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New Member

SRST and phone registration question


I've got approx 30 phones in a branch office with a 2620 configured with SRST. If the CCM at our HQ fails, and the phones register with the default router to run SRST, then since only a max of 24 phones are supported, what is the registration process? ie is it a race between the phones, with only the first 24 registering with the router succeeding?

I know that you set each phone in CCM to have SRST enabled/disabled, but am trying to figure out the correct way. Is the solution to simply choose beforehand, which 24 phones you want to run SRST on, and disable the feature on the other phones? Thereby when SRST is envoked, only 24 enabled phones will attempt, and succeed to register.


Cisco Employee

Re: SRST and phone registration question

I think the appropiate way is to choose before hand. If you don't you are correct it will be a race for the first 24 to register. This will give you no control over which 24 phones it will be. I suggest using the enable/disable option for srst under the phone config.

Thank you,


New Member

Re: SRST and phone registration question

Thanks for clarifying the issue.



New Member

Re: SRST and phone registration question

You may either only enable SRST on those 24 phones needed to fallback or do the following if all the ip phones are enabled for SRST:

1. you may separate 30 phones into 2 vlan groups, one able to fall back, the other one is not allowed to fallback. And then in CallManager-fallback configuration mode on the router, you may use "strict match" option for "ip source-address x.x.x.x" command, so that the other VLAN group will be restricted from falling back to the SRST.

2. Another way is that you use "access-list".


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