How to enable VM Multicast Traffic across multiple hosts on Cisco UCS

Recently I was asked to investigate an issue occurring on some VM’s within the environment I look after. The sysadmin informed me that he was unable to receive multicast traffic on VM’s in the same VLAN when they were separated on different hosts in the cluster. However, when the VM’s were on the same host, traffic flowed fine!

After doing some reading I found that the issue is with the UCS Fabric Interconnects. Here is what was needed to fix the problem.

Configure an IGMP querier on Nexus 5K

1. SSH into the first Nexus 5k switch.

2. Backup the configuration with.

3. Check the IGMP configuration on the VLAN that requires Multicast traffic.

IGMP0

4. As no querier is present, create one.

IGMP1

5. Check the configuration is now correct.

IGMP2


6. Repeat the above steps 1-5 on the second 5K switch.

Disable IGMP snooping on Nexus 1000v

1. SSH into your Nexus 1000v.

2. Backup the configuration as above.

3. Check the current IGMP snooping status, which is “Enabled” by default.

IGMP3

4. Disable IGMP snooping on the 1000v.

IGMP4

5. Verify that IGMP snooping has been disabled.

IGMP6

Conclusion

By default, VM’s in the same VLAN will only talk multicast traffic to each other if they reside on the same host. This is due to the way in which the UCS Fabric Interconnects handles multicast traffic and the fact that they only store MAC address information from the blade servers connected to them directly.

The work around is to is to enable an IGMP querier which will be used to facilitate multicast traffic on the upstream switches, for the specific VLAN in question.

I’m not entirely sure disabling IGMP snooping on the 1000v as a whole is a good idea, it might be best to do it on a per VLAN level, where it is required.

One comment

  1. user2345

    Hello,

    I have two questions:

    (1) Can you please advise what should be the IP for IGMP querier (i know this would depend on environment but what can qualify as IGMP querier address) in below command:

    IP igmp snooping querier x.x.x.x

    (2) In your above example, is Nexus 5000 connected with Upstream Switch: 1000 and that is the reason you disabled this on upstream switch ?

    Your help is highly appreciated.

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