Improving SRST Re-Convergence Time
When a phone re-registers in SRST to the local gateway, it can take more than 2 minutes by default when connectivity is restored to CallManager before the IP phone unregisters from SRST and reregisters to Cisco Unified Communications Manager.
This interval is controlled by the Connection Monitor Duration. This value is set globally (Enterprise Parameters) and overridden at the Device Pool.
Take a simple SRST configuration.
call-manager-fallback
 ip source-address 172.16.1.1 port 2000
 max-ephones 52
 max-dn 192
 system message primary CUCM is down... Run away!
All that is required on the CallManager side is to configure an SRST reference, and assign that reference under the device pool. Once the link to CallManager is disabled (or the CallManager service deactivated), the devices will SRST register. Changing the Connection Monitor Duration can improve the time it takes for a device to reregister with CallManager when the link is re-established. Note that setting the timer to 2 seconds won't automatically make all phones re-register in 2 seconds - convergence will certainly be improved, however the IP phones won't all automatically re-register that quickly. From a lab perspective, this could be useful to reduce the amount of time waiting for SRST-type scenarios...
SRST can be verified by either:
- Killing the link to CallManager; or
- Disabling the CallManager service; or, if the device is in production and such an aggressive option is not suitable
- Applying an inbound ACL on the interface or SVI to prevent the Phone from sending SIP/SCCP heartbeats to CallManager (thereby forcing the Phone into SRST)
Set the Connection Monitor Duration Timer globally
Override the value locally at the device pool
