Failover Clarification

Glenn Satchell Glenn.Satchell at
Wed May 9 13:02:09 UTC 2007

>From: "Benjamin Wiechman" <benw at>
>To: <dhcp-users at>
>Subject: RE: Failover Clarification
>Date: Tue, 8 May 2007 19:33:05 -0500
>-----Original Message-----
>From: dhcp-users-bounce at [mailto:dhcp-users-bounce at] On Behalf
>Of Simon Hobson
>Sent: Tuesday, May 08, 2007 5:20 PM
>To: dhcp-users at
>Subject: RE: Failover Clarification
>Benjamin Wiechman wrote:
>>Back to the original question... how well is a failover configuration going
>>to handle this type of network?
> From my limited knowledge of failover, not at all. Firstly the 
>balancing algorithms don't work well with very few available 
>addresses. Secondly IIRC there are issues where the surviving server 
>needs additional addresses when one server goes down - but I don't 
>deal with failover personally so I don't know the details.
>This is what I'm afraid of. If one server goes down, as far as I understand
>what I am reading, the remaining server will only hand out its share of
>addresses unless it is specifically placed into a partner-down state, at
>which point it will start using all the addresses in the pool. With only a
>couple of spare addresses this would have to done fairly quickly, or the max
>lease time would have to be lowered to an almost unmanageable time period.
>I also have not been able to determine whether the two peer servers will
>communicate to determine how many addresses are actually free. For example,
>if one server has 0 free leases and it receives a request for an IP, does it
>simply report that there are no free leases, or does it communicate with the
>other peer to determine if there truly are any free leases available?
>Basically what I am seeing is that we would likely want to increase our pool
>size to a point where we would have 5-10 available leases out of the pool. 

The two servers communicate with each other frequently, and go through
various pool balancing exercises so that they tend to keep an equal
number of free leases on each server.

When you're down to the last free IP one server will not reply because
it has no lease to offer. The other server should offer the available
lease that it has.

About a year ago I did some work for a client to set up a script which
ran on each dhcp server. When communication was lost for a certain
amount of time (30 or 60 minutes I think) it used the omshell interface
to switch the server to partner down. When the partner communications
returns dhcpd automatically return to failover mode by itself.

The details should be in the archives. It was a pretty simple script,
and it only got used once in a DR test where we manually switched off
the network between sites. This client had redundant networks, etc, so
we never had a real failure to test it out.


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