CCIE Voice Study

Since multicast is used for the streaming of MoH locally (for on-net calls) from the Cisco CME device, we can use both telephony troubleshooting and IP multicast troubleshooting commands to verify and troubleshoot MoH. Firstly, observe the behaviour of multicast group 239.0.0.1 below.

The following output of "show ip mroute" is from a steady-state - there are no calls currently on hold. Observe that group 239.0.0.1 specifies Gig0/0 as an outgoing interface, however traffic is pruned, and will therefore not be sent until an IGMP request is received downstream.

CME_ROUTER#sh ip mroute
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,

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The multicast MoH command under telephony service can be a cause for confusion. This command forces the Cisco CME device to generate multicast Music on Hold on the interfaces specified after the "route" argument.

The route argument is only required if the SCCP address bound under telephony service is different to the interface outbound towards the IP phones. This would be common, for example, if a loopback interface is used instead of the physical Ethernet interface.

When the outbound interface is also the SCCP source IP address:

telephony-service
  multicast moh 239.0.0.1 port 16834

When a loopback interface is used as the SCCP source IP address (for example, L0 = 172.16.1.1 and GigabitEthernet0/0 = 192.168.1.1):

telephony-service

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In CME, by default when 2 x on-net callers dial one another and one party initiates a hold, the party holding will not receive music on hold. This can be resolved by using Multicast Music on Hold. When using Multicast MoH, the CME router will transmit the audio stream on the physical IP interfaces of the specified router. Under the multicast command (in telephony-service), if the SCCP bound address under telephony-service is not the physical interface (for example, if a loopback is used), the outbound interface towards the IP phones must be specified. If they are not on the next directed subnet, multicast routing must be enabled and PIM dense or sparse mode.

telephony-services
  moh music-on-hold.au
  multicast moh 239.1.1.10 port 16384 ...

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