SRST (Survivable Remote Site Telephony) provides a mechanism for remote site IP phones to continue to operate in case of WAN failure. IP phones registered to a centralized CUCM cluster will continue to funtion if the WAN link is lost. SRST allows the remote site phones to re-register temporarily to the local SRST router and maintain basic functionality during the outage. This feature is typically accompanied with MGCP fallback (if the local remote site router is controlled via MGCP). MGCP gateways that lose their connection to their call agent can fallback to the H.323 application on the local IOS based router.
The following is a reference table for the number of maximum IP phones and DNs supported per Cisco IOS based platform when planning for SRST implementation: