DHCP Binding in IOS: Entries under DHCP client-id/hardware address

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Jan 17, 2011 10:49 PM
Jan 17th, 2011

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The string under the DHCP client-id/hardware address represents the ASCII equivalent of the client identifier. See the output of 'show ip DHCP bindings' on a router operating as a DHCP server:

IP address

Client-ID/ Hardware   address

Lease expiration

Type

192.168.1.3     

0063.6973.636f.2d30.3030.612e.6238.3939.2e31.3163.352d.566c.3130.31

Mar 16 1993 10:09 PM

Automatic

192.168.1.4

0063.6973.636f.2d30.3030.662e.3233.3339.2e36.3963.332d.566c.3130.31

Mar 16 1993 10:10 PM

Automatic

This string represents the ASCII equivalent of the client identifier. For example, if we convert the client ID (0063.6973.636f.2d30.3030.612e.6238.3939.2e31.3163.352d.566c.3130.31) by using the ASCII table, we will get cisco-000a.b899.11c5-Vl101. The format Cisco uses is cisco-<mac-address>-<interface>.

This is the ASCII value (in Hex) of the character. We can find a few ASCII codes below but we need to search for a full set.

Hex          Decimal          Character (symbol)

-----------------------------------------------------------------

0x30          48                    0

0x31          49                    1

0x32          50                    2

....

0x39          57                    9

0x61          97                    a

0x62          98                    b

0x63          99                    c

.....

0x7a          122                  z

-------------------------------------------------------------------------------------------------------------

For example, the string 'cisco' will be represented in ASCII (hex) as 63 69 73 63 6f

----------------------------------------------------------------------------------------c   i    s   c   o

So here in this example we assigned DHCP address to our vlan101 interface.

By default, DHCP implementations typically employ the client's MAC address for this purpose, but the DHCP protocol allows other options as well. In any case, in order for DHCP to function, we must be certain that no other client is using the client ID we choose, and we must be sure the DHCP server will accept it. What is termed the Client ID for the purposes of the DHCP protocol is whatever is used by the protocol to identify the client computer.

A client identifier (DHCP option 61) can be a hexadecimal or an ASCII value. By default, the client identifier is an ASCII value. That is what happens if we just use ip address dhcp without any options. We can override the default by using ip address dhcp client-id <interface> command, and then it will use the hexadecimal MAC address of the named interface as the client identifier.

There is also DHCP option 12 that is used to send the hostname to the Server. By default option 12 also will be sent with the configured hostname as the value, but we can change it with ip address dhcp hostname host-name.

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Posted January 17, 2011 at 10:49 PM
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