Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Easy question on DHCP design for branch, IP phones

Just to confirm:

Imagine large branch with 1400 cisco ip phones has connectivity via ds3 circuit, MPLS, to hub data center. CUCM 8.5 is on data center.

Then MPLS CE router goes down. At that point lets say DHCP is running in the MPLS CE in the branch. Even if DHCP is running in data center, there is no more connectivity to the DHCP server.

There is a 3945e available for srst and 16 pots lines connected to the router to provide survivability in the branch.


How the fact that there is no dhcp server available going to affect the 1400 Cisco ip phones and registration to the 3945e srst router once the connection to CUCM goes down the hill?  I know if the ip lease is not about to expire in the ip phones, that is ok the phones will hold onto their respective ips. But how about les say if a bunch of phones are about to get ip lease to expire?

Hall of Fame Super Silver

Easy question on DHCP design for branch, IP phones

This is going to depend on DHCP lease time, devices will attempt to re-negotiate IP address with DHCP server half-way thorough the DHCP least time, so imagine your DHCP lease time is 30 days, and the connection is lost you will have another 15 or so day before the phones need to re-negotiate DHCP address. If the DHCP server is still unavailable when the phones need to re-negotiate the address they will no longer function.  With this size of a branch office I would recommend defining DHCP server locally either on a server or a switch/router.

I belive there is a good section on how DHCP works in CUCM SRND doc, if you are intersted in more details.



New Member

Easy question on DHCP design for branch, IP phones

I have been through similar situation in the past.

As you well know, ther is no formula for this.

Well, here is what I would do

for this particular case, your brand office fully qualified to have a local DHCP server since it has lots of phones.

good thing is this branch office DHCP server can be a secondary DHCP server for other location.

I don't want to recommend router/switch as a dhcp server unless ther are small numbers of phones.

not because of capicity, for ease of management (at least for me)

CreatePlease login to create content