Unicast Flooding

Brian Shope blackwolf99999 at gmail.com
Wed Jun 17 16:32:44 CDT 2009


Recently while running a packet capture I came across some unicast flooding
that was happening on my network.  One of our core switches didn't have the
mac-address for a server, and was flooding all packets destined to that
server.  It wasn't learning the mac-address because the server was
responding to packets out on a different network card on a different
switch.  The flooding I was seeing wasn't enough to cause any network
issues, it was only a few megs, but it was something that I wanted to fix.

I've ran into this issue before, and solved it by statically entering the
mac-address into the cam tables.

I want to avoid this problem in the future, and I'm looking at two different
things.

The first is preventing it in the first place.  Along those lines, I've seen
some recommendations on-line about changing the arp and cam timeouts to be
the same.  However, there seems to be a disagreement on which is better,
making the arp timeouts match the cam table timeouts, or vice versa.  Also,
when talking about this, everyone seems to be only considering routers, but
what about the timers on a firewall?  I'm worried that I might cause other
issues by changing these timers.

The second thing I'm considering is monitoring.  I'd like to setup something
to monitor for any excessive unicast flooding in the future.  I understand
that a little unicast flooding is normal, as the switch has to do a little
bit of flooding to find out where people are.  While looking for a way to
monitor this, I came across the 'mac-address-table unicast-flood' command on
Cisco switches.  This looked perfect for what I needed, but apparently it is
currently not an option on 6500 switches with Sup720s.  Since there doesn't
appear to be an option on Cisco that monitors specificaly for unicast
floods, I thought that maybe I could setup a server with a network card in
promiscuous mode and then keep stats of all packets received that aren't
destined for the server and that also aren't legitimate broadcasts or
multicasts.  The only problem with that is that I don't want to have to
completely custom build my own solution.  I was hoping that someone may have
already created something like this, or that maybe there is a good reporting
tool for wireshark or something that could generate the report that I want.

Anyone have any suggestions on either prevention/monitoring?

Thanks!!

-Brian



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