DHCP reservation problem

[Infuscomus]

I am encountering a weird anomaly while trying to reserve DHCP address in a CISCO 3750.

Basically, for MAC address AAAA.BBBB.CCCC I want to reserve IP 1.1.1.7, which is in the DHCP pool range.

In the DHCP config I have added the reservation:

address 1.1.1.7 hardware-address AAAA.BBBB.CCCC 

However, instead of the 1.1.1.7 IP address, I get assigned another one 1.1.1.4 which in the show binding config shows as

1.1.1.4 hardware-address 01AA.AABB.BBCC.CC             Feb  Auto

1.1.1.7 hardware-address AAAA.BBBB.CCCC                  Inf    Manual

As it can be seen, the MAC address is shifted and a '01' is added at its start.

In the DHCP pool it displays correctly as:

address 1.1.1.7 hardware-address AAAA.BBBB.CCCC 

So what went wrong  and how can I fix this ?

[Peter Paluch · Accepted solution]

Hi,

In DHCP messages, clients usually identify themselves using the DHCP Option 61, also called the Client ID option. If this option is present in a DHCP client message, IOS will use the Client ID and not the MAC address of the client to look up the static binding.

The "01" at the beginning of the Client ID means that this is an Ethernet MAC address (basically, it is a type identifier). So in order to make your assignments work, replace the hardware-address AAAA.BBBB.CCCC with client-identifier 01AA.AABB.BBCC.CC and it should work.

You may also be interested in checking out these two threads that discuss the same issue:

https://supportforums.cisco.com/discussion/12261746/dhcp-static-lease-cisco-2811

https://supportforums.cisco.com/discussion/12246171/ask-expert-dynamic-host-configuration-protocol-dhcp-and-domain-name-system-dns

Best regards,
Peter

[Infuscomus]

I have removed the hardware-address-based binding.

I then cleared the binding for the .4 IP.

Then I re-added the binding with client-id instead of hardware-address.

Now binding shows:

1.1.1.7 hardware-address 01AA.AABB.BBCC.CC     Inf     Manual

which is correct.

From my initial tests it appears to be working correctly. The new IP address was assigned to the client.

[Terry Cheema]

Have you configured client-identifier:

Router(config)# ip dhcp pool Test

Router(dhcp-config)# host 10.10.10.10 255.255.255.0

Router(dhcp-config)# client-identifier unique-identifier << 01 followed by Mac address

Refer here and compare your config: http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfdhcp.html#wp1001108

If you still have issues post your complete DHCP config and sh ip dhcp bin

-Terry

[Infuscomus]

host cannot be configured under ip dhcp pool  --> This command may not be used with network, origin, vrf or relay pools.

Probably that would be fine on static a pool.