Tag Archives: dhcp

Office Phones Down on Monday Morning – DHCP Issue

If you are a Network Engineer, then having your phone ring early on a Monday morning is never a good thing. And that’s what happened this morning. Seems that none of our Cisco phones were working at the Corporate office…yep, none, as in 150 employees. (And it always seems to be the big offices…why does nothing go down at a little remote office that only has 3 employees?)

After talking to a couple of users (via cell phone of course), I realized that all of the phones were trying to get an IP address but were unable to.  (Phones were displaying:  Configuring IP). Well that is very interesting…and I recalled that the DHCP server was just replaced this past Friday. (Grrrrrr….how do you mess up a DHCP server?)

Well, it’s easy to mess things up if you don’t use the same IP address on the new server as the old server. Remember, DHCP is a layer two broadcast mechanism…the device booting up sends out a DHCP Request packet destined for all F’s, and the server responds.  Now, if the server is NOT on the same local network, then you need an IP helper statement such as this to properly forward the DHCP request…

interface Vlan11
description VoIP VLAN for Corporate office
ip address
ip helper-address

As you can see the switch will take any DHCP requests and forward as a unicast packet to the DHCP server. However, the new DHCP server had a different IP address. Why? I don’t know…however the server guy did fix the issue quickly, which was much appreciated. He simply changed the IP address of the new server to that of the old server…and boom, all of the phones started registering. You may be wondering about all of the PC’s on the network…they were working just fine as they were on the same local network as the DHCP server.

I then was able to finish shaving and get ready for the day. You just have to love Mondays…

DHCP Scope Configuration – Oops

So for the last couple of days, I’ve been in Nevada at one of our remote sites. (On a side note, the “middle of nowhere” pretty much describes all of Nevada!!) I was setting up a wireless bridge to connect separate parts of a large aggregate plant…it was a very busy few days. We kept running into problems, which took up a lot of time to resolve, but eventually we got things working. As I was testing the new subnet hanging off the bridge, I noticed that DHCP was not working…hmmm, very strange. I’ve configured DHCP many times over the years, and it just works. Time to troubleshoot…

First test was easy…I configured a static IP on my laptop and everything worked great. Next I drove over to the other end which housed the main switch and router, and plugged into a port configured for the new VLAN….and no DHCP. Say what? Hmmm…I must have made a mistake on my configuration…but the DHCP pool looks good…

Config for the DHCP pool

Config for the DHCP pool

And the subinterface configuration looks good too…

Sub-interface config looks good too

Sub-interface config looks good too

Very interesting…the only thing left was the DCHP excluded-address config, but that’s so easy, I know that’s not the problem.  But I checked it out anyways…

DHCP excluded-address config....oops

DHCP excluded-address config….oops

Say what?? How could I have messed that up? But I have to say, the configuration was doing exactly what I asked it to do…basically not handing out any IP’s!! So after a quick edit, everything was working properly…

The proper excluded-address configuration

The proper excluded-address configuration

So remember, most of the time, it will be the simple things that get you.