RE: Caching a sniffer

["Shawn Jackson" <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.

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 
> > 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.

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

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

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