What do the two status parameters in the show interfaces output mean?

Question

Accepted Answer

First parameter = physical layer (is there a carrier?); second parameter = data link layer (are protocol keepalives arriving?).

* The four show interfaces status/protocol states. *
Example: FastEthernet0/18 is up, line protocol is up (connected)

State Combination
Meaning
Likely Cause

Up / Up
Working correctly
Normal operation

Up / Down
Physical link OK, protocol problem
Encapsulation mismatch, error-disabled, or remote end problem

Down / Down
No physical connection
Cable not attached, other end administratively down

Administratively Down / Down
Manually disabled
shutdown command issued on the interface

Troubleshooting tips:

Up / Down: Check for encapsulation type mismatch, error-disabled state, or hardware problem on the remote end
Down / Down: Check cables, connectors, and verify the remote device is powered on
Admin Down / Down: Someone ran shutdown — use no shutdown to re-enable

Go deeper:

Duplex mismatch (Wikipedia) — a common cause of the up/down protocol state (CRC errors from a mismatch).

State Combination	Meaning	Likely Cause
Up / Up	Working correctly	Normal operation
Up / Down	Physical link OK, protocol problem	Encapsulation mismatch, error-disabled, or remote end problem
Down / Down	No physical connection	Cable not attached, other end administratively down
Administratively Down / Down	Manually disabled	`shutdown` command issued on the interface