FastEthernet Interface Status

bjberginz · ‎11-12-2006

As we all know, when doing a sh int, you get the physical status of an interface, and the status of the line protocol. In the context of Fast Ethernet interfaces I've been attempting to find out exactly what conditions must be met for the physical interface to be up or down, and what conditions must be met for the line protocol to be up or down.

I've looked at Cisco documentation with limited success. For example this document:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1904.htm

As far as the status of the physical interface, it says this:

"Indicates whether the interface hardware is currently active"

But that's quite general...I want to know EXACTLY what conditions need to be met in order for a FE interface to be considered physically "up", or "down". Does it have to do with seeing voltage on the line? With seeing NLPs or FLPs? I've not been able to find an answer.

Also on the page listed above, it is a little more specific about the line protocol:

"Indicates whether the software processes that handle the line protocol believe that the interface is usable (that is, whether keepalives are successful)"

So the status of the line protocol is all about the keepalives? But if a keepalive cannot be sent out on the wire...wouldn't that indicate that the interface is "physically down"? And since Keepalives are sent out (by default) every 10 seconds, wouldn't it take a while to determine if the line protocol was down?

If you have any more information on this, please let me know, or guide me to the documentation. Thanks.

jkeeffe · ‎12-10-2006

It's too bad no one answered your question because I have the same curiosity.

tracey.marshall · ‎12-13-2006

I am also very interested in this subject. I have a customer site, which is a school with maybe 800-900 students. They run CiscoWorks and have disabled snmp traps for line status. However, they are still getting around 150 alerts per day relating to switch ports. These come from various switches throughout the school, on different hardware and software. There are Cisco ip phones connected to some of the ports. I checked the logs and I am seeing line protocol down/up. It always seems to be the same ports generating the message. For example, on one particular switch it is always fasteth 0/3, 0/6 and 0/23. CiscoWorks was installed in July and the problem was first noticed then. It has probably always existed. It is not service effecting so was only recently reported to me. The messages are generated at night when there is no one at the school.

I will have my customer move one of the PCs that connects to an interface that is generating messages to see if the problem stays with the port or moves with the PC. Will also have the patch cable replaced.

Regards,

Tracey

sourabhagarwal · ‎12-13-2006

Generally physical interface in down state means faulty hardware, port, controllers.

fastethernet line protocol can go down due to several causes like speed/duplex mismatch, wrong cable type, faulty cable, faulty NIC, excessive collisions.

hope to help ... rate if it does ...

bjberginz · ‎12-13-2006

I appreciate the attempt to help sourabharawal, but this doesn't really answer the specific question, that being, exactly WHAT critieria have to be met in order for a Cisco router to consider a FastE interface up or down, or the line protocol up or down.

I've searched through Cisco docs and posted on multiple forums and have heard nothing but crickets. Seriously, is this knowledge a Cisco corporate secret, because no one seems capable/willing to answer it.

cghareeb · ‎12-13-2006

I'm sure I don't have the answer, but I tend to believe your assumption about NLPs causing the Interface Status to change (i.e. Link State = Interface Status).

Consider this:

A 10BaseT crossover cable connected between two switch interfaces can have any one of the four wires cut and the interface will continue to report Interface Status up (in fact, Line Protocol can stay up and traffic accepted). Any two cut/damaged, causes Interface Status to go down. If it were simply voltage or resistance, it seems like at most a single pair would have to exist between interfaces for Interface Status to stay up.

Just another $.02. Ramble complete.