DHCP Binding in IOS

Answered Question
Apr 13th, 2010

Using ' show ip DHCP bindings ' on a router operating as a DHCP server I have strange entries under the DHCP client-id/hardware address ( example below ) , should they correlate to the clients MAC address ?

IP address       Client-ID/              Lease expiration        Type

                 Hardware address

192.168.1.3      0063.6973.636f.2d30.    Mar 16 1993 10:09 PM    Automatic

                 3030.612e.6238.3939.

                 2e31.3163.352d.566c.

                 3130.31

192.168.1.4      0063.6973.636f.2d30.    Mar 16 1993 10:10 PM    Automatic

                 3030.662e.3233.3339.

                 2e36.3963.332d.566c.

                 3130.31

I have this problem too.
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Correct Answer by Shahaludeen N about 6 years 8 months ago

That string represents the ASCII equivalent of the client identifier. For eg. if you convert the client ID(0063.6973.636f.2d30.3030.612e.6238.3939.2e31.3163.352d.566c.3130.31) by using the ASCII table, you will get cisco-000a.b899.11c5-Vl101. The format cisco uses is cisco--. So here in your case I assume you assigned dhcp address to your 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, you must be certain that no other client is using the client ID you choose, and you 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 you just use ip address dhcp without any options. You can override the default by using ip address dhcp client-id <interface> command, 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 you can change it with ip address dhcp hostname host-name.

Shahal, CCIE

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Correct Answer
Shahaludeen N Tue, 04/13/2010 - 16:39

That string represents the ASCII equivalent of the client identifier. For eg. if you convert the client ID(0063.6973.636f.2d30.3030.612e.6238.3939.2e31.3163.352d.566c.3130.31) by using the ASCII table, you will get cisco-000a.b899.11c5-Vl101. The format cisco uses is cisco--. So here in your case I assume you assigned dhcp address to your 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, you must be certain that no other client is using the client ID you choose, and you 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 you just use ip address dhcp without any options. You can override the default by using ip address dhcp client-id <interface> command, 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 you can change it with ip address dhcp hostname host-name.

Shahal, CCIE

rwilletts Wed, 04/14/2010 - 00:24

Hi Shalal,

That's great ! - thanks for the reply

Can you walk me through how you converted (0063.6973.636f.2d30.3030.612e.6238.3939.2e31.3163.352d.566c.3130.31) to  cisco-000a.b899.11c5-Vl101

I can't quite see how the conversion works , I'm OK with dec to HEX etc ... but can't match the string ?

Thanks again for your help

Shahaludeen N Wed, 04/14/2010 - 19:39

Hi,

What I mean is the ASCII value(in Hex) of the character. You can find a few ASCII codes below but you can 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 eg. the string 'cisco' will be represented in ASCII (hex) as 63 69 73 63 6f

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

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

Does that make sense now?.

Regards,

Shahal.

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