Offer in a dhcp fo setup with split 255

[Peter Rathlev]

On Tue, 2011-11-08 at 11:27 +0100, Meike Stone wrote: 
> Yes but, in the normal state, each peer is answering queries from _its
> assigned_ leases?
>  
> The Load balancing is part of DHCP failover, and a simple set of rules
> are used to determine which peer should respond to the clients query.
> The algorithm (RFC-3074) is used to determine which peer will answer,
> and which peer will ignore the request.  The hash returns a number
> from 0 to 255, and each peer consults a table to determine, given the
> hash number, if it is a query it should process?

It apparently seems you're right -- that's how I would read RFC 3074
too, and draft-ietf-dhc-failover refers to RFC 3074. It does not however
seem to be how ISC DHCPd works. The man page says ISC DHCPd is based on
draft-ietf-dhc-failover, even though they took care to actually point
out that the implementation must not be assumed to conform.

We haven't seen problems from both servers answering (yet) as long as
they answer consistently among themselves.

When thinking about it, I would prefer that both servers answer instead
of only one server answering, since (transient or permanent) packet loss
might prevent one anwer from reaching the client. The other server has
no way of knowing if that happened.

I hope someone wiser can chime in here. :-)

> > cf. recent discussions on this list.
> Please can you point me there? I'm sorry, that I doggedly!
> I'm new in that topic and I want understand it right.

Recent question:
https://lists.isc.org/pipermail/dhcp-users/2011-October/013777.html

Refers to this earlier thread:
https://lists.isc.org/pipermail/dhcp-users/2009-July/009478.html

Excerpt from that thread:
> So, many DHCP servers ignore the server-identifier field supplied by
> the client, including ISC's, and instead reply based upon whether or
> not the server is able to provide for the client's request; which
> depends greatly upon the current state of leases that may or may not
> be actively assigned to the client in question.

So the servers answers if it can. And it can as long as the servers can
see each other via the FO link they "know" about each others leases and
can thusly answer. The FO protocol just makes sure that the answers from
the two servers are always the same (except Server ID).

If the two servers were to lose contact, each server would only reply
for it's own leases, since it does not know the state of the other
leases.

-- 
Peter