[c-nsp] Cat6509 CAM entries flapping

[James Sneeringer]

CatOS 8.1(1), Cat 6509, Sup2

We recently started seeing the following on one of our switches:

2007 Mar 08 17:14:59 %SYS-4-P2_WARN:Host 00-50-8b-dd-17-d1 is flapping
between port 6/11 and port 6/42

Port 6/42 is just a Windows server. Port 6/11 (and this may be the kicker)
is the "internal" side of a LocalDirector 430, which is running 4.1.1. The
"external" side of the LD430 is on this switch as well, but in a different
VLAN. We've been operating this way for a year with no apparent problems
until now.

As I understand it, dynamic CAM entries are learned based simply on the MAC
addresses on incoming frames. If it sees 00-50-8b-dd-17-d1 come in from port
6/42, it will insert that into the CAM table. If that MAC then shows up on
6/11, it changes the CAM entry accordingly. Nothing complicated.

Since the LD is just a bridge, a frame entering its external interface that
doesn't trigger any of the LD's binds will just pass through it unchanged,
so if a frame from the internal VLAN somehow managed to leak into the
external VLAN, the LD would (or could) bridge it back to the internal VLAN,
causing the CAM flapping I'm seeing.

Has anyone seen something like this before? Any suggestions on how to
troubleshoot it? I've mitigated it somewhat by adding static CAM entries for
the MAC addresses affected by this, but I'd like to find the root cause in
case other MACs start exhibiting the same problem. Thanks for any
suggestions.

-James

-- 
James Sneeringer                   JupiterImages Corp
Network/Phone Engineer             a division of Jupitermedia Corp
jsneeringer at jupiterimages.com      6000 N. Forest Park Dr.
309-688-8800 ext. 241              Peoria, Illinois 61614