Ethernet Mystery: connecting two Cisco routers with a straight cable.

Unanswered Question
Feb 24th, 2010
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Hi everybody.

I'm facing an ethernet cabling mistery.


My company owns a Cisco 2821 router.

This cisco is connected throught Fastethernet to a Cisco 2801 owned by a Telco operator.

Everything was working fine for almost a year.

The problem:

Last week end, after a power failure,  the link appeared to be down on the router ( Link Up, Protocol Down )

I solved the problem changing the cable between these two routers with a cross cable.

The previous cable appeared to be a staight cable !?!

The mystery:

I'm wondering... how could it work for almost a year?

Is there a way to activate some auto crossing protocol on a Fastethernet Interface (MDI-X?) or was there some mysterious changes on my network?

Interface configuration:

interface FastEthernet0/0
ip address  XXX XXX
ip access-group external3 in
no ip redirects
no ip unreachables
no ip proxy-arp
ip route-cache flow
duplex auto
speed auto
no mop enabled
standby 1 priority 80

Thanks for your help

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milan.kulik Wed, 02/24/2010 - 06:11
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so the routers were connected by a straight patch cord cable using on-board FE ports on both routers?

Or some HWIC used on the provider router, e.g.?

To the mystery:

Auto crossing is not 100% reliable in a case of power failures especially.



lyra-network Wed, 02/24/2010 - 06:18
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Exactly, the router were connected by a staight cable plugged in the onboard FastEthernet connectors (no 4ESW or any other module)

Best Regards,


Leo Laohoo Wed, 02/24/2010 - 13:44
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Hi Bruno,

MDI/MDIX is enabled by default.  During the reboot, probably someone made some configs to the router but didn't or forgot to save the config.

There are also some instances when appliances, such as routers, running different IOS have problem with auto-negotiation.  Particularly if the interface were "bounced".


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