DHCPREQUESTs and What the Logs Show

Martin McCormick martin at dc.cis.okstate.edu
Fri Feb 11 14:22:47 UTC 2011


	When trying to diagnose a situation in which one of our
networks has a lease imbalance and all others are okay, I notice
that one DHCP server will see something like:

Feb 10 15:56:53 dh1 dhcpd: DHCPREQUEST for 10.193.4.89 from 00:1d:70:97:59:cc via bce0

That network is not in the same subnet as the 
DHCP server so one would expect the interface to be that of the
router on the network segment where the request originated. Is
this correct?

	We have the Cisco DHCP helper on all our routers and
generally requests look more like:

Feb 11 00:00:02 dh1 dhcpd: DHCPREQUEST for 10.193.87.212 from 58:1f:aa:09:31:21 (Rachels-iPhone) via 10.193.87.254

Shouldn't every similar request always show the
router interface as the proper interface originating the
request on both servers, no exceptions?

	I also have noticed that in imbalance times, I will see
traffic via bce0 on our master DHCP server and via the router on
the slave.  Sometimes, this is reversed.

	The way I interpret what happens after that is that the
server that saw the traffic via bce0 tries to reply via bce0 and
nobody hears it. The other server will usually reply with a
unicast through the router and all is well if the client is
listening.

	Is there anything else besides problems at the DHCP
helper level that could cause us to sometimes miss the interface
address? I think I know the answer but this is time to be
extra analytical so as not to accumulate too much egg on one's
face.

	The only time we should see DHCPREQUESTs via the local
interface is when there is a client on the same subnet as the 2
DHCP servers.

	Thanks for any constructive responses.

Martin McCormick WB5AGZ  Stillwater, OK 
Systems Engineer
OSU Information Technology Department Telecommunications Services Group



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