Cisco DHCP with client-identifier 27 bytes

Posted on by zed

How configure the good value for a DHCP reservation with a client-identifier 27 bytes?

R1 will be the dhcp server with a DHCP POOL SERVER3. The client MAC Address will be : aacf.a2e3.aaff

Configuration ont the client :

 interface Ethernet0/0
 mac-address aacf.a2e3.aaff
 ip address dhcp

The problem here is to find the good value for the client identifier with 27 bytes (vendor-xxxx.xxxx.xxxx-Interface)

The first possibility is to find on the Internet a convertor Hex to ASCII.

The other one is to use the debug information on the client to find the good value with debug dhcp detail command.

Now we will shutting down the interface and no shut to generate a DHCP negotiation.

Now we see the good value here :

Retry count: 1 Client-ID: cisco-aacf.a2e3.aaff-Et0/0
 Client-ID hex dump: 636973636F2D616163662E613265332E
 616166662D4574302F30

The request is the following in ASCII : Client-ID: cisco-aacf.a2e3.aaff-Et0/0

In Hexadecimal : 636973636F2D616163662E613265332E616166662D4574302F30

Now you just need to configure the DHCP pool on the server and add “00” to the Hexadecimal value like this :

ip dhcp pool SERVER3
 host 192.168.30.103 255.255.255.0
 client-identifier 00636973636F2D616163662E613265332E616166662D4574302F30

Now the client can receive the IP address :

*Jul 23 17:44:36.638: DHCP: SRequest attempt # 1 for entry:
*Jul 23 17:44:36.638: Temp IP addr: 192.168.30.103 for peer on Interface: Ethernet0/0
*Jul 23 17:44:36.638: Temp sub net mask: 255.255.255.0
*Jul 23 17:44:36.638: DHCP Lease server: 192.168.30.13, state: 4 Requesting
*Jul 23 17:44:36.638: DHCP transaction id: B43
*Jul 23 17:44:36.638: Lease: 86400 secs, Renewal: 0 secs, Rebind: 0 secs
*Jul 23 17:44:36.638: Next timer fires after: 00:00:03
*Jul 23 17:44:36.638: Retry count: 1 Client-ID: cisco-aacf.a2e3.aaff-Et0/0
*Jul 23 17:44:36.638: Client-ID hex dump: 636973636F2D616163662E613265332E
*Jul 23 17:44:36.639: 616166662D4574302F30
<...>
*Jul 23 17:44:39.657: DHCP: Releasing ipl options:
*Jul 23 17:44:39.657: DHCP: Applying DHCP options:
*Jul 23 17:44:39.657: DHCP: Sending notification of ASSIGNMENT:
*Jul 23 17:44:39.657: Address 192.168.30.103 mask 255.255.255.0
*Jul 23 17:44:39.657: DHCP Client Pooling: ***Allocated IP address: 192.168.30.103
*Jul 23 17:44:39.730: Allocated IP address = 192.168.30.103 255.255.255.0

Client(config-if)#do sh ip int brief
 Interface IP-Address OK? Method Status Protocol
 Ethernet0/0 192.168.30.103 YES DHCP up up