IGMP, Globally or Locally?

Unanswered Question
Dec 7th, 2009

All, is IGMP Global or Locally?  The reason I ask, we are looking at an cisco ie 3000-8tc as a layer 2 switch attached to our 3750 stack.  Can the 3000 do snooping within the swtich itself, or must the 3750 stack have IGMP Snooping enabled too?

I have this problem too.
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Jon Marshall Mon, 12/07/2009 - 06:37

rwamstutz wrote:

All, is IGMP Global or Locally?  The reason I ask, we are looking at an cisco ie 3000-8tc as a layer 2 switch attached to our 3750 stack.  Can the 3000 do snooping within the swtich itself, or must the 3750 stack have IGMP Snooping enabled too?

Not sure what you mean by local or global. IGMP is enabled on any interface that runs PIM, nothing to do with L2 switches. However if you are referring to IGMP snooping which is the ability of a L2 switch to listen to IGMP queries and respones then you need to enable it on all L2 switches that are forwarding multicast packets.

Jon

rwamstutz Mon, 12/07/2009 - 06:40

what I mean by globally or locally....if I want to enable IGMP snooping on the IE 3000 to control/watch multi

-cast, do I also have to enable it on any L2/L3 3750 Switches within the network?  Can IGMP Snooping operate alone within the IE-3000

and not interfere with the 3750s in the same LAN?

Jon Marshall Mon, 12/07/2009 - 06:43

rwamstutz wrote:

what I mean by globally or locally....if I want to enable IGMP snooping on the IE 3000 to control/watch multi

-cast, do I also have to enable it on any L2/L3 3750 Switches within the network?  Can IGMP Snooping operate alone within the IE-3000

and not interfere with the 3750s in the same LAN?

You don't have to enable it across all switches for it to work on a particular switch but be aware that only on the 3000 will multicast traffic actually only be sent to devices that requested it. On the other switches mutlicast traffic within that vlan will be treated as broadcast traffic ie. flooded to all ports other than the one it was received on.

Jon

rwamstutz Mon, 12/07/2009 - 06:48

what do you mean. "be aware that only on the 3000 will multicast traffic actually only be sent to devices that requested it. "  do you  mean to those on that switch?

Is the scenerio I presented, recommened?  I dont want to enable IGMP Snooping on my 3750 Stack at this time.  The VLAN on the 3000 is also a vlan on my 3750 stack.

Jon Marshall Mon, 12/07/2009 - 06:57

rwamstutz wrote:

what do you mean. "be aware that only on the 3000 will multicast traffic actually only be sent to devices that requested it. "  do you  mean to those on that switch?

Is the scenerio I presented, recommened?  I dont want to enable IGMP Snooping on my 3750 Stack at this time.  The VLAN on the 3000 is also a vlan on my 3750 stack.

Without IGMP snooping or the older CGMP (not used on newer switches) a switch when it receives a multicast packet has to treat the packet as a broadcast packet because it does not know which of it's switchports want to receive the multicast traffic.

If you enable IGMP snooping on a switch then the switch can snoop on the IGMP conversation between a host and a router and it records the relevant switchport that the host is on. So now the switch knows where to send the multicast packet and it doesn't need to flood it to all ports.

If you are running multicast within a vlan and that vlan exists on both the 3000 and the 3750 switches then you need to enable IGMP snooping on both switches. If you only enable it for the 3000 then it will still "work" but on the 3000 only devices that requested the traffic will be sent it whereas on the 3750 all ports within that vlan will get a copy of the multicast traffic.

Jon

rwamstutz Mon, 12/07/2009 - 07:06

So if I enable IGMP snooping on the Ie-3000 for VLAN 199, but do not enable IGMP Snooping on the 3750 for vlan 199, only the device on the 3000 will get the snooping information, however the device on the 3750 without IGMP Snooping, will still continue to get any Multi-cast traffic coming off the 3000?

Jon Marshall Mon, 12/07/2009 - 07:16

rwamstutz wrote:

So if I enable IGMP snooping on the Ie-3000 for VLAN 199, but do not enable IGMP Snooping on the 3750 for vlan 199, only the device on the 3000 will get the snooping information, however the device on the 3750 without IGMP Snooping, will still continue to get any Multi-cast traffic coming off the 3000?

Ahhh, I see what you are saying now. Apologies, i was being a bit slow.

