A network administrator is reviewing the output of `show ip interface brief` on a Cisco router and sees the following for an interface: `GigabitEthernet0/1 192.168.1.1 YES manual up down`. What does the 'down' status indicate for the interface?
- AThe routing protocol is not operational on this interface.
- BThe interface has no IP address assigned.
- CThe physical layer (Layer 1) connection is not established.
- DThe interface is administratively shut down.
Show answer & explanationAnswer & explanation
Correct answer: C. The physical layer (Layer 1) connection is not established.
In Cisco IOS `show ip interface brief` output, the 'up' status for 'protocol' and 'down' status for 'status' (the first 'up' and 'down') typically indicates an issue at the physical layer. 'up down' means the interface is administratively up (not shut down), but the line protocol is down, often due to a cable issue, a disconnected device, or a speed/duplex mismatch.
Why the other options are wrong
- A. The routing protocol status is a higher-layer concern and would not directly cause the line protocol to be 'down' if the physical link is the issue.
- B. The output clearly shows an IP address (192.168.1.1) assigned to the interface.
- D. If the interface was administratively shut down, the output would show 'administratively down, down'. The 'up' indicates it is not shut down.
Cisco IOS Interface Status (up down)
In Cisco IOS `show ip interface brief`, an 'up down' status for an interface indicates that the interface is administratively enabled but the line protocol (Layer 2) is down, often due to a Layer 1 (physical) issue.
- First status ('up') refers to administrative state.
- Second status ('down') refers to line protocol state.
- Common causes for 'up down' include disconnected cable, faulty cable, or speed/duplex mismatch.
Memory trick: Up/Down is like a traffic light: Green to drive, Red means stop, but maybe the road is broken.