[c-nsp] Unicast traffic being sent to every port? Aging issue?

[Ray Van Dolson]

We have two Dell PowerConnect M6220 switches (A1 and B1).  They are not
cross-connected, but both have uplinks to the same subnet:

                      zfs1
                     /
                   +----+
                   | A1 |---------|
                   +----+     +-------+
                              | Cisco |------- linux1
                   +----+     +-------+
                   | B1 |---------|
                   +----+
                    / \
                  esx1 esx2

There's a host hanging off of A1 (zfs1) and several ESX hosts hanging
off of B1 (esx1, esx2, etc).  There's a host linux1 hanging off the
Cisco as well (actually many hosts, but for the sake of description

What's happening is, esx1/2 beging talking to zfs1.  All is well for a
while... but at some point, zfs1's MAC address expires from the CAM on
the switch (I guess that is what is happening).

At that point, the Cisco begins forwarding the unicast packets to all
its ports.  The result -- linux1, and all other hosts see the packets.
Occasionally, when we're dealing with a lot of traffic, this seriously
impacts performance.

My question here is.. what is the _right_ way to deal with this?  This
"flooding" can continue for many minutes at a time.. it isn't until an
ARP reply eminates from zfs1 that the CAM table is populated again and
the broadcasting stops.

I wonder if zfs1 would send back an ARP response quicker were it not
behind an additional switch (the PowerConnect)... 

Thanks,
Ray