Question : Problem: Ethernet Line Protcol?

On Cisco routers and switches the ethernet interface will show the interface status is up once link status is seen. But what causes the line protocol to go up on a full duplex ethernet interface?

Ethertype 9000 defines CTP but that assumes Ethernet v2 is being used and it would only function in a coax or hub environment.

I've also read:
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_tech_note09186a008048cffc.shtml#t3

Where it states "The Ethernet hardware sends this packet onto the Ethernet wire and then immediately receives this packet back again."

But on a full duplex link, it won't see it return. Debug confirms this as I can see the frame being transmitted but not received by a switch port.

Also, I haven't seen any of these frames coming out of typical workstation which means the switch port connected to a PC would never receive the keepalive and SHOULD then show the line protocol as down.

So what mechanism does Cisco use to indicate the line protocol is up?

Answer : Problem: Ethernet Line Protcol?

I guess this is it:

http://en.wikipedia.org/wiki/Autonegotiation