Call Flow with MGCP

Configuring SRST

You must configure SRST on the gateway and on CallManager. The CallManager configuration tells the IP phone that an SRST gateway is available. This is normally the tertiary device in the IP phone CallManager list.

Gateway Configuration

You enter SRST configuration mode by using the call-manager-fallback command. At a minimum, you must configure the source address and the number of e-phones and e-phone-dns for the SRST gateway to support. The source address is the IP address that the SRST gateway uses to communicate with the IP phones. Example 13-1 shows the initial SRST configuration.

Miami(config)#call-manager-fallback
Miami(config-cm-fallback)#ip source address 10.10.50.1
Miami(config-cm-fallback)#max-ephones 24
Miami(config-cm-fallback)#max-dn 48

 

The max-ephones command specifies how many IP phones can register with the SRST gateway. This value varies by router platform and the license you purchase. The max-dn command specifies how many virtual SRST voice ports are created. The number of DNs is limited by platform and memory; it is not limited to your license. This allows you to maintain multiple lines on your IP phones while in SRST mode. When you enter the max-dn command, the virtual voice ports are created immediately and reside in memory. These virtual voice ports are not saved in the config but are displayed when you enter a show dial-peer voice command, as shown in Example 13-2.

Miami#show dial-peer voice
VoiceEncapPeer20001
 peer type = voice, information type = voice,
 description = `',
 tag = 20001, destination-pattern = `',
 answer-address = `', preference=0,
 CLID Restriction = None
 CLID Network Number = `'
 CLID Second Number sent
 CLID Override RDNIS = disabled,
 source carrier-id = `',
 source trunk-group-label = `',
target trunk-group-label = `',
 numbering Type = `unknown'
 group = 20001, Admin state is up, Operation state is down,
 incoming called-number = `', connections/maximum = 0/unlimited,
 DTMF Relay = disabled,
 URI classes:
 Destination =
 huntstop = enabled,
 in bound application associated: 'DEFAULT'
 out bound application associated: ''
 dnis-map =
 permission :both
 incoming COR list:maximum capability
 outgoing COR list:minimum requirement
 Translation profile (Incoming):
 Translation profile (Outgoing):
 incoming call blocking:
 translation-profile = `'
 disconnect-cause = `no-service'
 advertise 0x40 capacity_update_timer 25 addrFamily 4 oldAddrFamily 4
 type = pots, prefix = `',
 forward-digits 0
 session-target = `', voice-port = `50/0/1',
 direct-inward-dial = disabled,
 digit_strip = enabled,
 register E.164 number with H323 GK and/or SIP Registrar = TRUE
 fax rate = system, payload size = 20 bytes
 supported-language = ''

 Time elapsed since last clearing of voice call statistics never
 Connect Time = 0, Charged Units = 0,
 Successful Calls = 0, Failed Calls = 0, Incomplete Calls = 0
 Accepted Calls = 0, Refused Calls = 0,
 Last Disconnect Cause is "",
 Last Disconnect Text is "",
 Last Setup Time = 0.

 

To limit the impact on the gateway, you should determine how many lines you actually need rather than configure the maximum possible. You can also use the limit-dn command to limit the number of DNs that can register per IP phone type. For example, if your 7960s have four lines, you might choose just to have the primary DN register with the SRST router to conserve resources. Table 13-1 lists the maximum number of e-phones and e-phone-dns supported in currently available platforms. Verify these numbers on Cisco.com, because they might change with new Cisco IOS versions or increases in router CPU or memory capabilities.

Table 13-1. Gateway SRST Support

Router Platform	Max E-Phones	Max E-Phone-dns
1751-V	24	144
1760, 1760-V	24	144
2600XM	36	144
2650XM, 2651XM	48	192
2691	72	288
2801	24	256
2811	36	144
2821	48	192
2851	96	288
3725	144	960
3745	480	960
3825	336	960
3845	720	960
CMM	480	800

 

CallManager Configuration

You must specifically configure an IP phone with an SRST reference for it to attempt to register with a gateway. You configure the SRST reference in the device pool. In most implementations, the SRST gateway is also the default gateway for the IP phones. In this situation, you can select Use Default Gateway in the device pool configuration. Figure 13-2 shows the device pool configuration for the Miami phones.

Figure 13-2. Configuring SRST Reference to Use Default Gateway

If the SRST gateway is not the default gateway for the IP phones, you must create an SRST reference that contains the IP address of the SRST gateway. You do this under the System>SRST menu option. Figure 13-3 illustrates an SRST reference configuration.

Figure 13-3. Creating an SRST Reference

If the number of IP phones at a remote site exceeds the capability of the SRST gateway, you can control which IP phones register by creating two device pools for the site. Configure one device pool with an appropriate SRST reference, and assign it to the IP phones that should register with the SRST gateway. Configure a second device pool with the SRST reference set to Disable. This device pool is assigned to IP phones that are not business critical, such as break room or conference room phones.

Dial Plan Considerations