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

    multicast in layer2 switches

    vanessa

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

        • 1. Re: multicast in layer2 switches
          Scott Morris - CCDE/4xCCIE/2xJNCIE

          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

          • 2. Re: multicast in layer2 switches
            Jared

            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

            • 3. Re: multicast in layer2 switches
              Scott Morris - CCDE/4xCCIE/2xJNCIE

              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

              • 4. Re: multicast in layer2 switches
                vanessa

                it seems that if the multicast only within one vlan, no ip pim need to be configured...is it right?  as long as the "ip igmp snooping querier address x.x.x.x" is configured.

                • 5. Re: multicast in layer2 switches
                  Jared

                  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.

                  • 6. Re: multicast in layer2 switches
                    vanessa

                    thanks.  I missed one sentence...if the "ip igmp querier address x.x.x.x" is configured on one of the switches that connect to the multicast source, it seems work if the client is on another switch that just in the same vlan as the source.  There is no other configures in other switches, just the default "ip igmp snooping".

                    • 7. Re: multicast in layer2 switches
                      Jared

                      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?

                      • 8. Re: multicast in layer2 switches
                        Scott Morris - CCDE/4xCCIE/2xJNCIE

                        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

                        • 9. Re: multicast in layer2 switches
                          Jared

                          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?

                          • 10. Re: multicast in layer2 switches
                            Scott Morris - CCDE/4xCCIE/2xJNCIE

                            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.