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DHCP lease renewal on IP phones

Patrick Ferriter EXPERT RESPONSE FROM: Patrick Ferriter

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QUESTION POSED ON: 03 June 2005
As most IP Phones get IP addresses via DHCP from the service provider (I am thinking ITSP) - what happens to voice on a VoIP connection when the IP phone's DHCP lease expires? Does it drop the call and then reconnect? Or does it re-extend the lease time of the IP address? If it extends the lease time, is there a maximum time after which it must re-lease the IP address?

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EXPERT RESPONSE
There are some IP phones that sit directly behind an integrated access device (IAD) such as a cable modem and act as a DHCP client on the WAN and a DHCP server on the LAN. The DHCP lease on the WAN interface will come from the local ISP (which also may be an ITSP). There are not many IP phones like this on the market, as the phone typically provides firewall and NAT functions when used in this mode.

One phone like this is the ZIP4x5 from Zultys. Most IP phones do not have this function built in and will likely get their DHCP IP address from a little router in the home office such as a Linksys or Netgear. In either case, the same question remains as to how IP phones behave when on a call and renewing the DHCP lease.

The answer is that this will depend on the IP phone manufacturer. Most phones (if lease renewal is successful) will have no interruption in the call. If lease renewal is not successful, the behavior again depends upon the phone manufacturer. One option is to keep the current call up and then indicate the failure to renew the DHCP address once the call clears and the other is to drop the call. I am not aware of any manufacturer that tries to artificially extend the lease from the phone side.


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