Phone IP Address in SRST Mode

[Syed]

Dear All,

As I understand phones that are in SRST mode will have the same DN information that they had from CUCM, but how about IP address information, does it change?? how will they get??

Thank you 

[Jaime Valencia · Accepted solution]

The ONLY thing that is assumed to fail for SRST, is the connectivity to CUCM.

The rest of the underlying network, including DHCP, is addressed separately. If the phone has a DCHP lease that is still valid, it should keep working, until the renew. That's just basic DHCP as with any other endpoint, server, computer, etc..

[Syed]

Thanks buddy!!! You answered it.

[Deepak Mehta · Accepted solution]

Hi Syed,

As Jaime (+5) pointed out ,the phones gets ip addresses from the DHCP and as long as DHCP lease doesn't expire your phone will still work even if you have CUCM as DHCP server.

You can refer to below doc-

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When a Cisco IP phone is connected to the Cisco Unified SRST system, it automatically queries for a DHCP server. The DHCP server responds by assigning an IP address to the Cisco IP phone and providing the IP address of the TFTP server through DHCP option 150. Then the phone registers with the Cisco Unified CallManager system server and attempts to get configuration and phone firmware files from the Cisco Unified CallManager TFTP server address provided by the DHCP server.

When setting up your network, configure your DHCP server local to your site. You may use your SRST router to provide DHCP service (recommended). If your DHCP server is across the WAN and there is an extended WAN outage, the DHCP lease times on your Cisco IP phones may expire. This may cause your phones to lose their IP addresses, resulting in a loss of service. Rebooting your phones when there is no DHCP server available after the DHCP lease has expired will not reactivate the phones, because they will be unable to obtain an IP address or other configuration information. Having your DHCP server local to your remote site ensures that the phones can continue to renew their IP address leases in the event of an extended WAN failure.

[Syed]

Superb information, Deepak. Thank you and appreciate it.,