Cisco switch (3560-48TS) DHCP issue

paradigm

paradigm

paradigm

Caporegime
Joined
26 Aug 2003
Posts
37,508
Location
Leafy Cheshire
Hi guys,

We've got a small comms cab in one of our warehouses that contains a 3560-48TS switch, and serves two departments (on two different subnets). Everything was working absolutely fine until about mid-way through last week, when clients have started to not pick up DHCP addresses from our DHCP server (Windows 2008 R2 box if it makes a difference). Giving clients a static IP gives them back connectivity, so its not as if data is completely failing.

Everything was working fine, and upon checking the config, nothing appears to have changed. More bizzarely, is that the DHCP server is still serving every other subnet without fail, so I have my doubts that the issue lies with the DHCP server.

I'm out of things to check though, and it's driving me nuts.
 
If I'm reading this right, the clients which are not picking up IP's from the DHCP now - are they all located in the second subnet which doesn't contain the physical DHCP server?
 
Basically, servers are on 10.1.1.0/24, management VLAN for network equipment is 10.0.0.0, and we have a number of departmental subnets 10.1.3.0/24 up to 10.1.12.0/24.

It appears it isn't a problem with the switches however, as even plugging directly into the core stack where the servers are directly connected, results in the same problem, of no DHCP leases being renewed. After a little more digging, it appears that it is only two subnets being affected 10.1.7.0/24 and 10.1.8.0/24.

They are being returned NACK responses from the DHCP server, doesn't matter what switch, port, or trunk I'm coming in on, the result is the same, those two vlans are only recieving NACK responses from the DHCP. All other subnets/scopes are fine.

Re-creating the scopes makes no difference, restarting the DHCP services or whole server makes no difference, restarting the switches makes no difference.

Oddly, the clients on those subnets are recieving the following in the event viewer:

The IP address lease 0.0.0.0 for the Network Card with network address 001A6B5D5E95 has been denied by the DHCP server 10.1.1.10 (The DHCP Server sent a DHCPNACK message).
 
Has something changed in your config regarding DHCP helper addresses or similar?

you could grab a wireshark capture of the DHCP exchange, from the server & the client if possible & see if something in the DHCP request is wrong, either from the client or once it gets to the server.
 
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