Phones Not Receiving DHCP IP

[bergquist]

Hello Experts -

We recently replaced our Call Manager servers, jumping from 4.1 to 7.1 after successfully running DMA backup/restore.

Most everything is working fine except when I've had to add a new ip phone to our system. The problem is that it cannot receive a DHCP ip address from the server.

I've checked all the settings that I can possibly find and everything looks like it is configured correctly. If I manually assign an ip to a phone (we are using mostly 7940's and 7960's), then the phone will boot and register fine and works like it should after flashing its image and getting the correct load and firmware.

Rebooting an existing phone in our environment (phones that were online when we had CM 4.1) will boot up more slowly, but it will finish and come online, so apparently it is caching that info from before.

The problem is with a new phone and the only way for me to get it to work is to manually assign the ip. No other changes were made to our environment, so I don't think it would be useful to try to capture any switch packets. I'm wondering if there is something I've missed with our DHCP setup.

Any advice would be appreciated. My TAC engineer basically gave up on me :-(

[bergquist]

Thanks Aaron. The TAC tech and I did run a capture on the phone, but I'll get on on the server end and see if that helps. I'll post an update when/if I ever find a resolution to this.

If any other ideas, please let me know. Thank you.

[Aaron Harrison]

Hi

What did the phone capture show you?

You presumably see a request go out; do you see anything come back?

Aaron

[bergquist]

According to Cisco TAC, it showed that there was no response which indicated a problem with DHCP, but the exact problem from the capture was apparently inconclusive. I'll try to get one on the server this afternoon to see what I can find.

[Aaron Harrison]

Hi

Yeah, sounds like the way forward - no reply from the server means:

1) didn't reach server

2) server didn't like the request (i.e. didn't match scope)

3) server response didn't get there

A cap should fill you in...

Aaron

[bergquist]

This is the capture we got from the Publisher:

7709    142.424216    0.0.0.0    255.255.255.255    DHCP    DHCP

Discover - Transaction ID 0x7fae8ceb

The phone will also show the 255.255.255.255 for it's DHCP server. Not sure what to make of it.

[Adam Thompson]

Would it be possible to post some screenshots of you DHCP server and scope configurations?

Thanks,

Adam

[bergquist]

Hopefully you can open this. Made with W7 snipping tool.

[Adam Thompson]

It looks to me that your DHCP scope is setup incorrectly.

In the screenshot that you supplied, you have 255.255.0.0 in the 'Subnet IPv4 Address' field. That field is where you are supposed to enter the network address for your DHCP scope. Example: 10.0.1.0

I have attached what your scope should look like.

Thanks,

Adam

Please rate if helpful

[bergquist]

Thanks Adam, that was a good catch. Cisco is the one that set that up, but you're right, it doesn't make sense so I changed it. However, my phone is still not pulling an IP and I restarted the DHCP service as well as erased the config on the phone, yet again.

[Aaron Harrison · Accepted solution]

Hmmm... can't believe we all missed that one!

Well - maybe I'm still being a bit slow, but shouldn't the IPV4 Subnet Address be 10.0.0.0? Since the subnet mask is 255.255.0.0, the address 10.0.1.0 is actually a valid host address, not the subnet address.

I suspect the input validation on this config page is either not good or doesn't exist, but the back-end DHCPD service will be rejecting the config.

Set the subnet address to 10.0.0.0 and restart DHCP monitor. Then if it doesn't work... on my system events relating to the DHCPD are in syslog/messages, maybe monitor that for errors when you restart DHCP Monitor.

Regards

Aaron

[Adam Thompson · Accepted solution]

Aaron,

You are correct, it should be 10.0.0.0. Hopefully changing this and then restarting the DHCP monitor service will get this working.

Thanks,

Adam

[bergquist]

Aaron / Adam -

That did it! You guys are awesome man! Thanks so much for your help on this (and everyone else too). I thought we had to be missing something fairly basic, but Cisco TAC is the one who originally assisted with the config of the DHCP in the first place...

You guys should let Cisco know if you need a job. I'll put in a good word for you

[Aaron Harrison]

Cisco? Wouldn't want to work there

Have to say it's a bit naughty of Cisco to throw a /16 subnet in for a CallManager, given that it will cave in if there are > 1024 devices on the subnet...

Good to hear it's working, credit to Adam for actually reading the screen grab properly!

[bergquist]

I selected both your answers as the corect one so I hope that is refected in your

points/feedback properly.

Really, a big thanks once again guys.

-Brian

[Aaron Harrison]

Can you post the actual capture file?

That's basically a headline, doesn't show the detail enough...