split scoping

[Glenn Satchell]

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>Subject: split scoping
>Date: Tue, 24 Nov 2009 11:39:30 -0800
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>From: "Ausmus, Matt" <mausmus at chapman.edu>
>To: <dhcp-users at lists.isc.org>
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> Hello All,
> 
>  
> 
> We're using ISC DHCP pretty much everywhere in our environment.  It has
> been suggested to me that we start using split scoping so I've started
> doing research on the issue.  ISC includes the draft failover mechanism
> which I'm leaning toward instead of just split scoping.  In fact, all
> the documentation I've found available only refers to setting up Windows
> DHCP servers split scoping but nothing with regards to ISC DHCP servers.
> Split scoping appears that it would work fine and be very quick to setup
> but would be not very elegant.
> 
>  
> 
> What are the pros and cons to using just split scoping between a couple
> of ISC servers vs. using the draft fault tolerance mechanisms built into
> ISC DHCPd?  1 obvious con is that DHCP leases would be split between 2
> different machines.  What are some others and are there any pros to this
> setup?
> 

Hi Matt

Split Scoping exists as a concept because Windows DHCP server doesn't
do failover. So split scoping is a poor mans way to increase
redundancy.

There are scenarios where split scoping would be perfectly adequate and
suitable. Others where the additional complexity of failover is
preferred.

ISC dhcpd failover has been worked on for many years, and I ran
networks using failover 5 years ago with 2000 PCs and 4000 IP Phones in
a corporate environment, where it is expected that getting an IP
address must Just Work. So I think the software is mature enough for
that.

One case where split scoping is good is where you have fixed-address
hosts, in that case it doesn't matter which dhcp server responds, as
the same IP address is always allocated.

On the other hand, if the client is getting a dynamic IP address, and
you don't care if that changes, then split scoping should be ok.

As others have pointed out you need some additional IP space for both
schemes. Not necessarily enough for the whole number of clients to run
on one server, but enough to cover the allocation of new requests
during the time that one (or more) servers may be down. If you use long
enough lease times (like a couple of days), then that may not be an
issue at all.

The advantages of failover is that it is tightly integrated into the
application, so at the dhcp lease level the two servers work together.

You could always try a test. Pick a couple of subnets and setup
failover on one and split scopes on another and see how it goes. Then
you can watch and work out which seems better for your environment.

regards,
-glenn
--
Glenn Satchell                            |  Miss 9: What do you
Uniq Advances Pty Ltd, Sydney Australia   |  do at work Dad?
mailto:glenn.satchell at uniq.com.au         |  Miss 6: He just
http://www.uniq.com.au tel:0409-458-580   |  types random stuff.