SRST Failover Time

[jefferymitchell]

I am testing one of our remote sites ability to dial 911 when the WAN link is down. My question is how long will it take for the phones at this location to re-register with the router to make an 911 call? The phones at this site normally register with a subscriber at the host site which is connected over Metro-E WAN connection.

[Manish Gogna · Accepted solution]

Hi Jeffery,

Typically, a phone takes three times the keepalive period to discover that its connection to CUCM has failed. The default keepalive period is 30 seconds.

If the IP Phone has an active standby connection established with a Cisco Unified SRST router, the fallback process takes 10 to 20 seconds after the connection with CUCM is lost. An active standby connection to a Cisco Unified SRST router exists only if the phone has a single CUCM in its Cisco Unified CM group. Otherwise, the phone activates a standby connection to its secondary CUCM.

NOTE: The time it takes for an IP Phone to fall back to the SRST router can vary, depending on the phone type. Phones such as the Cisco Unified IP Phone models 7902G, 7905G, and 7912G can take approximately 2.5 minutes to fall back to SRST mode.

If a Cisco Unified IP Phone has multiple CUCM systems in its Cisco Unified CM group, the phone progresses through its list before attempting to connect with its local Cisco Unified SRST router. Therefore, the time that passes before the Cisco Unified IP Phone eventually establishes a connection with the Cisco Unified SRST router increases with each attempt to connect to a CUCM. Assuming that each attempt to connect to a CUCM takes about 1 minute, the Cisco Unified IP Phone in question could remain offline for 3 minutes or more following a WAN link failure. You can decrease this time by setting the keepalive timer to a smaller value. You can configure the keepalive timer using the CUCM service parameter Station Keepalive Interval.

While in SRST mode, Cisco Unified IP Phones periodically attempt to reestablish a connection with CUCM at the main site. The default time that Cisco Unified IP Phones wait before attempting to reestablish a connection to CUCM is 120 seconds.

NOTE

If you want a Cisco IP Phone to come out of SRST fallback mode faster, you can manually reboot the Cisco IP Phone by pressing Settings and then **#**, or by going into the router mode CallManager-Fallback and entering Reset All.

Couple of useful links

http://www.ciscopress.com/articles/article.asp?p=1744068&seqNum=4

https://supportforums.cisco.com/discussion/10891036/srst-how-reduce-call-manager-fallback-time

Manish

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