Line Protocol - UP/DOWN status

Unanswered Question
Aug 28th, 2007


I was reviewing the logs on a cisco and noticed a router which had the FastEthernet port along with the Vlan port ,and they went to a down/up status multiple times in the past few days at the same time. The router's uptime is a few weeks.

What type of issues would cause these interfaces to "flap"?


I have this problem too.
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sundar.palaniappan Tue, 08/28/2007 - 12:30

If there is no device connected to a port or the connected device is powered off then the fastethernet port would go down. This usually happens when a PC is shut down the port goes down and when the PC is powered on again the port comes backup. The other reasons that can cause an interface to flap are faulty cable, bad port/transceiver, speed/duplex mismatches etc..

The only reason why a VLAN interface goes down is if there are no active ports in that VLAN. If you are seeing the fastethernet port and VLAN int go down at the same time then it's very likely the only interface that's part of that VLAN is that fastethernet interface.



sundar.palaniappan Tue, 08/28/2007 - 12:35

I just want to add one other thing to my last post. Often times flapping interface is a result of loose cable. Make sure the cable is seated tight.



danny9797 Tue, 08/28/2007 - 12:36

Makes sence, thanks a lot.

If a router is configured using ATM0 interface and this interface flaps, what are the usual causes for these types of scenarios?


sundar.palaniappan Tue, 08/28/2007 - 12:55

Is this an ADSL (ATM) interface?

Check the Cable Pinouts

To troubleshoot Cisco ADSL WIC Layer 1 issues, check the cable pinouts.

Make sure that the cabling is correct and that the Cisco ADSL WIC front panel LEDs function properly.

The Cisco ADSL WIC has an RJ-11 connector for the ADSL interface. It uses pins 3 and 4 to transfer data.

Note: For more information on cabling, interface pinouts, and status LED descriptions, refer to the DSL Modem/Router Interface Pinouts and Status LED Descriptions.

If the interface status shows down, the router does not see a carrier on the ADSL interface. This means that the pins used for the cable are wrong or that your ISP has not turned on DSL service for you. From the router prompt, issue the show interface atm command.

Router#show interface atm 0

ATM0 is down, line protocol is down

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