Survivable Remote Site Telephony part 2

This blog will discuss the Cisco SRST conversation that was started in the last blog. After a Cisco IP phones registers to the Cisco SRST gateway, the Cisco IP phones will continue to send keep alive messages to the primary Cisco Unified Communications Manager (CUCM) server. When the primary CUCM server is available, the connection monitor duration timer must expire before the phone re-registers back to the primary CUCM server. The connection monitor duration default is 120 seconds by default, but can be configured in the CUCM device pool configuration. Cisco IP phones maintain an active TCP connection with both their primary and backup CUCM server during normal operation. SRST technology assumes the Cisco IP phone has a valid configuration file. The configuration file of the phone is sent to the SRST router, allowing the router to dynamically load an ephone and ephone-dn configuration based on the configuration file. SRST 7.1 supports the following number of phones and directory numbers per router platform: Router Phones Directory Numbers 1861 15 48 2801 25 256 2811 35 144 2821 50 192 2851 100 288 3825 350 960 3845 730 960 6500 CMM 480 960 Let’s look at an example of a branch office with 50 phones using a 2811 series router. Management would need to decide which 35 phones would be supported in SRST during a WAN outage. Device pool #1 would be provisioned with an SRST reference, while device pool #2 would be provisioned without an SRST reference. Fifteen of the phones would not be available during the outage. If the same example site had (2) 2811 routers, the site could accommodate 70 phones total. The default gateway of the site is probably a virtual IP address of HSRP and could not be used because the phones would all be steered to the active router. Two SRST references would need to be configured to load share the SRST references between the two SRST routers. In the next blog, we will take a look at an SRST configuration.