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RE: Caching a sniffer
From: "David Gillett" <gillettdavid () fhda edu>
Date: Wed, 24 Mar 2004 12:15:52 -0800

-----Original Message-----
From: Shawn Jackson [mailto:sjackson () horizonusa com]

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

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.

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 
traffic.

  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.
 
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 
implement.

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

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