6.6 Multicast MOH from Branch Router Flash Implementation

This topic describes how to implement multicast MOH from the branch router flash.

Characteristics:

  • Works only with multicast MOH.
  • Based on MOH capabilities of SRST (Cisco IOS router requires feature set that supports SRST).
  • Cisco Unified Communications Manager must be configured to use multicast MOH.
  • IP phone is not aware that it listens to locally generated MOH.
  • Stream generated by MOH server is prevented from reaching the IP WAN.
  • Identical stream is generated locally at the branch sites.
  • Branch router can stream up to six MOH files.
    1. Only G.711 is supported.
    2. Each stream can be selectively enabled for multicast.

Multicast MOH from branch router flash is a feature that allows multicast MOH streams to be generated by gateways that are located at remote sites instead of being streamed from the main site to the remote site over the IP WAN.

Cisco Unified Communications Manager is configured for standard multicast MOH. Neither Cisco Unified Communications Manager nor the phones that are located at the remote site are aware that the stream generated at the central site is replaced by a locally generated stream. The multicast MOH stream that is generated by the centrally located MOH server is prevented from traversing over IP WAN, and the remote site router generates a stream that has the same attributes (codec, multicast address, and port).

Multicast MOH from branch router flash is based on multicast MOH, so you must configure Cisco Unified Communications Manager to use multicast MOH instead of unicast MOH. This configuration is recommended anyway to reduce the load at the MOH server by multicasting one stream that can be received by all devices, instead of streaming MOH individually for each endpoint in separate RTP sessions.

To generate a multicast MOH stream at the remote site, you use features of SRST or Cisco Unified Communications Manager Express. Therefore, the remote site router that generates the multicast MOH stream for the devices that are located at the remote site must be configured for SRST or Cisco Unified Communications Manager Express. SRST does not have to be active (there is no need for a fallback scenario), because an SRST gateway that is configured for multicast MOH streams MOH all the time, regardless of its state (standby mode or SRST mode). The same principle applies to Cisco Unified Communications Manager Express: Only multicast MOH must be enabled. No additional features have to be enabled, and no phones have to be registered.

Each SRST or Cisco Unified Communications Manager Express router can stream up to six different MOH files. You can configure each MOH file for multicast MOH or unicast MOH. Therefore, the maximum number of multicast MOH audio sources that can be used per remote site is limited to six. By providing different MOH files for each site, site-specific MOH files can be played for each site. SRST and Cisco Unified Communications Manager Express support only the G.711 codec.

When using multicast MOH also within the main site, you must enable multicast routing to allow the multicast stream to be routed from the Cisco Unified Communications Manager server network to the phone network or networks. If the MOH server and the IP phones are on the same network, multicast routing is not required, but such a scenario is not recommended for security reasons (servers should be separated from endpoints).

Multicast MOH from Branch Router Flash: Region Considerations

This section discusses how to configure codecs when you are using multicast MOH from branch router flash.

  • Locally generated MOH streams must be identical to the streams that are generated by Cisco Unified Communications Manager MOH servers:
    1. Same destination IP address (multicast address)
    2. Same destination port number
    3. Same codec
    4. Same packetization period
  • Multicast MOH from branch router flash supports only G.711.
  • G.711 must also be used for the stream generated by Cisco Unified Communications Manager MOH servers:
    1. Put Cisco Unified Communications Manager MOH server into a dedicated region via a dedicated Device Pool.
    2. Allow G.711 between the region of the MOH server and the region used by branch phones.
    3. Limit all other intersite calls to G.729.

When multicast MOH is used, IP phones and Cisco Unified Communications Manager are not aware that the IP phones listen to locally generated MOH streams. From a signaling perspective, the IP phone is instructed to listen to a certain multicast stream, and the local SRST gateway must generate a multicast MOH stream by using identical settings, such as destination address (multicast group), destination port, codec, and packetization period.

Multicast MOH in SRST gateways and Cisco Unified Communications Manager support only the G.711 codec. Therefore, G.711 must also be configured between the Cisco Unified Communications Manager MOH server and the branch IP phones. If Cisco Unified Communications Manager signals a codec other than G.711 to the IP phone, the IP phone cannot play the locally generated MOH stream because of a codec mismatch (the signaling is G.729, but the received RTP stream is G.711).

