We have a windows 2003 DHCP server. We recently had hardware problems and experienced a DHCP outage, we want to ensure this does not happen again by setting up a seconday DHCP server. I have read that the reccomended method is to configure two DHCP servers with the same scope, but add half of the range to the exclusions on one, and the other have to the exclusions on the other.
From my understanding of DHCP (correct me if I am wrong), the client sends a broadcast request to the subnet requesting an IP address, and when a dhcp server receives this request, it issues an IP. Therefore, is there no way of knowing which DHCP server is going to service the request?
This problem would seem even worse in an 80/20 setup, what if the DHCP server that serves the 20% fills up long before the server serving the 80%, and the 80% server dies? Ideally you would not want the 20% server to issue leases unless the 80% server is unavailable.
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by: CGretskiPosted on 2009-06-03 at 07:28:18ID: 24536932
It all depends on how many addresses you've got in your subnet (if it's /24 then 254 addresses) and how many you'll need to use.
Ideally you'd have a 100/100 setup, so either server has enough addresses for all your clients, but that only works if you've got plenty of free addresses.
There is no concept of a primary DHCP server, it is purely decided by which answers the request quickest. If the secondary DHCP server was on another site & the router was relaying the DHCP request then that delay could be enough to make the local one serve most requests.
Once a client has an IP address it will try renew it when it's close to expiry, and it will do this from the server that originally gave it the address. If that server cannot be contacted then it will keep trying until the address does expire, at which point it will begin broadcasting to any DHCP server for an address.