[c-nsp] dhcp failover - 3550 ???

[Kristofer Sigurdsson]

Marko Milivojevic, Fri, Aug 27, 2004 at 12:34:26PM -0000 :
> >Given 2x3550 EMI, could they be setup as DHCP servers running in some sort 
> >of failover configuration?
> >I'd assume not, but it doesn't hurt to ask ;-)
> 
>    Let me start with a disclaimer: This is just theory. I have not tried 
> this. I may be overseeing some limitation, but I may not. I don't have 
> spare 3550-EMI's at the moment to try. If someone has, please, feel free to 
> do so.
> 
>    You say you have 2 3550's. I presume they are "default gateways" for 
> local LAN and that they have L3 interfaces. Chances are, they are running 
> HSRP? If they do, my trick may work (with a certain limitation) :-)
> 
>    Configure 3550's L3 interfaces with IP addresses not used on LAN. 
> Configure HSRP with virtual IP that is used as gateway.
> 
>    When you configure DHCP server, it will not respond to client request 
> unless it has interface with IP address that would match replies from the 
> pool. In theory, active HSRP switch would respond to the query.
> 
>    Of course, limitation is when the primary fails. I am not aware of the 
> DHCP database synchronization between the two. You might be able to pull 
> something off in this sense, but I am not sure.

How about using an external DHCP server, configure it's IP address as a
helper address on the layer 3 interfaces?  The DHCP server would get a
request from, say, 10.10.10.254, it would look it up in it's subnetwork specifications,
see the 10.10.10.0/24 network and assign addresses from a designated range within
that network, while requests from the IP addresses not in that range (the "real"
L3 addresses) would go unanswered, as the DHCP server would not have a subnet specification
for those.

Of course, you could have two DHCP servers for redundancy, ISC DHCP 3 has some impressive
features in that department.

-- 
Kristófer Sigurðsson		   | Tel: +354 525 4103 / MSN: ks at rhi.hi.is
Netsérfræðingur/Network specialist | Reiknistofnun HÍ/University of Iceland