MS DHCP NACK with DHCP relay

[Adrian Ardelean]

Hi all,

I have some DHCP trouble since I subnetted my network with a 2921.

My clinets are in 172.16.2.0/23 and DHCP servers are in 172.16.5.0/24.

Sometimes, randomly I guess, I get NACK from my DHCP server, and if I look into DHCP logs I got something like this:

15,11/09/12,09:52:27,NACK,172.16.3.172,switchE51D12.host.com,A0CF5BE51D12,,0,6,,,,,,,,

15,11/09/12,09:52:28,NACK,172.16.3.172,switchE51D12.host.com,A0CF5BE51D12,,0,6,,,,,,,,

15,11/09/12,09:52:29,NACK,172.16.3.172,switchE51D12.host.com,A0CF5BE51D12,,0,6,,,,,,,,

15,11/09/12,09:52:29,NACK,172.16.3.172,switchE51D12.host.com,A0CF5BE51D12,,0,6,,,,,,,,

15,11/09/12,09:52:30,NACK,172.16.3.172,switchE51D12.host.com,A0CF5BE51D12,,0,6,,,,,,,,

15,11/09/12,09:52:31,NACK,172.16.3.172,switchE51D12.host.com,A0CF5BE51D12,,0,6,,,,,,,,

15,11/09/12,09:52:31,NACK,172.16.3.172,switchE51D12.host.com,A0CF5BE51D12,,0,6,,,,,,,,

15,11/09/12,09:52:32,NACK,172.16.3.172,switchE51D12.host.com,A0CF5BE51D12,,0,6,,,,,,,,

He is the interface settings i have:

interface GigabitEthernet0/2

description ....

ip address 172.16.2.1 255.255.254.0

ip helper-address 172.16.5.17

ip helper-address 172.16.5.26

ip policy route-map NET-2

duplex auto

speed auto

!

What am I missing?

Thanks!

[Abzal]

Hi,

Please check you may run out from available addresses on one of th server.

http://technet.microsoft.com/en-us/library/cc956919.aspxhttp://technet.microsoft.com/en-us/library/cc956919.aspx

Please rate helpful posts.

[Adrian Ardelean]

No, absolutely sure not, bacause I have an exclusion range that matches address pool, and it only serves to MACs I enter in reservations.

Forgot to mention, that MAC addr is from the switch where the client is plugged in

Something similar here: http://social.technet.microsoft.com/Forums/en-US/winserverNIS/thread/1377d9ff-e963-485d-b422-137fc71f1c57

but not solved...

[Abzal]

Explain your tnetwork topology how they are subnetted, how are connected dhcp server and workstations?

[Adrian Ardelean]

Two subnets: 172.16.5.0/24 and 172.16.2.0/23. In 2.0/23 are workstations and in 5.0/24 are servers.

The subnets are conected to 2921 router. Obviously, DHCP servers are in 5.0/24

[Sandeep Choudhary]

HI Adrian,

When a DHCP server receives a DHCPRequest with a previously assigned address specified, it first checks to see if it came from the local segment by checking the GIADDR field. If it originated from the local segment, the DHCP server compares the requested address to the IP address and subnet mask belonging to the local interface that received the request.

DHCP server will issue a NAK to the client ONLY IF it is sure that the client, “on the local subnet”, is asking for an address that doesn’t exist on that subnet.

The server will send a NAK EXCEPT in the following scenarios:-

1. Requested address from possibly the same subnet but not in the address pool of the server:-

This can be the failover scenario in which 2 DHCP servers are serving the same subnet so that when one goes down, the other should not NAK to clients which got an IP from the first server.

2. Requested address on a different subnet:-

   If the Address is from the same superscope to which the subnet belongs, DHCP server will ACK the REQUEST.

Regards

[Adrian Ardelean]

I ended up by putting DHCP server in workstations segment

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