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My problem:

Upon connecting the FastEthernet0 with a straight-through cable to a Catalyst switch, the line protocol comes up briefly and then goes down. Interface goes down too, I now have this on my show ip int brief:

Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0              unassigned      YES unset  down                  down 

I hereby state that the physical connection is down, and that there must be a problem with the ports or cabling.

Tried solutions:

Use a different cable
Use a different switch
Use a different port
Check Duplex settings and port speed

The problem persists, so I don't understand where the problem could lie, is it maybe port security on the switch? (I have no access to the switch, I'm doing this remotely)

Does anybody know the solution?

Edit: the logs show what happens upon connecting to the switch:

000072: *Mar 21 01:26:48.655: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up
000073: *Mar 21 01:26:48.659: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan10, changed state to up
000074: *Mar 21 01:26:49.651: %LINK-3-UPDOWN: Interface FastEthernet0, changed state to up
000075: *Mar 21 01:26:50.179: %DTP-5-NONTRUNKPORTON: Port Fa0 has become non-trunk
000076: *Mar 21 01:26:50.679: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to down
000077: *Mar 21 01:26:50.679: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan10, changed state to down
000078: *Mar 21 01:26:51.679: %LINK-3-UPDOWN: Interface FastEthernet0, changed state to down

Thank you in advance.

share|improve this question
up vote 1 down vote accepted

As you suspected, port security (MAC address limitation) will cause the router to connect and then disconnect immediately.

(The reason being, it connects electrically, but is not allowed to send traffic - the very first ethernet packet will be rejected and the switch places the port in a down state)

If you have the option to enable debug logging on the router, do so.

share|improve this answer
I will have to ask the IT partner to reconfigure the switch then... – glenn Feb 23 '12 at 14:22
I will keep this post up to date, I'll get back to you when the switch has been looked over. Thanks! – glenn Feb 23 '12 at 14:25
The switchports were in PortFast mode with BPDU guard enabled on a trunk line, this cannot work. The switch puts the port immediately in blocking mode, because it detects a BPDU error. – glenn Feb 24 '12 at 14:13

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