This question
should be used to assess the skills of any candidate, including
Network Administrators and Network Engineers, who will be
configuring or managing network hardware resources. A candidate
with one to two years of experience with switches should
be able to answer this question correctly. This question
addresses a problem that can crop up if using DHCP, especially
if DHCP policies are changed.
You
are managing a switch which serves as your local DHCP. When
looking at your switch logs, you see this message, over
and over again:
Invalid
ARP request : 10.4.4.48 on 10.4.4.48.
This
switch services the 10.4 subnet and has an address of 10.4.4.254
What
does this signify?
The candidate should state that the message signifies that
two machines on your network have the same IP address. The
switch is trying to map IP address to MAC addresses (the
actual hardware address of the network card), and is getting
back different answers from two different machines.
This often signifies
that you have a problem in your DHCP scope. For instance,
if the lease duration of the scope was changed, some machines
that had a longer lease might have its IP address given
to another PC because the server believes that the lease
is expired. This can also happen if someone mis-configures
a static IP address that falls into the DHCP scope, or if
two static IP addresses are configured to the same number.
The candidate
might respond that this indicates a loop somewhere in the
network. This is not a bad guess. If the IP address listed
in the error message were 10.4.4.254, this could be the
case. If switch A were connected to switch B, and switch
B to switch C, and by mistake switch c were connected back
to a different port on switch A, packets could make a large
loop and come back into switch A on another port. This would
give a similar error:
Invalid ARP
request: 10.4.4.254 on 10.4.4.254
This question
encapsulates what I would want a potential hire to know
about network address resolution, including what ARP is,
how it works, and how it can fail. In general, I would consider
this a fairly basic networking question. If I were hiring
for any position that would be configuring network resources,
I would expect this question to be answered correctly.
About the author
Glenn Basden has been in the technology field for about
a dozen years, and manages 50 servers and a dozen routers
on a day-to-day basis. He is currently focused on networking,
Citrix and Exchange, and re-certifying for Windows 2000.
