[cisco-voip] IP phones and DHCP

[Simon, Bill]

I find that our 7960 and 7940 phones (not sure about 7910, 7905, and 7935s)
disobey the DHCP protocol in a variety of ways.

First observation is that they're not using minimum, default, and maximum
lease times provided by the DHCP server.  In our network those values are
15000, 30000, 60000.  Thus at most I should see a phone talking to the DHCP
server every 250 minutes.  Almost all phones take the "minimum" lease time;
some renew even more frequently, especially at startup.  A phone I have been
observing renewed after three minutes then again at 15.

Recently we discovered a lot of traffic coming from certain phones.  They
were sending requests to renew their leases every 63 seconds.  A hard power
cycle has seemed to make this stop.

Lastly, one behavior of the phones breaks DHCP altogether.  If a phone once
received an IP address from a dhcp server but then can no longer talk to the
server at renewal time (or at startup if it has an old network config
stored) it keeps the IP address it once had.  This behavior might or might
not be good for the phone, depending on the circumstances, but it is
certainly bad for the network.  What if that IP address is reassigned by
DHCP to another client?  The correct behavior, as implemented by every other
DHCP client device out there, is to release the IP address if a renewal is
not given by the DHCP server.  

We have learned how to deal with most of these inconsistencies but if this
DHCP behavior is by design I would  consider it poor design.

Bill