Firewall Wizards: Re: Shomiti Taps, Cisco Port Mirroring and IDS

[Paul Cardon <paul () moquijo com>]

Ryan Russell wrote:

> > No, using a hub could lead to collisions and loss of packets when
> > combining the two directions.  Use a switch that can queue the packets.
> I would tend to disagree with that.  If a collision occurs, then no one
> gets the frame, neither the IDS nor the intended recipient.  Any lost
> frames by the IDS in that situation are the fault of the IDS.  On the
> other hand, because of the fact that switches do buffering and selective
> forwarding, there IS an opportunity for a frame to not get copied to the
> IDS.

I think you misunderstood what I was saying.  I didn't mean to say that 
a switch or a hub should replace the tap.  I was saying that instead of 
aggregating the two tap outputs into a hub, they should be aggregated 
into a switch.  I apparently was too terse in my response.
If you are tapping a full-duplex link then both directions of the 
original link get stuffed into the hub going in the same direction 
(full-duplex -> simplex) hence the potential for collision.  However, 
the collision occurs only on the hub where the two outputs from the tap 
are aggregated because the tap is a completely passive device and does 
not interfere with the original link in any way, so it is not true that 
the intended recipient will miss the frame, only the IDS will.
You are correct that with a switch, if the total traffic seen on the 
full-duplex connection exceeds what the aggregation switch can buffer 
then the IDS will not see all frames.
-paul

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