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Cisco C4500 DHCP problems - VOIP terminals


I have had some problems with the DHCP servers.

The siemens VOIP equipment requires the Option43 to be setuped, first the terminal gets the data vlan and the the voice VLAN address.

I have disabled the no ip dhcp conflict command, because I sometimes get power outage and reload of the device.

The problem I have, is that some phones are working, and after a while the loose the addres.

When I use the show ip arp command, someties I see several IP addresses from there, and sometimes there is an issue that the other IP

address is asociated with the mac address in the show ip dhcp binding command.

And in some cases one adapter is trying to get the address of the another VOIP terminal or phone.

Any help would be appreciated.

Thanks in advance.

4 Replies 4

Partha Dasgupta
Cisco Employee
Cisco Employee


Correct me if i m wrong.

- I understand 4500 is configured as DHCP server

-no ip dhcp conflict  configured on 4500 because some times there is power outage and 4500 gets reloaded

Please provide the configuration of DHCP service




Yes you are correct. 4500 is a dhcp server.

Here are the configs for the DHCP data and VOICE vlan

ip dhcp pool telefon-pool



   lease 7

ip dhcp pool DADA




   option 43 hex 010a.5369.656d.656e.7300.0000.0204.0000.0064.0000.0000.00ff

   lease 7

I have narrow something down with the ARP and the MAC agging timeout.

If I use the Wireshark to capture packets, I can see that, when I reset the L2 switches, C2960 that are connected to L3 C4500, and force the VOIP siemens telephone to renew the DHCP, I get the DHCP decline message fromt he VOIP telephone. It is always asking for an IP addres, with DHCP option 50 , with an address that is already in use.

Thanks in advance.

I have done the arp timeout on the DATA and VOICE VLAN SVI for 300 seconds, and the mac address aging time 500 secodns global. any help here ?


Yes , you can definately try this option and verify



I have tunned the ARP timeout on the Voice and Data vlan to 4 minutes. And setuped the mac agging to the value of 500.

I have cleared the IP dhcp binding database, and reload the router C4500 . In the Voice Vlan there are about 600 IP telephones.

And I have got some of them here more with the DHCP decline option, saying that they are requiesting and IP address that is already in use. When I do the cleari ip arp, and the clear ip dhcp binding on that particular IP , the Ip telephone gets and address and works fine.

After a couple of days the problem happens again.

Anyone with some ideas ?

Thanks in advance.

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