The answer is it depends on where the source is. If the source is on the 3000 then yes it could be a problem because the 3000 will not forward the multicast traffic to the 3750. If the source is on the 3750 then no it should not be a problem because the 3750 will flood the packets to all ports within vlan 199 which will include the port connecting to the 3000 and then the 3000 will simply forward to the ports that have registered an interest.

Note that i have been assuming that somewhere there is an IGMP querier ie. is PIM configured under the L3 interface for vlan 199 ? If not then you would need to see if the 3000 supported the IGMP snooping querier function.

If the 3000 doesn't support this or you haven't configured it, and you have not confiigured PIM under vlan 199 then all multicast packets on both switches will be treated as broadcasts.

Jon

rwamstutz Mon, 12/07/2009 - 07:51

so If I want to restrict multi-cast traffic from leaving the 3000, do I enable snooping on the 3000 and not the 3750?  do I enable it on the 3750 so that when muli-casts are send from the 3000, they dont hit the 3750?  The goal is to control the multi-cast traffic generated on ports on the 3000 on vlan 199, so they dont affect vlan 199 ports on the 3750.

Jon Marshall Mon, 12/07/2009 - 10:25

rwamstutz wrote:

so If I want to restrict multi-cast traffic from leaving the 3000, do I enable snooping on the 3000 and not the 3750?  do I enable it on the 3750 so that when muli-casts are send from the 3000, they dont hit the 3750?  The goal is to control the multi-cast traffic generated on ports on the 3000 on vlan 199, so they dont affect vlan 199 ports on the 3750.

Which switch is the multicast source attached to ?

Jon

rwamstutz Mon, 12/07/2009 - 10:37

the IE 3000 switch would be the switch that the multi-cast source is at....sending information to other

multi-cast devices in that switch, BUT sending to a specific IP on the 3750

Jon Marshall Mon, 12/07/2009 - 11:02

rwamstutz wrote:

the IE 3000 switch would be the switch that the multi-cast source is at....sending information to other

multi-cast devices in that switch, BUT sending to a specific IP on the 3750

Sorry not sure whaty you mean. You don't send multicast traffic to a specific IP because the destination is always the multicast group address.

Also what device is doing the IGMP queries ie. have you

1) enabled IGMP querier function on L2 switch

OR

2) enabled PIM on the L3 interface for vlan 199

Jon

rwamstutz Mon, 12/07/2009 - 11:05

The IGMP quierier function will be on the L2 (IE 3000 Switch).  If I want to contain the multi-cast traffic generated on vlan 199 of the IE 3000 switch from showing up on vlan 199 of the 3750 Switch without IGMP Snooping enabled..  What must I do?

Jon Marshall Mon, 12/07/2009 - 11:31

rwamstutz wrote:

The IGMP quierier function will be on the L2 (IE 3000 Switch).  If I want to contain the multi-cast traffic generated on vlan 199 of the IE 3000 switch from showing up on vlan 199 of the 3750 Switch without IGMP Snooping enabled..  What must I do?

If the 3000 is sending out IGMP queries then any device on the 3750 that wants to listen to the traffic will regsiter and then traffic will be forwarded down the link.

If you want to stop multicast traffic within the vlan then you should use a vlan access-list to deny any multicast packets for that group address crossing the interconnect.

Jon

rwamstutz Mon, 12/07/2009 - 12:12

but if i dont have IGMP Snooping enabled on the 3750, can devices on the 3750 register to the 3000?  if not, then does multi-cast traffic leave the 3000?

Jon Marshall Mon, 12/07/2009 - 12:39

rwamstutz wrote:

but if i dont have IGMP Snooping enabled on the 3750, can devices on the 3750 register to the 3000?  if not, then does multi-cast traffic leave the 3000?

Your'e mixing up IGMP snooping with IGMP. If the 3000 switch is an IGMP querier then it will send out IGMP queries within vlan 199. This means the query will also go to the 3750. Now it doesn't matter whether the 3750 has IGMP snooping configured or not because if a host attached to the 3750 answers the IGMP query to say it is interested then the 3000 will register the interconnect port as a port to send the multicast traffic out on. Being able to register with IGMP has nothing to do with whether or not IGMP snooping is configured.

Edit - if you don't have any devices connected to the 3750 that are interested in the traffic then no traffic will not be sent to the 3750 for that vlan.

Jon

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