Re: DHCP Option 82

Hi,

DHCP option 82 is an optional feature that allows the relay agent to insert
circuit specific information into a request being forwarded to a DHCP
server. This option 82 information can be used by the DHCP server to
implement policies (allocation of addresses based on the requesting subnet
(giaddress) as well as the circuit-id information (e.g: Port on the switch
on which the client resides). For instance, you can have 12 clients on a
common VLAN, and you would like to give 6 of them from a certain range
within the pool, and the other 6 clients from another range in the pool
based on the circuit-id (tied to the switch interface).

For instance:
You can create a pool for a subnet (172.16.12.0/24), within the pool you can
create classes. Each class will have a range of option 82 values, which will
match a range of circuit-ids of the client ports. Thus a client connected to
a certain port, the circuit-id of that port will determine to which class
the client belongs to and thus will issue the client an ip address from a
certain range within that pool. (172.16.12.1 - 172.16.12.20).

So the circuit-id matches option 82 hex value corresponding to the class
which in turn corresponds to a range of ip addresses within the
subnet(pool).

Sample Configuration(Cisco's Website):

Defines the DHCP classes and relay information patterns

ip dhcp class CLASS1

 relay agent information

  relay-information hex 01030a0b0c02050000000123

  relay-information hex 01030a0b0c02*

  relay-information hex 01030a0b0c02050000000000 bitmask
0000000000000000000000FF

ip dhcp class CLASS2

 relay agent information

  relay-information hex 01040102030402020102

  relay-information hex 01040101030402020102

ip dhcp class CLASS3

 relay agent information

! Associates the DHCP pool with DHCP classes

ip dhcp pool ABC

 network 10.0.20.0 255.255.255.0

 class CLASS1

  address range 10.0.20.1 10.0.20.100

class CLASS2

  address range 10.0.20.101 10.0.20.200

 class CLASS3

  address range 10.0.20.201 10.0.20.254

For more information on this topic check:

http://blog.internetworkexpert.com/2009/07/22/understanding-dhcp-option-82/

http://www.faqs.org/rfcs/rfc3046.html

Best Regards,
On Wed, Oct 28, 2009 at 10:30 AM, Antonio Saez <
antonio.saez.jimenez_at_gmail.com> wrote:

> Hi all,
>
>
>
> I am Reading the doc about dhcp option 82 and not sure I understand it
> completely. What is it used for? Is needed for dhcp snoop feature?
>
> Sorry for these basic questions but do not have switches to test it.
>
>
>
> Thanks all
>
>
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-- 
KJ
Blogs and organic groups at http://www.ccie.net