12-7 Survivable Remote Site (SRS) Telephony | Cisco Field Manual[c] Router Configuration

SRS Telephony provides Cisco CallManager (CM) fallback support in environments using Cisco IP phones at remote sites with a central CM.
If a WAN link to a remote site is lost, SRS Telephony configured on the remote router begins to handle calls for the IP phones.
SRS Telephony supports the following routers and resources: Cisco 2600 and 3620 series routers (up to 24 Cisco IP phones and 48 directory numbers), the Cisco 3640 router (48 IP phones and 96 directory numbers), and the Cisco 3660 router (144 IP phones and 288 directory numbers ).
The SRS Telephony router is listed as a last resort, after the primary and other secondary CallManagers.
It is good design practice to have both WAN (to the IP network) and PSTN connections at a remote or branch site. In the event of a WAN failure, the SRS Telephony router can still make calls over the PSTN connections.
SRS Telephony does not support the Cisco IP Softphone, Cisco uOne, or Cisco IP Contact Center. In addition, the Cisco 3660 router is unable to support Centralized Automatic Message Accounting (CAMA) trunks for 911 emergency services.

NOTE

SRS Telephony is a new feature available in Cisco IOS software release 12.1(5)YD1 for the Cisco 2600 and 3600 series routers. Although this is not a major release, this feature is described here because of its usefulness in IP telephony environments.

Configuration

Configure DHCP server entries for IP phones.
1. Create a DHCP pool entry for a phone:
```
 (global)  ip dhcp pool   name  
```
  The phone entry is named name (a text string).
2. Define the IP phone's address:
```
 (dhcp)  host   ip-address mask  
```
  The IP phone receives a reply with the ip-address and subnet mask.
3. Identify the IP phone's MAC address:
```
 (dhcp)  client-identifier   mac-address  
```
  The phone's MAC address is given as mac-address ( dotted - triplet format).
4. Define the TFTP server address:
```
 (dhcp)  option 150 ip   tftp-address  
```
  The IP address of the TFTP server used to download IP phone configuration information is set to tftp-address.
5. Set the default router used by the IP phone:
```
 (dhcp)  default-router   ip-address  
```
  The IP address of the default gateway on the IP phone's local network is given as ip-address.
Enable SRS Telephony:
```
 (global)  call-manager-fallback  
```
Specify a source address when talking to IP phones:
```
 (cm-fallback)  ip source-address   ip-address  [  port   port  ] [  any-match   strict-match  ] 
```
The router uses the ip-address as the source address when communicating with IP phones. An optional port number (the default is 2000) can be given. The router can accept registrations from IP phones using an IP address other than the fallback source address configured here using any-match (the default). Otherwise, the strict-match keyword forces the router to accept registrations only from phones that use the specific fallback source address. Usually, the fallback source address is the same address as the Ethernet interface where the phones are connected.
Set the maximum number of IP phones to support:
```
 (cm-fallback)  max-ephones   max-phones  
```
The maximum number of phones is max-phones (0 to 24, 48, or 144, depending on router platform; the default is 0). As soon as this is set, it cannot be lowered until the router is reloaded.
Set the maximum number of directory numbers to support:
```
 (cm-fallback)  max-dn   max-dn  
```
The maximum number of directory numbers is max-dn (0 to 48, 96, or 288, depending on router platform; the default is 0). As soon as this is set, it cannot be lowered until the router is reloaded.
(Optional) Set the IP phone keepalive time:
```
 (cm-fallback)  keepalive   seconds  
```
The router expects to see keepalive messages from each IP phone at intervals of seconds (the default is 30). If keepalives are not received for three consecutive intervals, the IP phone is considered out of service.
(Optional) Set a default destination number for incoming calls:
```
 (cm-fallback)  default-destination   number  
```
For incoming calls from a POTS voice port, calls are normally routed according to the called number information. If this is not present, calls can be routed to number (a valid directory number). Otherwise, the caller receives a secondary dial tone and must enter the extension number to complete the call.
(Optional) Set the global prefix for the IP phone dial plan:
```
 (cm-fallback)  dialplan-pattern   tag pattern   extension-length   length  
```
The global prefix is used to expand the IP phone extension numbers into a full E.164 number. The tag value (1 to 5) is used to identify multiple dial-plan patterns. The pattern (a string of digits followed by dots as wildcard digits) is used to complete the E.164 number. It can contain items such as the area code, a prefix, and any missing numbers to the left of the IP phone extension range. When incoming calls are displayed on an IP phone, they can be converted back to extension numbers. This is done using an extension number length (number of digits).
(Optional) Allow call transfer to a range of non-IP phone numbers:
```
 (cm-fallback)  transfer-pattern   pattern  
```
IP phones can be allowed to transfer calls to other IP phones (the default) or to non-IP phone numbers. The pattern (a string of digits, along with dots as wildcard digits) is used to specify the allowed destination numbers. Up to 32 transfer patterns can be configured.
(Optional) Set dial codes to access outbound POTS trunks during fallback:
```
 (cm-fallback)  access-code  {  fxo   e&m   bri   pri  }  dial-string  
```
During CallManager fallback, callers can access the POTS trunks for outbound calls by dialing a prefix dial-string (one or more digits). An access code can be set for each type of POTS trunk that is available. Many times, the dial string is set to 8 or 9, traditional access codes for trunks.
(Optional) Set a speed-dial number for voice mail:
```
 (cm-fallback)  voicemail   number  
```
By default, the "message" button on IP phones is disabled during CM fallback. To reach a voice-mail system through a fallback path (PSTN, for example), the voice-mail speed dial number can be configured. When the "message" button on an IP phone is pressed, the router dials number (a string of digits) to reach a voice-mail system.

Example

Figure 12-1 shows a network diagram for this example. An SRS Telephony fallback CallManager is configured to support a small branch office in the event that the WAN link to the branch is lost. The DHCP entries for two IP phones ( Joe and Fred ) are configured so that the phones can reboot and operate successfully. The TFTP server for the phones is at 192.168.243.100, and the default gateway is 192.168.243.1 (the SRS Telephony router).

Figure 0-7. Network Diagram for the SRS Telephony Example

graphics/12fig01.gif

CallManager fallback uses a source address of 192.168.243.1, port 2000, and expects the IP phones to register with it using a strictly matching address of 192.168.243.1. The maximum number of supported phones is 24, with up to 48 directory numbers.

A string of "859885" is added to the four-digit extension numbers of the IP phones. The phones are allowed to transfer calls to numbers matching the range "859555 . ". A voice-mail system is accessible by dialing 98850100, which accesses an FXO port using a leading 9.

  ip dhcp pool Joe   host 192.168.243.10 255.255.255.0   client-identifier 0020.e068.24ad   option 150 ip 192.168.243.100   default-router 192.168.243.1   ip dhcp pool Fred   host 192.168.243.11 255.255.255.0   client-identifier 0020.e068.7714   option 150 ip 192.168.243.100   default-router 192.168.243.1   interface fastethernet 0/0   ip address 192.168.243.1 255.255.255.0   call-manager-fallback   ip source-address 192.168.243.1 port 2000 strict-match   max-ephones 24   max-dn 48   dialplan-pattern 1 859885.... extension-length 4   transfer-pattern 859555....   voicemail 98850100   access-code fxo 9