Option 50 in failover mode

[Randall C Grimshaw]

Randall Grimshaw rgrimsha at syr.edu
________________________________________
From: dhcp-users-bounces+rgrimsha=syr.edu at lists.isc.org [dhcp-users-bounces+rgrimsha=syr.edu at lists.isc.org] on behalf of Simon Hobson [dhcp1 at thehobsons.co.uk]
Sent: Sunday, November 27, 2011 11:38 AM
To: Users of ISC DHCP
Subject: Re: Option 50 in failover mode

Bob Proulx wrote:

>When two dhcpd servers are configured in failover mode then it is
>critical that you have enough IP address space in your pool that
>either single server's pool is large enough for the entire network.

For the record, lest anyone believe this is universally true - it
isn't. It is true if you want "hands off" handling of failures :

>At this point several people will jump in talking about putting the
>single remaining server (after the other failed) into partner-down
>mode in order to move allocatable ip space from the down server to the
>remaining server.  That is all well and good but if you are available
>to do that then you would also be available simply to repair the
>failed server and get it back online.

Some people have scripted this - it can be automated.

Bear in mind that if you rely on each server having a separate pool
capable of serving the network (which incidentally is NOT failover),
then in the event of a server failing, all the clients using that
server will get new addresses which may or may not cause problems. It
may not be an issue if you have long leases, but if your leases are a
bit on the short side, you could cause a lot of your clients to
change address mid-session and thus drop all their connections.

With failover, once you put a server into partner down state, it can
handle the clients issued leases by the failed server without forcing
clients to change addresses.

--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.
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