problems with DHCP server for IP phone defined in switch 3750

[filiberto.aguirre]

Hi

End user has an stack of 5 switches (3750) where a DHCP server is defined for IP phones (and others DHCP server). There are period of times where many messages about ip cconflict are shown and the phone lost connectivity with CM, so we need to apply the command " clear ip dhcp conflict * " and  " clear arp " in order to solve this issue.

There are more than 90 ip phones conected, but the " sh ip dhcp binding " just show a few ones ip leased.

I have no found the source of  this issue.

any idea about this ??

regards

SW#sh ip dhcp binding

IP address       Client-ID/              Lease expiration        Type
                 Hardware address
172.16.21.76     0100.1956.d667.fa       Sep 01 2011 11:14 AM    Automatic
172.16.21.77     0100.1956.d666.69       Sep 01 2011 01:11 PM    Automatic
172.16.28.108    0128.6aba.08bf.12       Sep 01 2011 10:05 AM    Automatic

ip dhcp pool VoIP210
   network 172.16.21.0 255.255.255.0
   default-router 172.16.21.254
   option 150 ip 172.16.22.210 172.16.22.211

[lgijssel]

With these symptoms, there is only one logical conclusion possible:

A rogue dhcp server is running in your customers network.

This should be easy to detect by checking some phones and verify the ip address of the dhcp server.

regards,

Leo

[filiberto.aguirre]

Hi Leo,

Thanks a lot for your recomendation.

I have already checked 3 IP phones network configuration; one of the appears in the result of " sh ip dhcp binding"

the others two not.  There is no difference in dhcp server IP shown ( IP address of switch core)

I attach screenshots.

sw#sh ip dhcp  binding

IP address       Client-ID/              Lease expiration        Type

                 Hardware address

172.16.21.76     0100.1956.d667.fa       Sep 01 2011 11:14 AM    Automatic

172.16.21.77     0100.1956.d666.69       Sep 01 2011 01:34 PM    Automatic

172.16.28.108    0128.6aba.08bf.12       Sep 01 2011 10:05 AM    Automatic

172.16.28.109    01f4.0b93.46ae.68       Sep 01 2011 04:20 PM    Automatic

regards

[lgijssel]

First of all, you may want to know why there are so few leases on the "real" dhcp server.

The others must be somewhere else.

In the worst case you could even have a split-brain situation in which the two servers have the same ip address.

Perhaps there is something wrong with your stack setup?

Try to ping from a few workstations to the ip address of the dhcp server and verify the mac address.

This should always be the same. Be sure to test from the correct vlan. Happy hunting!

regards,

Leo