Question about DHCP.

Simon Hobson dhcp1 at
Mon Jul 7 22:10:14 UTC 2008

Luis Fernando Lacayo wrote:

>Question  A.
>I have two DHCP servers (dell 2950 with 2 CPU's and 4G or RAM) set up as
>a failover.
>I had a subnet that had ran out of available leases and I had to clear
>the old leases. I stopped but DHCPD servers and deleted both of the
>lease files on each of the two servers.


>any advice is welcomed.

That was (IMHO) a pretty dumb thing to do, and may well cause you 
much grief in the future - depending on your network, client churn 
rate, and lease times.

You now have a situation where your DHCP servers do not know what 
promises they made, and they will be at liberty to try and allocate 
leases to any new devices coming onto the network for addresses that 
are already in use. When this happens, the address will be marked as 
abandoned if the current device responds to pings. Thus you may well 
end up with abandoned addresses.

Further, as devices come up to renew their leases, if the address has 
been marked abandoned then IIRC the server will NACK the request and 
force the client to get a new one. Given the number of addresses in 
use (to exhaust the address pool, I assume you have a number of 
clients close ot the size of the pool), there is a good chance that 
further addresses will be marked as abandoned.

Abandoned addresses are only retried as a last resort, so the result 
could be that you end up with no free leases and a high churn rate.

On the other hand, if no new devices appear, then the servers will 
populate their lease databases as the clients renew.

A second problem awaits you if you have been using dynamic DNS, since 
you have deleted the records of what DNS entries the DHCP servers 
have created. You can expect more fun with stale entries from that.

In future, should this happen again, I would suggest that you simply 
delete some old leases (assuming you are sure the devices are no 
longer on the network).

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