[ipv6hackers] Best Network Tap - for home

[Gert Doering]

Hi,

On Fri, Jul 12, 2013 at 03:52:32AM -0700, Mark ZZZ Smith wrote:
> > We've been trying to tell you: This doesn't work for GigE.
> 
> Why not? I already proved that Auto MDI/MDI-X is optional by quoting the IEEE spec, and I've already made a GigE X-over that worked with autoneg enabled on both GigE interfaces (which also worked as a 10/100 x-over).

Because I already said that there are no dedicated "RX" or "TX" lines in
a GigE port that you could passively tap into.  All 8 wires are used 
bidirectionally, and the use of the wires is negotiated between the GE
transceivers.

> 3 GigE's connected would be an issue with autoneg in Sander's scenario, but no more than 100BASE-T would be. If you switched off Autoneg on the GigE attached to the passive port of Sander's device, it'll use it's own clock and would probably receive packets.

You can't switch off link negotiation on 1000BaseT

> Has anybody here tried what I'm suggesting, or is there no actual evidence that it won't work, just speculation. I'm speculating it might, given that it appears to work for 100BASE-T.

100BaseTX works fully different - dedicated wires for RX and TX, and no
(except for MDIX) negotiation on the *use* of the wires.  So you can very
easily wire each pair on the TX side of an extra port, so one tap port
sees the "RX" and one port sees the "TX" data from the original link
(and since the other direction of the tap port is not connected, the 
signal emitted by the 100BaseTX card won't interfere with the original link)

10/100 negotiation and MDI/MDIX on the TAP ports could be interesting.

> I don't 100% accept things that I've been told, too many times it ends up being not completely true, or dependent on a context that has been forgotten or not passed on.

Feel free to test.  Or read up how FE vs. GE uses the wires on the cable,
and think about about it.

Gert Doering
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