DHCP troubleshooting help needed

[RightIsWrong]

Situation: Decommissioning a server and moving roles (DHCP) from it to another server in the AD.

Problem: New server will not respond to DHCP requests and pass out address.

Steps taken so far:
Transferred the DHCP database/scopes to the new server and authorized it in AD.
Verified using "netsh dhcp show server" that it is authorized and showing up on the network. Unauthorized old scope and attempted to release/renew unsuccessfully.
Searched the Google to no avail
Crawled with a flattened forehead from banging my head against my desk and my tail between my legs to AT 🙂

Please help me find the (probably) simple step that I missed so that I can feel like an idiot.

 

[spidey07]

What are the old and new IP addresses of the servers and are the clients on the same subnet as the servers?

If there is a router between clients and servers there will be a dhcp relay agent (ip helper in cisco terms) and that needs to be changed to point to the new dhcp server.

 

[jlazzaro]

have you hooked up a sniffer and confirmed the DHCPDISCOVER and DHCPOFFER messages are being seen?

 

[Dug]

It could be that you used the same database between both DHCP servers at the same time. Make sure other server is off.
Assuming you are on the same subnet, remove the current scope (after backing it up). Then create a new one from scratch as a test, using different IP #'s.
If the test works, then delete that scope and try to bring up the old database.
Also make sure your DNS sees the DCHP server correctly and vice versa.
Check that your clients don't have login scripts that may interfere

 

[RightIsWrong]

Originally posted by: spidey07
What are the old and new IP addresses of the servers and are the clients on the same subnet as the servers?

If there is a router between clients and servers there will be a dhcp relay agent (ip helper in cisco terms) and that needs to be changed to point to the new dhcp server.

I was thinking about something in the routing tables but I haven't been able to verify this because of the size of the company that I work for (>25k). Different people are assigned different roles and never the two shall intermingle.

I will start there. If that doesn't work, I will go down the line and try jlazzaro's and dug's suggestions.

Should add that I did verify the login.bat and there was nothing more than drive mappings and that the "new" server was already in production and is just being given the DHCP role so DNS shouldn't be an issue.

Not sure if I can try the scope test that Dug is suggesting however. We are a global company and users from all over the world are hitting our servers 24/7/365. Even routine maintenance is scheduled months in advance and with this being something that is just being thrown at me.....

 

[spidey07]

Well if there is a router between them then it is the relay agent on the router that needs to be changed. You can have it pointed to both dhcp servers and then you can control which server is doing dhcp buy starting/stopping the service.

DHCP is a layer2/layer3 broadcast protocol and a router won't pass it unless told to do so.

So it really has nothing to do with routing tables. If the clients and server are on different subnets then the router must be told what to do with dhcp traffic and where to send it. That's what the relay agent/ip helper is for.

Be careful though, a change like this takes some really good planning so that you don't have duplicate addresses out there. Windows is generally pretty good about pinging an address before using it but you shouldn't rely on this.

 

[Dug]

Can you make the new server a backup dhcp server to the old one? Wait for everything to propogate.
Then take the old dhcp server offline and see what happens.
I know that's not optimal but, considering the size of the company, you can't afford downtime.
The only other thing I can think of is that you already have a backup dhcp server that is switching rolls and not allowing the new one to take its place.
But it does sound like the location problem like spidey07 mentioned. It would be helpful to know your IP addresses, and router info.

 

[RightIsWrong]

Originally posted by: Dug
Can you make the new server a backup dhcp server to the old one? Wait for everything to propogate.
Then take the old dhcp server offline and see what happens.
I know that's not optimal but, considering the size of the company, you can't afford downtime.
The only other thing I can think of is that you already have a backup dhcp server that is switching rolls and not allowing the new one to take its place.
But it does sound like the location problem like spidey07 mentioned. It would be helpful to know your IP addresses, and router info.

Thanks for the directional help.

I spoke with someone in the Cali offices where the N. American network team is located and confirmed that the routers are directing traffic.

Unfortunately, I don't have access to the routers to be able to see anything. The company figures that the less hands trying to stir the pot the more likely the recipe will be followed as written.

I'll update this post once I am able to confirm this and test it so that anyone else that might run into this issue and searches might be able to get the help that you have been to me.