[c-nsp] Redundant DHCP Server

Peter Rathlev peter at rathlev.dk
Tue Jun 2 06:29:06 EDT 2015

Hi Mohammad,

On Tue, 2015-06-02 at 11:05 +0300, Mohammad Khalil wrote:
> Usually I rely on DHCPD package on Linux distributions to configure my
> DHCP server , but the issue is that my client wants his MLS to do that
> job 

That's a fair argument. We use IOS routers as DHCP servers for some
remote locations where deploying a server would be overkill.

Insisting on using IOS as a DHCP server does mean that they will be
severely limited regarding functionality though.

> Is there a way that I can remove the unused releases from the database
> like my case where I am using two hosts only ?

Leases ("bindings" in IOS) could be cleared/removed with a CLI command:

  Router# clear ip dhcp binding

Choosing what bindings to clear can be tricky. Until the first renew
there is, as far as I can tell, no way of knowing if the lease was
actually taken by the client in IOS.

If the client doesn't renew the lease it should automatically disappear
from the bindings list when it expires after at most one lease-time

> Is there a way that I can make the DHCP assign addresses for the
> clients in a round-robin fashion?

Both servers always offer a lease and the client decides which to
accept. That would typically be the offer first received. Having two
similar DHCP servers at roughly the same distance to the client should
balance the leases between the two servers.

The two DHCP servers never exchange information so there's no way of
letting them coordinate which of them answers what clients.

There is also apparently no way of delaying answers from one server,
like you can with e.g. ISC DHCP. But that would of course do the
opposite of what you want. :-)

All in all I think you have to accept that each server registers each
client on every DISCOVER/OFFER exchange. But since you would need room
for all clients on each server it shouldn't be a problem.


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