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RE: Caching a sniffer
From: "Shawn Jackson" <sjackson () horizonusa com>
Date: Thu, 25 Mar 2004 11:21:40 -0800

  You've confused routine learning of source MAC addresses 
with the "learning" mode that ports go through while spanning 
tree is reconverging. It's true that packets won't be 
forwarded in the latter case, but 
resetting the MAC table (forgetting learned source MACs) need 
not force a spanning-tree reconvergence, and if it doesn't 
then the traffic will flood temporarily (as source MACs are 
re-learned) rather than be dropped.

On a link status change STP will converge, which means 30 seconds of
blackout time while BPDU's are sent across the network. If you acting
like a 'reset' then STP will converge. If we just drop the MAC table
then a few ARP's later it should be A-OK, I didn't get them confused, I
just wasn't clear on what you were saying, sorry.

  Flooding unknown destinations to all ports (except the origin) 
*includes* forwarding to uplink/inter-switch ports.  So hosts on 
the afflicted switch will still be able to reach hosts on 
other switches, and vice versa.

This would be just like Cisco's (and others) solution to the problem.
When the switch is just blanketing the ports with traffic, (minus the
valid actually in the table) is basically the same. In this case, it
still wouldn't stop the sniffer but would keep the switch system
operating.

  I think this suggestion was that the MAC flood would 
trigger the aging, so it keeps pace automatically.  That, in 
turn, requires the malicious 
host that's flooding bogus MAC addresses to continue doing 
so, in order 
to try to keep valid MACs from being retained in the table 
(long enough 
to keep their traffic from flooding and being seen by the sniffer).

So there would need to be a algorithm to detect the MAC flood and
respond by increasing the age time for records in the table. Once the
flood stops aging would return to normal. Correct?

Shawn Jackson
Systems Administrator
Horizon USA
1190 Trademark Dr #107
Reno NV 89521

www.horizonusa.com
Email: sjackson () horizonusa com
Phone: (775) 858-2338
       (800) 325-1199 x338

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