On Mon, Jun 12, 2000 at 10:20:08AM -0500, c.spurgeon@mail.utexas.edu wrote:
> Although it may sound odd, this is most likely an example of
> auto-negotiation performing as designed.
Thanx,
This explained the reason we had to manually configure an Alteon
switch to Full duplex while connecting to a 7206VXR...
IS there a specific reason way Cisco doesn't do auto negotiation on
the FastEthernet interfaces on their routers??
>
> Cisco Ethernet router interfaces typically do not support auto-neg,
> however most switch ports DO support auto-neg. In this case, the
> auto-neg standard defines a scheme called "parallel detection" to make
> it possible for an auto-neg device to deal with a link partner that
> does not support auto-neg. Call the auto-neg device "A" and the
> non-negotiating link partner at the other end of the link "B."
>
> Parallel detection on "A" uses either normal link pulses (for 10 Mbps)
> or the signal characteristics of 100 Mbps systems to figure out what
> speed "B" is running at. However, since "B" is not sending auto-neg
> signals, there is no way for "A" to determine any other
> characteristics, such as the duplex setting.
>
> According to the Ethernet standard, when "A" uses parallel detection
> to determine the speed of the link partner, then "A" must assume that
> the link partner is operating in half-duplex mode. In this case you
> have to assume something, and half-duplex was apparently considered
> the safe assumption.
>
> Therefore, when you manually configure a non-negotiating interface to
> operate at full-duplex, you need to manually configure the device at
> the other end of the link to also be in full-duplex mode. Otherwise,
> when left to itself the auto-neg process will result in the
> half-duplex misconfiguration and link errors that you have seen.
>
> Parallel detection only applies to 10 or 100 Mbps twisted-pair
> Ethernet links. All twisted-pair Gigabit Ethernet devices are required
> to support auto-negotiation, since the auto-neg system is also used to
> configure some essential signaling timing characteristics over a
> 1000BASE-TX link.
>
> -Charles
>
> Charles E. Spurgeon / UT Austin Networking Services
> Email: c.spurgeon@mail.utexas.edu
> Ethernet Web Site: http://www.ots.utexas.edu/ethernet/
-- ------------------------ Hendrik Visage hvisage@envisage.co.za
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