DHCP Failover - initial Configuration

perl-list perl-list at network1.net
Thu Aug 9 13:13:46 UTC 2018

That could be really bad.  MCLT only affects the behavior of the "secondary" failover peer when the "primary" isn't present and how long a "recover-wait" period lasts, as far as I know.  If you set your lease expiry time to 7200 and the length of your snooping/aging setup the same, that could work in most cases.  However, clients are in control of what lease time they want to use.  They can and do, at times, request different lease times than are offered by the DHCP server which could throw your snooping/aging system off.  All that said, I'd think you should set MCLT to the same as your lease length in this situation for maximum compatibility.

----- Original Message -----
> From: "Philippe Maechler" <plcmaechler at gmail.com>
> To: "Users of ISC DHCP" <dhcp-users at lists.isc.org>
> Sent: Thursday, August 9, 2018 2:15:25 AM
> Subject: Re: DHCP Failover - initial Configuration

> Hello Simon, hello list

> On Wed, 8 Aug 2018 at 19:24, Simon Hobson < [ mailto:dhcp1 at thehobsons.co.uk |
> dhcp1 at thehobsons.co.uk ] > wrote:

>> > Server restarts
>>> Currently we restart the service every 5minutes if something changed. When we go
>>> for failover, we should reload server one and if it synced to his partner, we
>>> can reload the server two. How does server two know, that the server one is up
>> > to date and everything is synced?

>> After a restart it will take time for the servers to resync. You'll need to
>> adapt your management system to hold off on restarts. Hopefully someone more
>> familiar with failover will be along soon with more details, but from things
>> said on here, there are some cases where the servers can take a while before
>> they get back to fully normal operation.

> Yes, I'm already testing a way for checking the server state before a reload.
> The current idea is, that our reload script first checks via omapi the
> failover-state from the other server. If the server is in ready and in sync, we
> do the reload. otherwise we wait another few minutes. Since we already rely on
> omapi for other things, this shouldn't be much magic :)

> Something else you mentioned, mlct. On of our access system is doing something
> like dhcp-snooping/dhcp-aging. When a client successfuly logs on with a DORA
> sequence, the clients mac address is allowed to communicate for a given time.
> unfortunately this time is hardcoded in the access system and not learnt from
> the DORA sequence. If we have a lease time of 7200s but an mlct of 3600,
> clients would first get a lease time of 1h and on a Request/Ackownlede a lease
> time of 2h. Would that work if we set mlct==lease-time? What are the benedits
> and drawbacks from such a configuration?

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