DHCP design question

Unanswered Question
Jun 20th, 2007



Because IP telephony devices are configured to use and rely on a DHCP server for IP configuration information, you must deploy DHCP servers in a redundant fashion. At least two DHCP servers should be deployed within the telephony network such that, if one of the servers fails, the other can continue to answer DHCP client requests.


The above is Cisco recommendation on DHCP server.

But as we know is that DHCP is single point of failure we can not deploy two of them, otherwise phone may get duplicate IP address since the so called redundant dhcp servers may designed to have same scope.

I also copy the following from another cisco link to prove my concept which is only 1 dhcp allowed.



DHCP Server

Only one DHCP server per Cisco Unified CallManager cluster should exist.


Do you think the 1st statement from srnd recommending two dhcp servers is misleading?


I have this problem too.
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Aaron Harrison Wed, 06/20/2007 - 11:46

No - Redundant DHCP is possible in many ways depending on what DHCP server you run:

For Windows servers you have two main options

1) Clustering (typically requiring specific hardware)

2) Create two scopes (identical) on two seperate servers. On server A, create an exclusion that covers the first half of the scope. On Server B, create an exclusion that covers the second half of the scope. This prevents one server from allocating addresses from the other server; and also stops one server from sending out NAKs to DHCP requests/renews intended for the other server.


Server A:



Server B:



I've used this second setup lots of times with no problems.



Please rate helpful posts..

ciscoforum Wed, 06/20/2007 - 13:53

Thanks Aaron

But I have 1 more question with 2nd option:

Assuming there are phones over the wan and with the ip helper-address to point to these two servers, meantime there are ip phones in the same vlan as dhcp server which use broadcast to get the ip address.

My question is:

1.from remote site , if the first listed dhcp server is full, will the phone send the dhcp to the 2nd server?

2.At local lan, if the broadcast being received by two servers at the same time, and one server address is used out which may send no address available kind of message to the phone. Meantime the other server still has addresses available, will this cause problem?

m.s.tyler27 Thu, 06/21/2007 - 05:04

I would think that it would go to the next server as long as you have both listed as IP helper addresses on the Router interface. On large LAN environments packed full of VLANs, that is pretty standard operation.


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