What makes the networking device link to go up ?

[from88]

Hello,

Could someone remind  mewhat makes the link on the network device to go up.

If we've optical connection. What's the first requirement. If the port see power in RX direction the switch should immediately turn the interface UP ?

Of course there autonegotiation then comes in play. But if we pretend that autonegotiation in both sides is OFF.

Thanks!

[Leo Laohoo · Accepted solution]

If the switch sees an RX then the port goes up.
However, it doesn't guarantee that the other side can see the RX either. If one core of the cable is broken then only one side of the link will have an UP and the other side (the side that can't see the RX) will have a DOWN.