Re: Firming Up Understanding of DHCP Details

And only the first L3 hop/RA/helper-address interface will look into or be
concerned with the Option82 or lack of Option82 information?

On Tue, May 11, 2010 at 12:12 AM, Tyson Scott <tscott_at_ipexpert.com> wrote:

> Yes is the answer to your lengthy email. Option 82 information is not
> inserted without DHCP snooping enabled also.
>
> Regards,
>
> Tyson Scott - CCIE #13513 R&S, Security, and SP
> Technical Instructor - IPexpert, Inc.
> Mailto: tscott_at_ipexpert.com
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>
>
> -----Original Message-----
> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
> Justin Mann
> Sent: Tuesday, May 11, 2010 12:33 AM
> To: ccielab_at_groupstudy.com
> Subject: Firming Up Understanding of DHCP Details
>
> GS,
>
> First ever post, appreciate any feedback as I don't currently have a
> sniffer/WS setup in my remote rack to clarify some of this but will
> otherwise continue labbing my own tests - around some of the finer DHCP
> implementation questions and verification that my current understanding is
> accurate...
>
> - IOS acts as a DHCP RA when the "ip-helper address x.x.x.x" is set on an
> interface and it will then unicast the DHCP request to the configured
> helper
> address after first updating the "giaddrr" field of the DHCP packets with
> the address of the interface on which it was received (this will be used by
> the server to determine the appropriate pool and unicast reply destination)
> - Additionally the IOS RA can update the Option 82 information in the
> forwarded packet if the task states the the server will use it be enabling
> "ip dhcp relay information option" either globally or per interface (this
> is
> not done by default).
> QUESTION: Will setting this when the DHCP server does not support Option
> 82
> break the request?
> QUESTION: At this point, in a multi-hop topology, does anything else need
> to be set on the additional L3 interfaces in the path to allow this to work
> if the DHCP server requires Option 82 information?
>
> - If an IOS RA receives a DHCP packet with Option 82 Information but
> nothing
> set in the "giaddr" field (ie 0.0.0.0) it will drop the packet, this could
> happen as a result of the "ip dhcp snooping information option" which is
> enabled by default on IOS switches whenever DHCP Snooping is configured -
> if
> this might happen I need to set one of several "dhcp relay information
> option trust" options on the L3 RA or I can disable the insertion of the
> Option 82 info on the switch
> QUESTION: Will IOS switch ports still update this Option 82 information
> even withOUT DHCP Snooping configured? NOTE: I didn't have a sniffer set
> up to check this.
> QUESTION: I only need to be worried about the first L3 hop/IOS RA correct?
> Assumption is that the first L3 RA/helper-address configured router will
> unicast the packet across the network to the server, transit interfaces
> won't be checking the Option 82 info or "giaddr" fields in the unicast
> packet correct? Or do I need to be concerned about each hop in transit and
> how it handles the Option82 information?
>
> Apologies for the lengthy email - sincere appreciation for anyone who has
> time to pick through and provide answers or clarification (or maybe
> confirmation) where required.
>
> - j
>
>
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Received on Tue May 11 2010 - 09:26:13 ART