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SRST on single cluster

I read about srst being implemented on remote sites cluster over WAN, where WAN connection goes down to the main CUCM cluster then srst will kick in on the remote sites for the remote phones

1) Now if I only got a single cluster cucm and one cucm on the single cluster, is it advisable or ok to configure srst on that single cluster so that IP phones can still call each other , NDD and IDD when cucm goes down?

2) Also if srst has kick in and after a while the cucm on that single cluster goes online again, how long will the phones in srst mode shift back its registration to cucm? Can that period of time be adjusted like shorten it?

Arun Kumar
Cisco Employee


It's good to configure SRST if you need High Availability incase remote site looses connectivity to central CUCM (one node or multiple node cluster). Remote site phones will register to local SRST Router and will be able to make and receive calls (but IP Phone may not have all the features in SRST compared to while registered to CUCM).

An IP phone that connects to the SRST over a WAN reconnects itself to CUCM as soon as it can establish a connection with CUCM over the WAN link. However, if the WAN link is unstable, the IP phone switches back and forth between the SRST and CUCM. This situation causes temporary loss of phone service (no dial tone). These reconnect attempts, known as WAN link flapping issues, continue until the IP phone successfully reconnects itself to CUCM. These WAN link disruptions fit into two classifications: infrequent random outages that occur on an otherwise stable WAN and the sporadic, frequent disruptions that last a few minutes.

To resolve the WAN link flapping issues between CUCM and SRST, CUCM provides an enterprise parameter and a setting in the Device Pool Configuration window that is called Connection Monitor Duration.

The default for the enterprise parameter specifies 120 seconds. Use the enterprise parameter to change the connection duration monitor value for all IP phones in the CUCM.



Thanks for your reply Arun, that answers my 2nd question, so by default 120seconds or 2minutes of stable time is needed for CUCM to take over the ip phones from SRST state

How about my first question? If SRST is still needed if I only have one Cluster and one CUCM on the cluster?

Kindly check my first reply again and let me know if any query after that.

I already checked your first reply and it says nothing regarding my first question if it is advisable to configure SRST on a single cluster where there is only one cucm on that single cluster

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