dhcp failover question

[Julie Xu]

Hi,

 

I am dreaming to put a large subnet, assume it is 10.1.0.0/20. 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?

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

-          Can I setup them behind LB?

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

 

Any comments will be appreciated

 

Thanks in advance

 

 

Julie Xu

 

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[Simon Hobson]

Julie Xu <[hidden email]> wrote:

> I am dreaming to put a large subnet, assume it is 10.1.0.0/20. 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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[Rob Moser]

Simon Hobson <[hidden email]> wrote:
   
> Julie Xu <[hidden email]> wrote:

> > -          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.

Well to be fair, most modern load-balancers will have some method of sending the same client persistently to the same back-end server.  But how to achieve that would be a question for the folks who built/run your load-balancer; dhcpd doesn't have anything to do with it.

     - rob.
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[Simon Hobson]

Rob Moser <[hidden email]> wrote:

>>> -          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.
>
> Well to be fair, most modern load-balancers will have some method of sending the same client persistently to the same back-end server.  But how to achieve that would be a question for the folks who built/run your load-balancer; dhcpd doesn't have anything to do with it.

But what happens if a server goes down ? Presumably the LB will re-route requests to a different server - which of course will have no knowledge of what the others servers have been doing.

So you either have to have non-overlapping ranges across the servers - in other words, the same as just running multiple independent servers and the LB does nothing useful for you.
Or the servers have to have knowledge of what the other servers have been doing - in other words, like running failover.


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