IGMP Snooping for same VLAN Traffic

[jon_panes24]

Hi,

If the source and the receiver of the multicast traffic is on the same VLAN, would IGMP snooping work?

Lets say there is no active L3 SVI interface on the switch and the traffic is just flowing between 2 hosts on the same VLAN.

Thanks!

[Peter Paluch]

Hi Jon,

The IGMP Snooping functionality really depends on the presence of an multicast-enabled router sending out IGMP Query packets in the VLAN where the receivers are located. It is not that important whether the source of the multicast is in the same VLAN, or a different VLAN; what is important is to have a device that performs the function of an IGMP Querier.

Assuming that you have a very simple multicast deployment without any multicast-enabled routers, and just perform a simple multicast streaming in a single VLAN, then the IGMP Snooping would not work because there is no router present that would be sending out IGMP Membership Query packets. Without them, the hosts will not periodically respond, and IGMP Snooping will not be able to maintain an up-to-date table of multicast receivers.

For these deployments, Cisco switches usually support a feature called IGMP Snooping Querier; this feature allows the switch itself to send out IGMP Membership Query packets if there are no other multicast-enabled routers present.

Check out this guide for more context:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_55_se/configuration/guide/scg_2960/swigmp.html#pgfId-1193337

Feel welcome to ask further!

Best regards,
Peter

[Joseph W. Doherty]

I haven't read Peter's reference, but in case it's not noted, or not clear, if you have multiple switches, you only need one querier for a VLAN (just as there only needs to be one multicast router interface). Additionally, if you don't have multicast querier, on an active IGMP snooping switch, it would suppress sending multicast to any ports (as it would appear no hosts want it). So, if you don't have a querier, insure IGMP snooping is disabled.

[jon_panes24]

Hi Peter,

Thanks for the quick response.

If i understand it right, there has to be a L3 device for IGMP, it will be the one to check if hosts wants to receive mcast traffic. 

I have a diagram attached. If the host connected on SW2 wants to receive Mcast traffic would it receive it?

Lets say on a scenario that there is the router and where the router is removed.

Thanks!

Jon

[Joseph W. Doherty · Accepted solution]

"If i understand it right, there has to be a L3 device for IGMP, it will be the one to check if hosts wants to receive mcast traffic. "

As Peter explained, if IGMP snooping in active, you need a querier. Normally that is a L3 device, but many switches that support IGMP snooping can perform this role too. I.e. a querier is needed, but it doesn't have to be on a router or L3 switch.

For your diagram, if IGMP snooping is enabled on both switches and there's a querier (your router), yes hosts on either switch should obtain the multicast stream. Again if IGMP snooping is enabled but now the querier is not present (your router), hosts on either switch should not obtain the multicast stream.

Lastly, if your switches do not have IGMP snooping enabled, all hosts will receive any multicast stream regardless of whether there's a querier or not, this assuming the multicast stream is placed on those switches. (I.e. if the multicast stream was being placed on the switches by the router, it would use its querier to determine if a multicast stream is desired by any host. If not, if would not forward the multicast stream to the switches.)

[jon_panes24]

Hi Joseph,

Thanks for the quick response.

" Again if IGMP snooping is enabled but now the querier is not present (your router), hosts on either switch should not obtain the multicast stream. "

Is this behavior platform specific? Im currently testing GNS3 using IOU L2 (i86bi-linux-l2-adventerprisek9-15.1a.bin). Though im not particularly sure whether it supported but the commands are there. 

I've attached a diagram. IGMP snooping is enabled on the transit switches, IOU4 is configured with "ip igmp join-group 239.1.1.1". On IOU3 im sending a multicast ping "ping 239.1.1.1", what im seeing is that the multicast traffic reaches the receiver. Im not sure whether this is a IOU limitation. I currently dont have real gear to test it on. In this scenario multicast is like broadcast for a VLAN, it reaches all configured ports for the same vlan.

[Joseph W. Doherty]

I'm unfamiliar with your GNS3 setup, so unable to comment on how well it represents physical equipment.

As to "Is this behavior platform specific?", I would say that's always possible, including bumping into "bugs".

[Peter Paluch · Accepted solution]

Hi Jon,

I am not sure how closely the IOU/IOL images approximate the hardware behavior.

However, you might find this information interesting: On IOS-based Catalyst switches, IGMP Snooping automatically deactivates itself if there is no IGMP Querier present in the VLAN. As a result, the Catalysts will simply flood all multicasts as if they were broadcasts. If an IGMP Querier later comes into the VLAN, either as a standalone router, or some switch configured for this functionality using the IGMP Snooping Querier feature, IGMP Snooping reactivate itself and will kick into action.

This is in stark contrast to Nexus switches. On them, IGMP Snooping does not deactivate itself automatically in the absence of an IGMP Querier. As a result, without a Querier, your multicast won't work at all.

So in your particular case - I would not rely too much on the behavior of the IOU image since that one only approximates how the hardware operates and might not get everything with 100% accuracy, but if you had a true Catalyst-based network without an IGMP Querier, you would actually see the multicasts truly leaking out through every port in the given VLAN.

Best regards,
Peter

[Joseph W. Doherty]

"On IOS-based Catalyst switches, IGMP Snooping automatically deactivates itself if there is no IGMP Querier present in the VLAN."

Ah, didn't know that.

[jon_panes24]

"On IOS-based Catalyst switches, IGMP Snooping automatically deactivates itself if there is no IGMP Querier present in the VLAN."


Thanks Peter, thats probably why the multicast is flying everywhere in my setup. Do you have a Cisco documentation regarding this behavior?