multicast traffic received by the switch would be transmitted out
    EVERY port, just like a broadcast.

    I **think** that you can enable "ip multicast-routing" on the switch with
    really turning on multicast. IP multicast routing is done using PIM
    (Protocol Independent Multicast). While ip multicast-routing will enable
    multicast support on the switch PIM must be enable on each layer 3 interface
    using an "ip pim <mode>" command. If an "ip pim" command is not enabled on
    the interface, it will not route multicast traffic, which is what you want
    in this case. This will enable layer 3 IGMP on the layer 3 vlans and give
    you some visibility into what is going on with IGMP.

    You should also read the RFCs dealing with IGMP, and read up on PIM and
    multicast from Cisco. Cisco has a multicast resources page here.
    Thrill5, Feb 8, 2008
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  3. Hoffa

    sreemakam Guest

    Hi Fredrik
    "igmp snooping querier" is used in cases where L3 multicast routing is
    not necessary. In your case, since you are not doing L3 multicast
    routing, "igmp snooping querier" should suffice. I do not understand
    why multicast channels would lose their channels from time to time
    using "igmp snooping querier".
    As you mentioned, the alternative approach to "igmp snooping querier"
    is enabling PIM on the vlan.
    sreemakam, Feb 15, 2008
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