SIP SRST allows a Cisco voice gateway to provide SRST support to SIP phones that are registered to a SIP proxy server. To accomplish this, the SIP SRST gateway acts as a SIP registrar. Ideally, the SIP phone should support dual registration. This allows it to be registered with the SIP proxy server and the SIP SRST gateway simultaneously. If the SIP phone does not support dual registration, the SIP SRST gateway cannot route incoming calls to the SIP phone until the SIP phone establishes registration with the SIP SRST router. You can place outbound calls, but they will experience some delay while the SIP phone is waiting for a response to INVITE messages that are sent to the SIP proxy server.
SIP SRST 3.4 added support for back-to-back user agent (B2BUA). B2BUA supports more features than a redirect server, which was used previously. SIP SRST 3.4 supports calls between SIP phones, between SIP and SCCP phones, and between SIP phones and router voice ports.
Configuring SIP Registrar Server
You can use the voice service voip command to configure the SIP registrar server on the SIP SRST gateway. You must also specify the call flows that you want to enable. Example 13-9 shows the steps for enabling the SIP registrar server and allowing all call combinations.
Miami(config)#voice service voip Miami(config-voi-serv)#allow-connections h323 to h323 Miami(config-voi-serv)#allow-connections h323 to sip Miami(config-voi-serv)#allow-connections sip to h323 Miami(config-voi-serv)#allow-connections sip to sip Miami(config-voi-serv)#sip Miami(config-serv-sip)#registrar server
The allow-connections commands allow the gateway to act as a B2BUA. The registrar server command initiates the gateway as a SIP registrar server.
Configuring a Voice Register Pool
After you enable the SIP registrar server, you create a voice register pool that is used to determine which devices can register with the SIP SRST gateway. You can specify allowed devices by a range of IP addresses, a specific IP address, or a specific MAC address. You can configure multiple pools. Example 13-10 shows a register pool that allows all IP phones in the 10.10.50.0/24 subnet to register with the SIP SRST gateway.
[View full width]! ! Set global voice register parameters ! Miami(config)#voice register global Miami(config-register-global)#max-dn 10 Miami(config-register-global)#max-pool 10 ! ! Create voice register pool ! Miami(config)#voice register pool 1 Miami(config-register-pool)#id network 10.10.50.0 mask 255.255.255.0 Miami(config-register-pool)#call-forward b2bua busy 5000 Miami(config-register-pool)#call-forward b2bua noan 5000 timeout 30 Miami(config-register-pool)#preference 2 Miami(config-register-pool)#proxy 10.1.10.10 preference 1 monitor-probe icmp-ping Miami(config-register-pool)#voice-class codec 1
The proxy command in the voice register pool is the address of the SIP proxy server. This is the preferred server, so the preference is set to 1. The preference for the SIP SRST gateway is set to 2. The optional monitor-probe icmp-ping keyword instructs the SIP SRST gateway to monitor the availability of the proxy using pings, allowing quicker failover. Response Time Reporter (RTR) monitoring is also supported.
In addition, you can configure COR lists, dialplan-patterns, and outgoing voice translation rules to a voice register pool. Apply outgoing translation rules before the dialplan-pattern or the alias command is used in a voice register pool.
Part I: Voice Gateways and Gatekeepers
Gateways and Gatekeepers
Part II: Gateways
Media Gateway Control Protocol
Session Initiation Protocol
Connecting to the PSTN
Connecting to PBXs
Connecting to an IP WAN
Influencing Path Selection
Configuring Class of Restrictions
SRST and MGCP Gateway Fallback
Using Tcl Scripts and VoiceXML
Part III: Gatekeepers
Part IV: IP-to-IP Gateways
Cisco Multiservice IP-to-IP Gateway
Appendix A. Answers to Chapter-Ending Review Questions