I have a phone on the same subnet as the CUCM MoH (Multicast enabled) server that plays the annunciator when call is placed on hold. The phones' MRG has multicast checked, the MoH server has Multicast checked, the MoH server is registered, voice media streaming service running. The MoH server is the only server I have in its entire MRGL.....If I uncheck "Allow multicast" in MRG, caller then hears MoH. If I recheck the box to enable multicast MoH, the user on hold hears ann. Again, the phones are on the same VLAN/subnet as the CUCM server so it is not a network issues- also, since I hear the ann when on hold, when multicast is enabled, that tells me CUCM realizes it cannot source multicast MoH and defaults to the ann tone. Same results if I set up the Subscriber as Multicast MoH. Also, since uncheck/checking "Allow Multicast" in MRG turns MoH on/off, I know my music file is good. I've reviewed the enterprise and services parameters for CallManager and Voice Media Streaming, but couldn't find any fields that I thought could cause it......
- New install - CUCM 7.1(3)-2000
Actually in most modern switches (post 2002 deployments), IGMP snooping is on by default. When IGMP snooping is enabled, multicast is NOT treated as broadcast. When snooping is enabled you need a querier on that L2 VLAN. A router interface can be a querier by turning on PIM or on a Cisco switch you can configure the switch itself to be a querier for a L2 VLAN. So you have 3 options:
1) Turn on PIM on the L3 interface
2) Turn off IGMP snooping on that VLAN
3) Enable a querier on that VLAN
If it is isolated to a single VLAN, I would use option 3. Older switches like Cat4k's with Sup2's don't even support snooping so IGMP is off by default on those devices - in those cases, you are exactly right. But if you have newer switches and haven't done of the 3 options above, multicast will not work.
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