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Home > Archive > Voice over IP Cisco > August 2005 > IP phones and DHCP
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IP phones and DHCP
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| Simon, Bill 2005-08-19, 5:45 pm |
| 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
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| Wes Sisk 2005-08-19, 5:45 pm |
| Bill,
Last I tested:
the phones did dhcp renew at 50%,75%,.... of lease duration. If the phone
does not receive an offer in response to a renewal it does continue to use
the current address. If the phone sends a renewal and receives a DHCP NAK,
it stops using the current address. If lease duration expires without a
successful renewal the phone stops using the address.
what phone load are you using? what dhcp options are you providing back to
the phone?
/Wes
-----Original Message-----
From: cisco-voip-bounces@puck.nether.net
[mailto:cisco-voip-bounces@puck.nether.net]On Behalf Of Simon, Bill
Sent: Friday, August 19, 2005 10:10 AM
To: 'cisco-voip@puck.nether.net'
Subject: [cisco-voip] IP phones and DHCP
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
________________________________________
_______
cisco-voip mailing list
cisco-voip@puck.nether.net
https://puck.nether.net/mailman/listinfo/cisco-voip
| |
| Marc Hering 2005-08-19, 5:45 pm |
| I've noticed that behaviour also on my 7960's...
-----Original Message-----
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.
| |
| Simon, Bill 2005-08-19, 5:45 pm |
| Phone load is 7.1(2.0)
Our DHCP options look like this:
netaddr => '10.64.0.0'
netmask => '255.255.248.0'
max_lease_time => '60000'
min_lease_time => '15000'
default_lease_time => '30000'
options => ('dhcp' ('broadcast-address' '10.64.7.255') ('routers'
('10.64.0.1')) ('subnet-mask' '255.255.248.0') ('tftp-server-name'
'XXXXX.phone.psu.edu') ('domain-name' 'phone.psu.edu') ('option-150'
('10.20.30.40')))
default_options => ('dhcp' ('ntp-servers' ('clock.psu.edu'
'clock.aset.psu.edu')) ('domain-name-servers' ('128.118.25.3'
'128.118.141.32' '146.186.163.66' '130.203.1.4')))
-----Original Message-----
From: Wes Sisk [mailto:wsisk@cisco.com]
Sent: Friday, August 19, 2005 10:26 AM
To: Simon, Bill; cisco-voip@puck.nether.net
Subject: RE: [cisco-voip] IP phones and DHCP
Bill,
Last I tested:
the phones did dhcp renew at 50%,75%,.... of lease duration. If the phone
does not receive an offer in response to a renewal it does continue to use
the current address. If the phone sends a renewal and receives a DHCP NAK,
it stops using the current address. If lease duration expires without a
successful renewal the phone stops using the address.
what phone load are you using? what dhcp options are you providing back to
the phone?
/Wes
-----Original Message-----
From: cisco-voip-bounces@puck.nether.net
[mailto:cisco-voip-bounces@puck.nether.net]On Behalf Of Simon, Bill
Sent: Friday, August 19, 2005 10:10 AM
To: 'cisco-voip@puck.nether.net'
Subject: [cisco-voip] IP phones and DHCP
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
________________________________________
_______
cisco-voip mailing list
cisco-voip@puck.nether.net
https://puck.nether.net/mailman/listinfo/cisco-voip
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