Solved: Multicasting in same Vlan broken

glenthms · ‎10-10-2011

Currently we do not have multicat routing enabled on either of our 2x Sup720-10G 6509 Switches. We have no intention of doing so either. I am looking for clarification on how mutlicasting operates within a Vlan.

IOS: s72033-ipservicesk9_wan-mz.122-33.SXH2a.bin

Our Vlan has IGMP snooping enabled. Here is the output.

6509Switch#sh ip igmp inter vlan 25

Vlan25 is up, line protocol is up

Internet address is 10.1.1.254/24

IGMP is disabled on interface

Multicast routing is disabled on interface

Multicast TTL threshold is 0

No multicast groups joined by this system

IGMP snooping is globally enabled

IGMP snooping CGMP-AutoDetect is globally enabled

IGMP snooping is enabled on this interface

IGMP snooping fast-leave (for v2) is disabled and querier is disabled

IGMP snooping explicit-tracking is enabled

IGMP snooping last member query response interval is 1000 ms

IGMP snooping report-suppression is enabled

I am getting reports from our app engineers that multicasting in this Vlan does not work 100% as follows.

Using 238.0.0.50 and a ttl of 1, it works consistently

Using 232.10.10.10 and a ttl of 1, it doesn't work at all

Using 238.5.5.55 and a ttl of 1, it worked a single time, then never again

I am looking to understand what I should be looking for. I will be enabling 'ip igmp snooping querier' on this vlan later this week, however as I understand it. This command simply will relay the igmp join messages from switch to switch due to the lack of mrouter in our environment.

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

So, what can we look at to help troubleshoot this issue? Looking for general suggestions or comments or even an idea of what commands would help locate the issue here. Thanks!

Also to note, we do not want to disable IGMP due to this one concern that I can't find documented.

If we disable IGMP snooping on the vlan, what happens to multicast packets with a TTL > 1. Do they get flooded to the entire switch and all Vlans? Or does it stay within the Vlan?

Jon Marshall · ‎10-10-2011

If you disable IGMP snooping mutlicast packets with TTL > 1 do not get flooded to other vlans but they will be flooded to all ports within that vlan. You would need to enable multicast routing to get across vlans.

However as far as IGMP snooping is concerned, it is a passive thing. IGMP snooping listens to responses from IGMP queries. If there is nothing to make the queries then IGMP snooping is ineffective as it has nothing to listen to.

If you have not enabled PIM on the vlan interface and you have not statically configured which port(s) should receive the multicast packets then the packets should be flooded to all ports within that vlan even with IGMP snooping configured.

You need to configure the IGMP snooping querier so that IGMP snooping has something to listen to.

Jon

View solution in original post

Peter Paluch · ‎10-10-2011

Hello Glen,

Using 238.0.0.50 and a ttl of 1, it works consistently

Using 232.10.10.10 and a ttl of 1, it doesn't work at all

Using 238.5.5.55 and a ttl of 1, it worked a single time, then never again

Your switch is performing IGMP Snooping. The 238.0.0.50 works because it maps to the multicast MAC address 01:00:5e:00:00:32, and the entire range of MAC addresses 01:00:5e:00:00:xx is exempted from IGMP Snooping restrictions on Catalyst switches as it corresponds to the link-local multicast scope 224.0.0.x. You should avoid using such addresses in your multicast applications, as these streams will be flooded to all stations in the VLAN, negating the optimization provided by the IGMP Snopping. The 232.10.10.10 and 238.5.5.55 do not work or work only in an intermittent manner because they are subject to IGMP Snooping. Without a device sending IGMP Queries (a router or a switch using the local querier feature), the stations will send the IGMP Membership Report only once, when they initially join the group. These IGMP messages will be intercepted by the switch but without a querier eliciting these messages over and over again, they will soon expire and not get renewed, which is the most probable reason why the 238.5.5.55 worked a single time only.

Regarding the IGMP querier function on the switch, it is not about relaying the IGMP messages but rather about originating the IGMP Membership Queries on the switch itself. As the IGMP Snooping relies on processing the IGMP Membership Report / Leave messages, they must be elicited from the group members using the IGMP Membership Query messages themselves. If you want to keep the IGMP Snooping, which you definitely should, and if you have no multicast-enabled router in your VLAN, you must activate the IGMP querier function.

Without the IGMP Snooping running, multicasts will be flooded to all ports in the same VLAN, however, they will not be forwarded to another VLAN, regardless of the TTL.

Best regards,

Peter

View solution in original post

Jon Marshall · ‎10-10-2011

If you disable IGMP snooping mutlicast packets with TTL > 1 do not get flooded to other vlans but they will be flooded to all ports within that vlan. You would need to enable multicast routing to get across vlans.

However as far as IGMP snooping is concerned, it is a passive thing. IGMP snooping listens to responses from IGMP queries. If there is nothing to make the queries then IGMP snooping is ineffective as it has nothing to listen to.

If you have not enabled PIM on the vlan interface and you have not statically configured which port(s) should receive the multicast packets then the packets should be flooded to all ports within that vlan even with IGMP snooping configured.

You need to configure the IGMP snooping querier so that IGMP snooping has something to listen to.

Jon

Peter Paluch · ‎10-10-2011

Hello Glen,

Using 238.0.0.50 and a ttl of 1, it works consistently

Using 232.10.10.10 and a ttl of 1, it doesn't work at all

Using 238.5.5.55 and a ttl of 1, it worked a single time, then never again

Your switch is performing IGMP Snooping. The 238.0.0.50 works because it maps to the multicast MAC address 01:00:5e:00:00:32, and the entire range of MAC addresses 01:00:5e:00:00:xx is exempted from IGMP Snooping restrictions on Catalyst switches as it corresponds to the link-local multicast scope 224.0.0.x. You should avoid using such addresses in your multicast applications, as these streams will be flooded to all stations in the VLAN, negating the optimization provided by the IGMP Snopping. The 232.10.10.10 and 238.5.5.55 do not work or work only in an intermittent manner because they are subject to IGMP Snooping. Without a device sending IGMP Queries (a router or a switch using the local querier feature), the stations will send the IGMP Membership Report only once, when they initially join the group. These IGMP messages will be intercepted by the switch but without a querier eliciting these messages over and over again, they will soon expire and not get renewed, which is the most probable reason why the 238.5.5.55 worked a single time only.

Regarding the IGMP querier function on the switch, it is not about relaying the IGMP messages but rather about originating the IGMP Membership Queries on the switch itself. As the IGMP Snooping relies on processing the IGMP Membership Report / Leave messages, they must be elicited from the group members using the IGMP Membership Query messages themselves. If you want to keep the IGMP Snooping, which you definitely should, and if you have no multicast-enabled router in your VLAN, you must activate the IGMP querier function.

Without the IGMP Snooping running, multicasts will be flooded to all ports in the same VLAN, however, they will not be forwarded to another VLAN, regardless of the TTL.

Best regards,

Peter