[cisco-voip] weirdness with DHCP and 7960 phones..

[Simon, Bill]

I can see now why the phones do this (I guess as protection from problematic
DHCP servers), but it caught us off guard.  We figured that it should be up
to the DHCP server to remember lease information, not the device.  For
example, if a phone is unplugged from the network and power removed, when it
comes back up we expected a DHCP Discover to be broadcast to initiate a new
lease.  At that point, if the DHCP server wanted to offer it the same IP it
had before, no problem.  Instead the phone starts up with a DHCP Renew
(unicast).
 
 

---
Bill Simon - bills at tns.its.psu.edu - (814) 865-2270
http://tns.its.psu.edu/ <http://tns.its.psu.edu/> 



 


  _____  

From: Wes Sisk [mailto:wsisk at cisco.com] 
Sent: Monday, April 10, 2006 5:11 PM
To: Simon, Bill
Cc: 'cisco-voip at puck.nether.net'
Subject: Re: [cisco-voip] weirdness with DHCP and 7960 phones..


This was an interesting one - We found that MS dhcp server would NOT send
DHCP NAK for a renewal.  So long as the phone does not receive a NAK it will
continue to use the same address.  If the phone does not receive any
response it will continue to use the current address until the lease
expires.

Microsoft's behavior is to allow redundant DHCP servers in the same subnet.
If one server dies (controlling the lower half of the range) the redundant
server continues to function offering addresses from the upper half of the
range.  But both dhcp servers must receive all DHCP Discovers and Renews for
this method to work and servers must not respond with a NAK.

/Wes

Simon, Bill wrote: 

It's true, the 7960s do not obey the DHCP protocol.
 
If they cannot talk to a DHCP server, they will use whatever previous
address they had.
 
We have complained to Cisco before about this disregard for the protocol and
were told it is "by design".
 
Our solution was to make gigantic DHCP pools for phones so that there is
plenty of address space available and it is unlikely that we will have IP
conflicts.
 

---
Bill Simon - bills at tns.its.psu.edu <mailto:bills at tns.its.psu.edu>  - (814)
865-2270
http://tns.its.psu.edu/ <http://tns.its.psu.edu/> 



 


  _____  

From: Tim Reimers [mailto:tim.reimers at asheville.k12.nc.us
<mailto:tim.reimers at asheville.k12.nc.us> ] 
Sent: Monday, April 10, 2006 4:41 PM
To: cisco-voip at puck.nether.net <mailto:cisco-voip at puck.nether.net> 
Subject: [cisco-voip] weirdness with DHCP and 7960 phones..





Hi everyone--- 

We had a weird occurrence in the last few days I thought I'd run past the
group to see what everyone thought... 


A vendor (who shall remain nameless) rebuilt our CCM -- and in the process,
it turns out, failed to reinstall the DHCP server/scope and TFTP options..

All worked fine until we had a power glitch of some sort and the phones
rebooted... 

At that point- well, that's when things get weird.. 

You'd think that NONE of the phones would come back up --- 

but about 20 out of 75 did just that...  with no DHCP server. 

Mine, which was one that did it, is NOT set for static addressing (none
are)... 

Smoehow, it retained it's IP settings, though it took a while to boot... 


my question???  Wassup with that? I thought "no dhcp, no boot', no
NOTHING....for ALL of the phones on that subnet? 

Why did some come up and work and others not? Is there some magic in some of
the boot code? 

(except perhaps a phone with a 169.254.x.x address, trying to find a CCM, a
load ID ,etc..) 

Tim 



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