DHCP failover setup with several relay agents

Robert Blayzor rblayzor.bulk at inoc.net
Mon Sep 22 14:10:59 UTC 2008


We have a set of DHCP servers setup now that properly work in a  
failover setup. (best we can tell)

In reading the docs and a lot of posts on the list, it seems as if  
this type of setup only works with local LAN based broadcast setups  
where the servers can see all requests coming in.

In our setup, we have no local broadcast DHCP clients and all of our  
requests come pointed to one server IP address from different DHCP  
relay agents (ie: Cisco routers).

The question is, can a DHCP failover setup work for relay agents if  
requests are only directed at one server?  Now, I know that this is  
not the perfect failover setup, as if the primary failed, the agents  
would have to be directed toward the working server.  This can easily  
be fixed with the L3 switch we have in front of the servers that can  
direct traffic for a virtual IP instead of the physical IP's if the  
active server fails.

I think the problem is however that if we just keep sending all  
requests to one server, the IP pools will be "half" of what they  
normally are because the "failover" setup is actually a "load balanced  
with failover" setup.  If that's the case then would it be safe to  
assume them that failover only properly works in a broadcast based  
environment only?

Would it make more sense in this type of setup to just run one active  
server and use something like rsync to send the leases file over to a  
secondary "standby" server?   I was hoping that in DHCP failover one  
could actually be a primary and one could be a standby, but from the  
docs, it doesn't sound like that will work because the secondary unit  
will hold half of the available leases per pool.

-- 
Robert Blayzor, BOFH
INOC, LLC
rblayzor at inoc.net
http://www.inoc.net/~rblayzor/





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