DHCP Relay & Option 82

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Aug 25 19:00:39 UTC 2010

Alan Bryant wrote:
>On Wed, Aug 25, 2010 at 11:19 AM, Bruce Hudson <Bruce.Hudson at dal.ca> wrote:
>>     My assumption, largely inferred I admit, was that the network was
>>  still flat and that they were trying to reconfigure their IP addresses
>>  to facilitate a future split of the the network. That was why they
>>  were using option 82 to determine where the breaks will be, making the
>>  upcoming split less painful.
>You are exactly right. We currently have a flat network and are using
>this as the next step in getting it routed.

To be honest, I'm not convinced the intermediate stage is likely to 
be that productive. It's likely to cause network upheaval across the 
whole network, and still won't prevent more upheaval when you do 
convert to a routed network. Not only that, but if there are any 
badly behaved clients then you also run the risk of some interesting 
effects when you do switch.

For example, suppose the client doesn't act on the change of subnet 
when it switches from being at (say) to - ie the address is still valid, but the subnet mask 
(and router) have changed ? I don't know if there are any clients 
that badly behaved, but I do know there have been many reports of 
bugs where changes made after a lease was initially acquired where 
not put into effect by the client.

Personally, I'd have tackled it by picking on sections of the network 
and converting them one at a time - sorting out any problems as we 
go. Set up the DHCP in advance (all subnets declared, fixed address 
statements in place etc). One problem with this approach is that your 
old and new subnets cannot overlap.

Also, this may be tempered by the resources available to you - eg if 
you have to get outside contractors in to re-configure 
routers/switches etc and so it would cost a lot to do it piecemeal.

Simon Hobson

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