dhcp failover question

Simon Hobson dhcp1 at thehobsons.co.uk
Thu Feb 22 20:47:06 UTC 2018

Julie Xu <J.Xu at westernsydney.edu.au> wrote:

> I am dreaming to put a large subnet, assume it is This is for dynamic range. Not reversed ip.
> Assume, I have 4 dhcpd servers. I want put this large subnet for all of them. Can I:
> -          Can I give 4 dhcpd servers subnet statement same? Same gateway address, same boardcast address?

Yes - you normally want to be giving the same gateway etc to all clients.

> -          Can I setup different pool range for every dhcpd server? Average every server has a 1000 dynamic range start with different node number?

Yes, each server can have a different range - but they should NOT overlap.

> -          Can I setup them behind LB?

No. You cannot load balance requests, because it would mean clients randomly contacting a server that knew nothing about it's lease.

> -          Can I not use LB, but, setup VIP around the 4 server? If so, how can I do it?

No, each server must have a unique address.

What you can do is setup the servers in failover pairs (only pairs, you can't have 3 or more sharing a single pool) - pairs are configured per pool, not per subnet. There are different ways of doing this. Lets call the servers A, B, C, D

You could split the subnet into two pools, about 2000 addresses in each. All the servers have the SAME subnet declarations, but the pools (ranges) are different. Servers A and B run a failover pair for one pool; servers C and D run a failover pair for the other pool.

Or you could split the subnet into 4 pools, and split them along these lines :
A & B, B & C, C & D, D & A

Or you can run four independent servers, each managing about 1/4 of the addresses. If one server fails, then clients will eventually get a new address from one of the other servers. This is a fairly simple setup to manage, and as long as you have roughly 1/4 of the addresses spare and are prepared for clients to change address if a server is down for too long then it works without the overhead of failover.

And for anything with a static address, you can put the same host statements in more than one server.

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