Survivable Remote Site Telephony


Survivable Remote Site Telephony (SRST) is a Cisco Unified Communications Manager (CUCM) call processing backup mechanism that allows Cisco IP phones to register to a Cisco router. A Cisco router in SRST mode provides call setup and call teardown services similar to CUCM, but only when connectivity to the CUCM servers provided in the Cisco Call Manager group are not reachable. SRST technology is widely used in the centralized call processing model where branch sites do not have any local call processing capabilities. Cisco IP phones at branch locations register with CUCM over wide area network (WAN) links. All call setup and call teardown functions of the branch site Cisco phones are carried over the WAN links. Call setup and call teardown (call control) bandwidth requirements are benign (under 1kbps). Table 3-12 of the CUCM 7.x SRND recommends the amount of call control bandwidth dependent upon the number of users at the site. If WAN connectivity is lost between CUCM and the branch site, Cisco IP phones will re-register to their SRST reference provisioned in the device pool configuration of the Cisco IP phone. Cisco IP phones send a TCP keepalive to their primary CUCM server every 30 seconds by default. If 3 keepalives are missed, the Cisco IP phone will re-register with another CUCM server in their Call Manager group configuration and attempt to register with the SRST reference if no CUCM servers are available. The SRST failover process occurs faster if there is an active event on the Cisco IP phone device (e.g. Cisco IP phone was taken offhook). The SRST reference can be configured to the default gateway of the DHCP scope or an administratively defined IP address. In many deployments, the default gateway of the site is the only router(s) at the branch location. The default gateway of a branch location may be a virtual IP address shared between two Cisco routers using a virtual router redundancy protocol (HSRP, GLBP, or VRRP). As of this writing, the current version of SRST is 7.1. The SRST Administration guide maps SRST release versions to the Cisco IOS version required for the SRST version. In the next blog, we will continue our SRST conversation with the number of Cisco IP phones and directory numbers supported by different router models. Links SRST Administration Guide

Join the Network World communities on Facebook and LinkedIn to comment on topics that are top of mind.

Copyright © 2009 IDG Communications, Inc.