Frame Tagging

sthaug at sthaug at
Mon Aug 12 16:28:04 UTC 2013

> I did try the suggestion of using the DHCP Helper address option on a 
> procurve switch, and it made no difference. Looking at the server log, 
> what I was seeing was that dhcp on eth0 was seeing the reponses on 
> eth0.2, and then sending out wrong network/ NACK packets about them. 
> Kind of crazy.

Then, assuming the Procurve switch is doing the right thing, something
is fundamentally wrong with your network configuration.

A DHCP relay agent converts DHCP *broadcast* requests from the client
to DHCP *unicast* requests going towards the server - and performs the
opposite conversion for unicast replies from the server towards the
client. I assume this is what the DHCP Helper address option on the
Procurve does.

The significant change of using a DHCP relay agent, from the server's
point of view, is that no special "magic" broadcast handling is needed.
Everything is just plain vanilla unicast traffic.

If a DHCP server on eth0 is seeing its own responses on eth0.2 (and
assuming eth0.2 is supposed to be in a different broadcast domain than
eth0) then either the server is misconfigured or those two broadcast 
domains are bridged somewhere else.

Steinar Haug, Nethelp consulting, sthaug at

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