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

[Marko Milivojevic]

> 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.


Marko.