SRST (DHCP and option 150)

Unanswered Question
Feb 11th, 2008

I am trying to setup my Cisco 2800 for SRST. According to Cisco documentations, one of the requirements for SRST is to enable DHCP option 150 with the IP address of the CallManager as the TFTP server.

If SRST is used when the CallManager is unreachable, why do we still need to define the Call Manager as the TFTP server for option 150?

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Michael Owuor Mon, 02/11/2008 - 04:46

Because in this case the phones are expected to register with CallManager under normal operation, they need to be configured to receive their configuration files from the CallManager TFTP server. If you have configured SRST in Callmanager, the said configuration files will indicate that the phones are to register with the CallManager servers first, and if those become unavailable, then they should register with the SRST gateway.

So we still need to define the CCM as the TFTP server for option 150 in order to allow the phones to receive their configuration files that will let them know the which CCM and which SRST gateway they should register with.

Hope that helps.

davetucker Mon, 02/11/2008 - 05:30


DHCP Option 150 is required for the phones to know where to find the TFTP Server. This is usually CallManager because CM holds all the Phone Firware Loads and Configuration Files for the phones.

When Phones fail-over they use the latest configuration file they had recieved from CallManager. They do not require a TFTP server during a WAN outage.

Cisco recommend that you either use the SRST router or a local DHCP server, as DHCP lease expiry during extensive WAN failure will result in the phones being unable to pick-up an IP Address.

Hope this helps, if so, please rate.



m.casamayor Mon, 02/11/2008 - 06:37

Thanks for the reply.

What happens if an IP phone reboot during a WAN outage? I know that the phone will be assigned an IP address because of the DHCP service enabled on the SRST router, but does the option 150 have any purpose in this case? Does the phone still need to download the latest configuration when it reboots?

davetucker Mon, 02/11/2008 - 06:52


The phone doesn't need a reachable TFTP server when rebooting during WAN outage. It will use the Firmware and Configuration files that are held locally on the phone.




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