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RE: Caching a sniffer
From: "Shawn Jackson" <sjackson () horizonusa com>
Date: Thu, 25 Mar 2004 10:27:00 -0800

1. Dump the entire MAC table.  Switch acts as if power on 
reset just 

Seams logical, but I've never seen it implemented. It would
halt traffic while learning resumes, in addition if other checks 
needed to run (Spanning Tree/CDP) it would take much longer.

  No.  Traffic will flood while learning restarts from 
scratch. Nothing was said about dumping STP/CDP statuses.

Traffic will halt because the switch can't forward at the point it's 
trying to learn the network.

2. Stop learning.  All previously learned MAC addresses
remain, and so only traffic for unrecognized MAC addresses 
gets sent to all ports.

That would damage the network. If a new client fires up, they
would not get added to the switches tables and not receive any 

  Destinations not in the table normally get flooded, not 
dropped. Dropping this traffic is possible, but not a normal 
part of the action being suggested.

If the system fires up on the afflicted switch. If it's in another
of the network all systems and paths on the afflicted switch would be
because the switch would use it's current MAC->Port table, and not add
MAC's to the port.

3. Partial Purge of table.  Some portion of the table gets purged 
and the switch continues, treating those purged MAC 
addresses as if 
this was the first time they were seen.  Depending upon how the 
purged addresses are selected - oldest first, youngest first, 
random, lowest MAC addresses, highest MAC addresses or something 
else - will cause the switch to act differently for 
different users.

Seams a better solution out of the bunch, could be a pain to

  Some switches routinely age unused entries out of the 
table. Accelerating this process if the table fills shouldn't 
be too hard.

You can usually modify aging in the configuration. But the aging time
would have to
outpace the flood to keep the table clear, this would result in more
traffic and load
for the switch.

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

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

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