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RE: Caching a sniffer
From: "Shawn Jackson" <sjackson () horizonusa com>
Date: Thu, 25 Mar 2004 12:50:38 -0800

  Not really.  I'm a switch, and I've just seen a new source MAC 
address, which I want to insert into my table.  Uh-oh, the 
table is full.  Instead of just failing to learn the new 
address, I'll 
"age out" the oldest address in the table prematurely to make 
space for the new one.  (If I track their ages so I can age them 
out normally, finding the oldest one shouldn't be any problem.)
  I didn't "detect the flood" or alter the normal aging time. 
 I just discarded some old information instead of discarding 
the new information just received.  (In essence, this means 
treating the MAC table like a FIFO cache.)

I looked through some old docs on Cisco switches. To my knowledge the
switches will automatically adjust the learning rate of the CAM table in
much the same fashion your describing, but it doesn't really mitigate
the effects of a MAC Address flood. Cisco called this attack "Forwarding
Table Exhaustion", or at least that is what was in my notes.

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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