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14895 Views 10 Replies Latest reply: Sep 3, 2009 4:56 AM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

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multicast in layer2 switches

Aug 18, 2009 1:06 PM

vanessa 11 posts since
Jul 24, 2008

do i just need configure "ip igmp snooping" in the global mode to enable multicast on layer 2 switches?

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    1. Aug 18, 2009 3:34 PM (in response to vanessa)
    Re: multicast in layer2 switches

    Multicast traffic will flow whether IGMP snooping is enabled or not.  The snooping process allows the switch to more efficiently distribute multicast frame (e.g. not flooding to ports that don't need it).

     

    Depending on the switch model you are talking about, snooping may well be enabled by default.  Check the command reference guide for the switch/release you are running.  Look for the "ip igmp snooping" command and look at the DEFAULTS section to see whether on or not.

     

    HTH,

     

    Scott

  • Jared 5,498 posts since
    Jul 27, 2008

    Actually, I found that enabling ip igmp snooping alone did not give me the result I wanted.  I also had to enable ip Multicast-routing on a layer 3 device to get this to work.

     

    This is the doc that helped me to get this working in my environment.

     

    http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a008059a9df.shtml

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    3. Aug 19, 2009 11:12 AM (in response to Jared)
    Re: multicast in layer2 switches

    Well, that's true.   I suppose I assumed that multicast was configured in the L3 stuff.    Without PIM enabled on at least one L3 interface there, you won't get anyone in charge of IGMP.  No IGMP = no igmp snooping. 

     

    HTH,

     

    Scott

  • Jared 5,498 posts since
    Jul 27, 2008
    Currently Being Moderated
    5. Sep 1, 2009 8:25 AM (in response to vanessa)
    Re: multicast in layer2 switches

    Not really.  In my situation, I have several fiber links that go through out the building and have switches on each fiber link.  So all of these switches need to hook up with a querier for multicast to work correctly.  In order to get the multicast router (querier) to work, I had to enable multicast routing and enable pim on the inside interface of the vlan only.  Pim is not enabled on my outside interface, so multicasts are not routing accross the WAN, but I did have to have it on the inside (vlan) interface.

     

    But I never tried the explicit command.  You may want to try it out and see if you can see multicast sessions.

  • Jared 5,498 posts since
    Jul 27, 2008
    Currently Being Moderated
    7. Sep 2, 2009 6:43 PM (in response to vanessa)
    Re: multicast in layer2 switches

    It makes sense.  But that config is like a static route.  Now, what happens if your multicast source is not connected directly to the switch but is a couple of hops away?

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    8. Sep 2, 2009 6:51 PM (in response to Jared)
    Re: multicast in layer2 switches

    What difference would that make?  IGMP snooping is designed to make Layer2 networks more efficient.  By default, a switch will flood multicast frames out to all available (VLAN) ports.  IGMP snooping restricts that to "only the interested parties".

     

    that way if I get stuck being the only dweeb on a network that does NOT want the cool multicasts, I don't need to receive it anyway! 

     

    Multiple hops away though is a PIM forwarding function, nothing to do with IGMP.

     

    HTH,

     

    Scott

  • Jared 5,498 posts since
    Jul 27, 2008

    There is no difference, that was the point I was trying to make.  Ealier in this thread, the point was made that there has to be mulitcast routing enabled on a L3 device and have at least one interface have PIM enabled so that someone would be in charge of IGMP.  Now, if I am not mistaken, another post stated to statically configure on a switch a source of who is in charge of IGMP instead of running PIM.  That is something I had not done before and I have always enabled PIM on my internal interface to get IGMP to do what I want it to do.

     

    So, if I had a switch that was statically configured with a source of who is in charge of IGMP instead of using PIM, would not that source need to be statically configured on all Layer 2 switches in your network so they could all know who is in charge of igmp?  Maybe there is a gap in my own understanding.  I implemented a PIM config at 40 sites so that when we imaged PCs across the network, we would get a true multicast and not a broadcast.  For my situation, enabling ip multicast routing and enabling PIM were the 2 things that made it work.

     

    Maybe I could have enabled multicast routing and used the static command and got the same result?

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    10. Sep 3, 2009 4:56 AM (in response to Jared)
    Re: multicast in layer2 switches

    If it's a Layer3 switch and you want the switch to move traffic (route) then you'll need to enable PIM and multicast-routing.  If it is simply a Layer2 switch, you have nothing to do (or if it's a L3 switch participating only in L2 mode).

     

    Example:

     

    emanon-Bellerive#sh ip igmp snooping
    Global IGMP Snooping configuration:
    -------------------------------------------
    IGMP snooping                : Enabled
    IGMPv3 snooping (minimal)    : Enabled
    Report suppression           : Enabled
    TCN solicit query            : Disabled
    TCN flood query count        : 2
    Robustness variable          : 2
    Last member query count      : 2
    Last member query interval   : 1000

     

    Vlan 1:
    --------
    IGMP snooping                       : Enabled
    IGMPv2 immediate leave              : Disabled
    Multicast router learning mode      : pim-dvmrp
    CGMP interoperability mode          : IGMP_ONLY
    Robustness variable                 : 2
    Last member query count             : 2
    Last member query interval          : 1000

     

    Vlan 10:
    --------
    IGMP snooping                       : Enabled
    IGMPv2 immediate leave              : Disabled
    Multicast router learning mode      : pim-dvmrp
    CGMP interoperability mode          : IGMP_ONLY
    Robustness variable                 : 2
    Last member query count             : 2
    Last member query interval          : 1000

     

    Vlan 100:
    --------
    IGMP snooping                       : Enabled
    IGMPv2 immediate leave              : Disabled
    Multicast router learning mode      : pim-dvmrp
    CGMP interoperability mode          : IGMP_ONLY
    Robustness variable                 : 2
    Last member query count             : 2
    Last member query interval          : 1000

     

    Vlan 950:
    --------
    IGMP snooping                       : Enabled
    IGMPv2 immediate leave              : Disabled
    Multicast router learning mode      : pim-dvmrp
    CGMP interoperability mode          : IGMP_ONLY
    Robustness variable                 : 2
    Last member query count             : 2
    Last member query interval          : 1000

     

    emanon-Bellerive#sh run | in multicast|pim
    emanon-Bellerive#

     

    So I haven't done anything to my config, yet it's active for snooping.  PIM/DVMRP is the default method.  If your switch supports "older" technologies still, you can use "ip igmp snooping vlan (#) mrouter learn cgmp" as well.

     

    HTH,

     

    Scott

     

    PS.  That was a 3750 switch.

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