DHCP scope duration for VoIP phones

[srosenthal]

Will the Cisco VoIP phones keep working if they loose contact with the DHCP server that provides them their IP address?  I have the timeout set for 8 days on the scope so I assume that as long as the phone does not loose power then it should continue to work for 8 days.  Is that correct?

Also, these phones are using extension mobility so I did not know if when logging into and out of the phones if they interact with the DHCP server or not.

Seth

[dkosekrti · Accepted solution]

Hi,

If the lease is set to 8 days, the dhcp server will provide the same address to the device for 8 days, beyond that the device will receive a new address (or the same address) once it restarts.

The DHCP address will remail on any device until it is restarted. If it can not reach the DHCP server at the time of the restart it will not get an IP address.

[Aman Soi]

Hi Seth,

In addition to what Mr. David has said,after user logout from phone by pressing services button using EM,phone restarts.

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/8_5_1/ccmfeat/fsem.html#wp1146344

I think IP address would change.

regds,

aman

[Chris Deren]

Just a small addition to above comments. DHCP lease re-negotiation cycle is half the set lease time, so if lease is set to 8 days phones will attempt to renegotiate the IP address after 4 days. I normally recommend setting DHCP at 30 days in a stable DHCP environment, Cisco has some good guidance in the SRND you may want to read.

HTH,

Chris

[Gordon Ross]

In addition to what the others have said, the DHCP client in Cisco phones is technically broken.

Once a DHCP client reaches the end of it's lease, if it can't contact a DHCP server to obtain a new lease (which may give a new IP address) the client should stop using the IP address it has.

The Cisco phones don't obey this, and continue to use the, now expired, IP address. We've seen phones still using IP addresses months after they stopped being able to see the DHCP server.

GTG

[srosenthal]

I appreciate all the answers.  We will test to see what happens.

Thank you.