To ensure that Cisco Unified Communications Manager sends signaling messages to the phone and instructs it to listen to a G.711 stream, configure regions in this way:

  • Put the Cisco Unified Communications Manager MOH server or servers into a dedicated region (for example, MOH).
  • Put all branch devices into a site-specific region (for example, Branch-1).
  • Allow G.711 between regions MOH and Branch-1.
  • Make sure that region Branch-1 is limited to G.729 for calls to and from all other regions.

Multicast MOH from Branch Router Flash: Address and Port Considerations

Because a single MOH server can stream multiple multicast MOH files, you must specify an initial multicast address and port that is used for the first stream. In addition, you must choose whether to increment the IP address or port on additional streams. It is recommended that you increment on IP addresses instead of on ports. If there are multiple MOH servers within a network, you have to make sure that they do not use overlapping multicast IP addresses and ports for their streams.

When you increment on IP addresses, each stream consumes one IP address. In other words, each audio source requires four IP addresses. When you increment on ports, you must consider the RTCP. For each audio stream, two separate RTP ports are reserved: one for the actual audio transmission and one for (the optional) RTCP. Therefore, when you increment multicast MOH on ports, each stream consumes two ports.

Note

It is important to know the used multicast IP addresses and ports when you choose the option to prevent multicast traffic from entering the IP WAN by access lists.

If you are not sure about the used multicast addresses and ports, you can configure traces for the Cisco IP Voice Media Streaming Application service. Make sure that you check the Service Initialization check box in the trace configuration. Then restart the Cisco IP Voice Media Streaming Application service.

When analyzing the trace output, you will find this kind of information:

CMOHMgr::KickStartMultiCastStream (1) Starting Multicast stream, asID = 1, conferenceID = 1, codecType = mulaw, Multicast ip:port = 239.1.1.1:16384|
<CLID::Cluster><NID::10.1.1.1>
CMOHMgr::KickStartMultiCastStream (1) Starting Multicast stream, asID = 1, conferenceID = 1001, codecType = alaw, Multicast ip:port = 239.1.1.2:16384|
<CLID::Cluster><NID::10.1.1.1>
Note

The number after the KickStartMultiCastStream identifies the audio source. For each enabled codec, you will find information about the used multicast IP address and port. The node ID shows the IP address of the MOH server. In this example, only G.711 mu-law and G.711 a-law codecs are enabled. Only one audio source (audio source 1) is multicast-enabled. There is a single MOH server at 10.1.1.1.

Example: Implementing Multicast MOH from Branch Router Flash

The figure shows a sample scenario for implementing multicast MOH from branch router flash.

Multicast MOH packets generated by the Cisco Unified Communications Manager MOH server should not be sent over the IP WAN. Branch router should generate multicast MOH packets.

In the example, an MOH server is located at the main site. This MOH server is configured for multicast MOH. Multicast routing is enabled in the whole network, including in the IP WAN link to the remote site.

The main site router, however, should no longer route multicast MOH to the remote site. The remote site SRST gateway should instead generate multicast MOH streams to the phones that are located at the remote site.

Cisco Unified Communications Manager is not aware that the multicast packets that are generated by the MOH server at the main site are filtered on the IP WAN interface and then locally generated by the remote site SRST gateway. Therefore, Cisco Unified Communications Manager will instruct the IP phones that are located at the remote site to join the multicast group IP address that is configured at the Cisco Unified Communications Manager MOH server. To allow the phones to receive MOH for the multicast group IP address that they join, you must configure the SRST gateway to use the same multicast address and port that is used by the Cisco Unified Communications Manager MOH server that is located at the main site.

The figure shows the configuration of the previously discussed scenario.

It is assumed that the baseline configuration provides multicast routing in the whole network and that the Cisco Unified Communications Manager MOH server is already configured for multicast MOH.

Now the multicast MOH stream that is sent toward the remote site needs to be blocked, and multicast MOH from branch router flash needs to be implemented at the remote site.

Therefore, the SRST configuration of the remote site router is extended to include multicast MOH. The SRST configuration uses the same multicast IP address and port that are configured at the Cisco Unified Communications Manager MOH server that is located at the main site.

