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Multicast traffic being blocked on voice vlan

Hi guys,

We have had an issue reported by our users that their tannoy feature has stopped working. According to the phone system vendor, this feature requires multicast to be enabled.

Over the weekend that it stopped working, we had a lot of work going on and the switch (2960S and 3560) configs were rebuilt from the ground up.

I've included the important info that I can think of below.

Each access switch port now has the following config applied:

switchport mode access
switchport nonegotiate
switchport voice vlan 300
switchport port-security maximum 1 vlan access
switchport port-security maximum 1 vlan voice
switchport port-security aging time 2
switchport port-security violation restrict
switchport port-security aging type inactivity
srr-queue bandwidth share 10 10 60 20
queue-set 2
priority-queue out
mls qos trust device cisco-phone
mls qos trust cos
auto qos voip cisco-phone
storm-control broadcast level 10.00
storm-control action shutdown
spanning-tree guard root
service-policy input AutoQoS-Police-CiscoPhone

The voice vlan across the whole site is 300 and the phone system is on an access port in this VLAN.

Our central switches have an interface on this vlan, but there should be no traffic reaching this.

The interface config for the VLAN is the same as before the re-config of the switches.

What could be blocking multicast traffic?

 

Thanks.

1 Reply 1

Jon Marshall
Hall of Fame
Hall of Fame

It would be helpful if you had the original configurations :-)

The issue may be that by default IGMP snooping is enabled on all vlans.

But for it to work properly you need something to make the queries ie. -

1) PIM enabled on the L3 interface (SVI) for that vlan. PIM would normally be enabled if you wanted to route the multicast traffic which from your descrption I'm assuming you don't

or

2) most modern switches support an IGMP querier function which means you don't have to enable PIM on the L3 SVI

This may be your best solution.

The alternative is to disable IGMP snooping for that vlan in which case the switch treats the multicasts as broadcasts.

See this link for more details of all the above -

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/68131-cat-multicast-prob.html

if you decide to use the IGMP snooping querier function the relevant configuration guide for your switch/IOS version will have the details.

Jon

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