Cisco IOS DHCP Client Identifier

Most host devices that use DHCP client will use their MAC address as the client ID. Cisco IOS however uses one of their own formats, this can be a problem if you need to get an IP address from the DHCP server based on your MAC address. For example, some cable providers check the MAC address of a client before handing out an IP address.

Of course we can change this behavior on Cisco IOS to make it use the MAC address when it requests an IP address. Let’s take a look at the default behavior and how we can change it.

First of all, here’s the MAC address of R1:

R1#show interfaces FastEthernet 0/0 | incl bia    
  Hardware is MV96340 Ethernet, address is 001d.a18b.36d0 (bia 001d.a18b.36d0)

Before I enable DHCP client on this interface, let’s enable a debug. This will allow us to see the client ID:

R1#debug dhcp detail 
DHCP client activity debugging is on (detailed)

Great, now let’s enable DHCP client:

R1(config)#interface FastEthernet 0/0
R1(config-if)#ip address dhcp

Within the DHCP request you will see the Client ID:

DHCP: SRequest attempt # 1 for entry:
Temp IP addr: 192.168.12.8  for peer on Interface: FastEthernet0/0
Temp  sub net mask: 255.255.255.0
   DHCP Lease server: 192.168.12.2, state: 4 Requesting
   DHCP transaction id: 16F3
   Lease: 86400 secs,  Renewal: 0 secs,  Rebind: 0 secs
   Next timer fires after: 00:00:03
   Retry count: 1   Client-ID: cisco-001d.a18b.36d0-Fa0/0
   Client-ID hex dump: 636973636F2D303031642E613138622E
                       333664302D4661302F30
   Hostname: R1

The Client ID starts with cisco, has the MAC address in the middle and ends with the interface number. Now let’s change it so it only uses the MAC address:

R1(config)#interface FastEthernet 0/0
R1(config-if)#ip address dhcp client-id FastEthernet 0/0

The command above tells the router to use the MAC address on the FastEthernet 0/0 interface as the client ID. You could use the MAC address of another interface if you want. Here’s what the DHCP request looks like now:

DHCP: SRequest attempt # 1 for entry:
Temp IP addr: 192.168.12.7  for peer on Interface: FastEthernet0/0
Temp  sub net mask: 255.255.255.0
   DHCP Lease server: 192.168.12.2, state: 4 Requesting
   DHCP transaction id: 95
   Lease: 86400 secs,  Renewal: 0 secs,  Rebind: 0 secs
   Next timer fires after: 00:00:03
   Retry count: 1   Client-ID: 001d.a18b.36d0
   Client-ID hex dump: 001DA18B36D0
   Hostname: R1

This is looking better, the client ID is now only the MAC address. That should solve the problem. I hope this lesson has been useful, if you have any questions feel free to leave a comment in our forum.