To stop multicast MOH that is generated by the main site Cisco Unified Communications Manager MOH server from being sent over the IP WAN, you can choose one of three options:

  • Set TTL to a low enough value at the Cisco Unified Communications Manager MOH server: If the TTLvalue in the IP header of the generated multicast MOH packets is set to a low enough value, the packets will not be routed out to the IP WAN. However, if the IP WAN link is one hop away from the Cisco Unified Communications Manager MOH server, and if the main site phones are also one hop away from the server, this method cannot be used, because the main site IP phones would also be affected by the dropped packets. In the current example, TTL is set to 1, and it is assumed that the IP phones are in the same VLAN, like the Cisco Unified Communications Manager MOH server.
  • Filter the packets by an IP ACL: At the main site router, you can configure an ACL that drops the multicast MOH packets at the IP WAN interface.
    Note

    Make sure that you verify the actually used multicast IP addresses and ports. As described earlier, the base address and port configuration determines the method that is used to increment the base number (on IP address or port), the codecs that are enabled for MOH, and the audio sources that are multicast-enabled.

  • Disable multicast routing at the IP WAN interface: You can disable multicast routing at the IP WAN interface, which prevents multicast packets from being routed out on that interface.

At the branch router, the multicast MOH stream is sent out on the interface that is specified in the ip source address command in call-manager-fallback configuration mode (or in telephony-server configuration mode, when Cisco Unified Communications Manager Express in SRST mode is used). Therefore, the multicast MOH stream that is generated at the branch router does not have to be blocked at the branch router WAN interface.

Configuration Procedure for Implementing Multicast MOH from Branch Router Flash

Implementing multicast MOH from branch router flash includes the steps that are shown in the following figure.

  1. Enable multicast routing in the network.
  2. Configure multicast MOH in Cisco Unified Communications Manager:
    1. Configure MOH audio sources for multicast MOH.
    2. Configure MOH audio server for multicast MOH.
    3. Enable multicast MOH at the media resource group or groups.
  3. Enable multicast MOH from branch router flash at the branch router.
  4. Implement a method to prevent multicast MOH streams from being sent over the IP WAN:
    1. Configure maximum hop value to prevent multicast MOH streams from being sent over the IP WAN.
    2. Use IP ACL at IP WAN router interface.
    3. Disable multicast routing on IP WAN router interface.

The configuration procedure for implementing multicast MOH from branch router flash begins with enabling multicast MOH (Steps 1 and 2). Once multicast MOH works as desired, the configuration is modified so that the multicast MOH stream is generated locally at the branch router (Step 3) and the multicast MOH stream that is generated by the MOH server is prevented from being sent to the IP WAN.

When enabling multicast MOH at the MOH server, make sure that you set the maximum hop value of the multicast-enabled MOH audio source or sources to a high enough value to allow the multicast MOH packets to be sent all the way to the remote phones.

Note

This is the base configuration where you use multicast MOH over the IP WAN.

When choosing option 4a to prevent the multicast MOH stream of the MOH server from being sent to the IP WAN, you must use a low enough value to ensure that the multicast MOH packets that are generated by the MOH server do not reach the IP WAN. If the IP WAN is as many hops away from the MOH server as the voice VLAN at the main site, then you cannot use the maximum hop value to prevent the MOH packets from reaching the IP WAN because that would also disable multicast MOH at the main site. In such a scenario, you have to use other options such as 4b or 4c. You can only use option 4c (disabling multicast routing) if there are no other applications in the network that require multicast routing. Option 4b (using access lists) can always be used.

Note

All IP phones must be able to access the main site Cisco Unified Communications Manager MOH server from their MRGL. This access is required as soon as multicast MOH is configured, regardless of whether multicast MOH from branch router flash is used. If the remote site IP phones do not have access to the Cisco Unified Communications Manager MOH server from their MRGL, Cisco Unified Communications Manager cannot instruct the IP phones to join the multicast group and will make the phone use tone on hold instead of MOH. Furthermore, you must check the Use Multicast for MOH Audio check box at the MRG that includes the multicast-enabled MOH server (see Step 2c). Finally, make sure that the G.711 codec is used between the MOH server and the branch phones, because SRST multicast MOH supports only G.711.