[mrtg] Re: Any ethernet switch which always floods?

[Josh Howlett]

How about setting the MAC address on each station to be the same?  I'm
not sure whether this would result in frames being forwarded to all
ports, or just a single random port.

josh.

On Thu, 18 Oct 2001, SHOLAAS Margaret G wrote:

> We considered setting up a script to do that, but concluded we couldn't do
> it fast enough. Say the switch's forwarding table iss empty and a SYN comes
> in port 1 from station A to station B. That packet would get flooded to all
> ports. The switch would then create a forwarding table entry indicating
> station A is on port 1. Then the SYN-ACK comes in port 2 from station B to
> station A. The switch looks up station A in the forwarding table, finds it
> listed as accessible via port 1, forwards that packet only to port 1 instead
> of flooding, and so the Sniffer, IDS, etc. miss it. The response often comes
> back in milliseconds, so we could never flush it fast enough to keep from
> missing packets. I just wish there were a way to disable keeping a
> forwarding table, but so far there doesn't seem to be switch which allows
> that, not surprising because apart from this weird application, you'd never
> want it to do that.
> 
> We were also hoping we could set the forwarding table timeout to 0 so an
> entry it would timeout the moment it got in there, but we couldn't find a
> way to set it on the Cisco 3500XL.
> 
> Thanks for thinking about this for me!
> 
> -----Original Message-----
> From: Josh Howlett
> 
> It just occured to me you could force flooding by flushing
> the bridge table on the switch whenever it learns anything.  With an
> empty bridge table the switch *must* repeat out on every port.  Or maybe
> would this force the switch into the learning state, in which this
> wouldn't be of much use.
> 
> In any case, automating this wouldn't be that easy, tho not impossible.
> 
> josh.
> 
> 

---------------------------------------
Josh Howlett, Network Supervisor,
Networking & Digital Communications,
Information Systems & Computing,
University of Bristol, U.K.
0117 928 7850 | josh.howlett at bris.ac.uk
---------------------------------------


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