DHCP - Cisco Client ID format

Cisco devices that act as DHCP clients will by default use a proprietary client ID format. The format is the following:

cisco-5254.0006.dbd9-Gi0/1

It begins with the word "cisco" followed by the MAC address, and the interface number. This value is actually stored in the DHCP message in ASCII code. In the output of various show commands, this client ID may be displayed as a hexadecimal value as well. For example:

DHCP#show ip dhcp binding 
Bindings from all pools not associated with VRF:
IP address          Client-ID/	 	    Lease expiration        Type
		    Hardware address/
		    User name
192.168.12.2      0063.6973.636f.2d63.    Mar 02 2002 12:24 AM    Automatic
                    6330.372e.3132.3265.
                    2e30.3030.302d.4661.
                    302f.30

Note that the client ID is the following hex value displayed above:

0063.6973.636f.2d63.6330.372e.3132.3265.2e30.3030.302d.4661.302f.30

If we were to translate this hex value to ASCII, we get cisco-5254.0006.dbd9-Gi0/1 which is the initial client ID of the client.

Cisco devices can use a wide variety of Client ID formats beyond this default. The following context sensitive help from the CLI of a Cisco router shows the various options:

R1(config-if)#ip dhcp client client-id ?
  Async              Async interface
  Auto-Template      Auto-Template interface
  BDI                Bridge-Domain interface
  BVI                Bridge-Group Virtual Interface
  CDMA-Ix            CDMA Ix interface
  CTunnel            CTunnel interface
  Dialer             Dialer interface
  GMPLS              MPLS interface
  GigabitEthernet    GigabitEthernet IEEE 802.3z
  LISP               Locator/ID Separation Protocol Virtual Interface
  LongReachEthernet  Long-Reach Ethernet interface
  Loopback           Loopback interface
  Lspvif             LSP virtual interface
  MFR                Multilink Frame Relay bundle interface
  Multilink          Multilink-group interface
  Null               Null interface
  Tunnel             Tunnel interface
  Vif                PGM Multicast Host interface
  Virtual-PPP        Virtual PPP interface
  Virtual-Template   Virtual Template interface
  Virtual-TokenRing  Virtual TokenRing
  ascii              Client-ID as ascii string
  hex                Client-ID in hex
  vmi                Virtual Multipoint Interface

R1(config-if)#ip dhcp client client-id

The default is the ascii option. However, you can actually change the string you use by issuing anything you like as the client ID.

The MAC address can be configured to be used as the client ID using the following interface command:

R1(config-if)#ip dhcp client client-id gigabitethernet

https://forum.networklessons.com/t/cisco-ios-dhcp-client-identifier/1284/7?u=lagapidis

https://networklessons.com/cisco/ccie-enterprise-infrastructure/cisco-ios-dhcp-client-identifier

https://www.cisco.com/c/en/us/td/docs/ios/12_2sb/12_2sba/feature/guide/sbhcpcf.html#:~:text=By%20default%2C%20the%20client%20identifier,address%20of%20the%20named%20interface.&text=(Optional)%20The%20interface%20name%20from,the%20MAC%20address%20is%20taken.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr/command/ipaddr-cr-book/ipaddr-r1.html#wp2790763288

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr/command/ipaddr-cr-book/ipaddr-i1.html#wp1820979811

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