Auto MDI/MDI-X + conference rooms + bored == loop

James Hess mysidia at
Fri Mar 26 23:13:11 CDT 2010

On Fri, Mar 26, 2010 at 9:29 PM, Chuck Anderson <cra at> wrote:
> So basically, the problem is the core switches implement a proprietary
> loop-prevention protocol that sends "beacon" frames out every 500ms,
> and if a certain number of these special frames come back (exceeds
--> loop first, but I'm beginning to think that this protocol is crap and
> I should just disable it and let the core ride out the loop in the

Ah, nasty..  it seems like you definitely should want to keep the
beacon frames from getting injected then. Taking down core links ought
to be harder than 1 user emitting a few frames.   A malicious user, or
a naive user with a malicious trojan on their computer could try to
send fake beacons, to cause trouble.  I for one might start thinking
if the beacons can be sunk from end user ports by brute force, using a
 Layer 2 ACL.

I wonder if RFC 5556, IETF TRILL specs, or  802.1aq/802.1Qbb /
Datacenter Ethernet  / Bridging  standards  and more  robust
standards-based loop avoidance standards will ever get finalized,
considering they have been drafts for over 5 years,   it seems like
the standardization is very sluggish.
A new protocol is probably the right solution,  but it might not be
ready until 2015 at this rate.

> Anyone know if Auto MDI/MDI-X is inherent or required in 1000Base-T?
> It would be nice if I could shut it off.

Auto MDI/MDI-X  is an optional feature in the 1000BaseT standard.
Automatic negotiation of speeds and duplex, is mandatory due to 802.3ab,
but not auto-crossover

You  can get that here
Clause  40.4.4   in IEEE 802.3-2008 -- Section Three
states the following:

"40.4.4 Automatic MDI/MDI-X Configuration  Automatic MDI/MDI-X
Configuration is intended to eliminate the need for crossover cables
between simi
lar devices. Implementation of an automatic MDI/MDI-X configuration is
optional for 1000BASE-T   devices. If an automatic configuration
method is used, it shall comply with the following specifications. The
  assignment of pin-outs for a 1000BASE-T crossover function cable is
shown in Table40–12 in 40.8.


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