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Periodicaly dhcp ip phone problems

Level 1
Level 1

Hi all,

     We are having some problems related to the lease of ip addresses to the phones, the situation is the next one:

    There are some ip cisco phones 7941,7911 that can not get an dynamic IP address provided commonly by a

    router 2851 (the gateway of the ip voice network), the state of the phones stays on "GETTING IP ADDRESS"

    and can not continue to get in a registered state, we solved this problem executing this commands:

          clear ip dhcp bindings *

          clear ip dhcp conflicts *

   After that, phones get IP address normally, dhcp server has enough IP addressess, this problem happens each

   3 months, and we to solve it applying the same commands previously mentioned, is there another way to solved it

   with out to have apply the same commands each determinated time?.

   I will apreciate so much any input that can help,

9 Replies 9

paolo bevilacqua
Hall of Fame
Hall of Fame

This can happes when, for example, the router crashes. While the router is rebooting, some but not all phones will release address, once the router is back up the phones will to renew the address, but the router will be unable to allocate du to ping conflict with a phone that has not released address.

A big factor causing is this is having a small subnet compared to number of phone, eg 240 phones ond a /24.

In my opinion the best apparoch is check why the router is crashing in first place.

I am not very sure that the router gets crash..., I think that the customer would tell us about that,

I can not underestand very well why all ip addresses are wasted,

can you explain it again please, thanks a lot!

Hi IngGerardo013

In addition to the good tips from Paolo, He told about a likely cause of your problem, when he referred the "router crashes", mean a situation where the system has detected an unrecoverable error, and has restarted itself.

Regarding the DHCP ensure the Pool is not running out of the IP addresses. Check how many phones there are in your environment against your DHCP pool configuration.

Please remember to rate helpful responses and identify helpful or correct answers.

Bruno Rangel
Please remember to rate helpful responses using the star bellow and identify helpful or correct answers

Level 1
Level 1

Hi IngGreardo013,

Prior to clearing the bindings and conflicts, did you notice anything strange in the "show ip dhcp bind" and "show ip dhcp conflict" output?

What is the lease configuration on the dhcp pool? Is it infinite? Please paste the output of the dhcp config from "show run"


Arun Kumar

Please rate useful posts !!

We have less than 254 phones, and there are many conflict ip addresses when we execute the show ip dhcp conflicts.

ip dhcp pool voice



   option 150 ip

   lease 7



Where are your excluded addresses??

conf t

ip dhcp excluded-address low-address [high-address]

we have these excluded address:

ip dhcp excluded-address,

today happended again, 3 phones can not registered, they stayed in "Configuring IP address" state, I typed the command clear ip dhcp conflicts * to solve (for some days) the problem, and I executed the command show ip dhcp conflicts and show us conflicted IPs (like 15 Ip address phones)


What can be the problem??

Try using "no ip dhcp conflict logging" under global config.  This way you will not have to manually clear these.  The DHCP server will still ping to make sure the address is available for assigning so this shouldn't cause any issues.

Ok, I will try it and I will monitor the phones, thank you Brian

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