IGMP Snooping

Answered Question

I have a 3750 that's currently strictly layer 2 (it does have Advanced IP but it's not doing routing at the moment), it has 4 vlans on it, 3 of which carry data and 1 just for management. I also have this switch plugged into our core with a trunk, the trunk only carried the one vlan for management....the other 3 vlans only exist on this switch.

The servers that sit on this switch do most of their communication with multicast and currently I have IGMP snooping disabled, whenever I enabled it alot of servers stop receiving multicast traffic.

What's the best way of enabling igmp snooping in that scenario? Do I have to assign the SVI's IP addresses and enable routing for igmp snooping to work properly?

I have this problem too.
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Correct Answer by Joseph W. Doherty about 8 years 6 months ago

What I suspect is happening, you probably don't have an active IGMP Querier. Normally this function is provided by multicast routers, but many manageable switches can perform this function too. The 3750 has the command "ip igmp snooping querier" for this purpose, since it's disabled by default. This command does depend on having an IP address; using an SVI IP address is one of its options. See "Configuring the IGMP Snooping Querier" within http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_44_se/configuration/guide/swigmp.html#wp1089644 for more information.

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Correct Answer
Joseph W. Doherty Mon, 06/02/2008 - 16:06

What I suspect is happening, you probably don't have an active IGMP Querier. Normally this function is provided by multicast routers, but many manageable switches can perform this function too. The 3750 has the command "ip igmp snooping querier" for this purpose, since it's disabled by default. This command does depend on having an IP address; using an SVI IP address is one of its options. See "Configuring the IGMP Snooping Querier" within http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_44_se/configuration/guide/swigmp.html#wp1089644 for more information.

Joseph W. Doherty Mon, 06/02/2008 - 17:22

No, I wouldn't think you need to enable routing and multicast routing, but as to usage of a loopback as the querier address, the documentation is a bit unclear whether the loopback might be considered the global address in "If there is no IP address configured on the VLAN interface, the IGMP snooping querier tries to use the configured global IP address for the IGMP querier."

Thinking in unicast terms, "Configure an IP address on the VLAN interface. When enabled, the IGMP snooping querier uses the IP address as the query source address." makes sense, but multicast isn't unicast, so with IGMP queries, a loopback address might work fine, but I don't have first hand experience to say so with 100% certainty.

Jon Marshall Tue, 06/03/2008 - 00:13

Joseph

Must admit my first thought was to enable an IGMP querier but i was confused as to why without it things stopped working.

If you are running IGMP snooping but not with a querier then as far as i am aware this should make no difference and the multicast traffic is just treated as broadcast traffic by the switch.

Any ideas as to why running IGMP snooping broke the server communication ?

Jon

Joseph W. Doherty Tue, 06/03/2008 - 02:28

Jon,

Without seeing or testing what's going on, I can only hazard a guess, but I suspect IGMP snooping without a IGMP querier might be behaving somewhat like a flood and prune multicast protocol that doesn't see any responses to IGMP queries and times out sending to the subnet. Although in this case, unlike multicast routing, time outs are the individual ports. In other words, the server clients announce the multicast group join, which IGMP snooping detects and opens their port for multicast, but without a periodic IGMP query and response for the IGMP snooper to detect, the snooper then believes the multicast client has silently left the multicast group (IGMPv1) and then blocks the port for that multicast.

[edit]

When I say open or block a port for multicast, of course, it's really whether the switch will replicate the multicast frame/packet for that port.

Ok so so far it seems to be working...all the servers are getting data.

However...when I do a show ip igmp snooping querier detail it shows the admin state as disabled...does that mean igmp snooping is not enabled on these vlans and multicast is still acting as broadcasts??

Here's an example (I masked the IP):

Vlan 500: IGMP switch querier status

--------------------------------------------------------

admin state : Disabled

admin version : 2

source IP address : *.*.*.*

query-interval (sec) : 60

max-response-time (sec) : 10

querier-timeout (sec) : 120

tcn query count : 2

tcn query interval (sec) : 10

operational state : Disabled

operational version : 2

tcn query pending count : 0

Joseph W. Doherty Tue, 06/03/2008 - 06:37

"However...when I do a show ip igmp snooping querier detail it shows the admin state as disabled...does that mean igmp snooping is not enabled on these vlans and multicast is still acting as broadcasts??"

Unsure. If might be because there isn't an external querier detected on that VLAN or that you're not using a VLAN IP for the querier IP. What do show ip igmp snooping and show ip igmp snooping groups display?

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