Difference Between Interface States

 Q: What does "Interface is up, line protocol is down" mean?

Answer:
It means the physical layer (Layer 1) is working—the interface has detected a valid electrical or optical signal—but the data link layer (Layer 2) is not operational. Common causes include VLAN or trunk configuration problems on Ethernet links, encapsulation mismatches on serial links, keepalive failures, or other Layer 2 configuration mismatches.

Difference Between Interface States

StatusMeaningCommon Cause
Administratively down, line protocol downInterface is manually disabledshutdown command
Down, line protocol downPhysical connection failedCable unplugged, device off
Up, line protocol downPhysical layer is OK, Layer 2 failedEncapsulation, VLAN, trunk, keepalive, or protocol mismatch, Errors..speedmismatch
Up, line protocol upInterface is fully operationalNormal state

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