Know the difference between a port and an IP address.
Check the physical path first
Copper Ethernet and fibre are different media. The supported speed and distance depend on the cable, transceivers and interface standard. Single-mode fibre is generally used for longer optical paths; multimode uses different optics and distance limits. Match the supported optics and fibre type rather than assuming any fibre cable will work.
A link light is useful evidence, but it does not prove that addresses, VLANs or applications are correct. On Cisco IOS, show ip interface brief summarises interface status. “Administratively down” means the interface has been shut in configuration. no shutdown enables it; it cannot repair an unplugged cable.
Read the counters
show interfaces can reveal errors, drops, speed and duplex. Full duplex supports simultaneous transmission and reception. A mismatch can cause poor performance and errors. A counter is a clue: compare it before and after traffic, inspect both ends, and check the cable before changing settings.
Auto-negotiation and auto-MDI/MDIX support depend on the devices. For a beginner simulated lab, use the cable and ports specified in the exercise. In a real network, do not force speed/duplex independently on one end without understanding the other end.
Verification commands
Use these viewing commands to inspect the idea in this lesson. Start in the environment shown above each example.
show interfaces
show ip interface briefCommand reference · enter in the stated environment; output varies by device.
Practical exercise
An interface says administratively down. Which configuration action addresses that state, and what should you check afterwards?
Show the answer and explanation
Select the intended interface and enter no shutdown in a lab. Then check physical and protocol status. A disconnected peer or cable fault can still leave the link down.
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Technical references checked on 25 October 2026. These are original teaching explanations, not official Cisco course material. See how content and lab validation are described.
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