Cisco Switch Commands
Step 3 of 838%

Ports, Interfaces & VLANs

A link light only proves physical connection. It does not prove the device is in the right VLAN, negotiating cleanly, or reaching the rest of the network.

After this page, you should be able to:

  • Use interface commands to find up/down ports
  • Read connected, notconnect, disabled, and err-disabled states
  • Verify access VLAN assignment
  • Understand why link state does not prove correct network placement

show ip interface brief: quick Layer 3 sanity check

Open SW2, go to the CLI tab, and make sure the prompt is SW2#. If it says SW2>, type enable first.

Type this exact command:

show ip interface brief

On a router or Layer 3 switch, this summarizes IP addresses and interface status. On a Layer 2 access switch, you may mostly see and the management IP. That is normal.

show interfaces status: the help desk favorite

Stay on SW2 at the SW2# prompt. Run this command:

show interfaces status
Port      Name               Status       Vlan  Duplex Speed Type
Fa0/1     Uplink to SW1      connected    trunk a-full a-100  10/100BaseTX
Fa0/2     PC1                connected    10    a-full a-100  10/100BaseTX
Fa0/3     Printer1           connected    20    a-full a-100  10/100BaseTX

This is often the first command to run when someone says a device is plugged in but not working. It shows link state, , , speed, and the port description when one exists.

Expected result for the known-good lab: Fa0/1 is the trunk to SW1, Fa0/2 is PC1 in VLAN 10, and Fa0/3 is PRINTER1 in VLAN 20. If your output does not match that, first check cabling before changing configuration.

Interface descriptions in Packet Tracer

Packet Tracer note

Packet Tracer's 2960 switch does not support show interfaces description. On real Cisco switches, that command is useful. In this lab, use show interfaces status and the Name column, or use show running-config and find the interface block.

show interfaces status
show running-config

show vlan brief

Stay on SW2#. Now check which VLANs exist on the access switch and which access ports are assigned to them.

show vlan brief

If a printer should be in VLAN 20 but the port is in another VLAN, the cable can be perfect and the device can still fail. are logical neighborhoods.

For the known-good SW2 config, PRINTER1 is described on Fa0/3 and that interface has switchport access vlan 20. Since PRINTER1 uses 192.168.20.50, VLAN 20 is the correct printer-side network.

Watch out for voice VLANs: phone deployments often use an access for the PC plus a separate voice VLAN for the phone. Do not flatten that setup because you only looked for one VLAN.

Checkpoint

  • You can identify connected, notconnect, disabled, and suspicious port states.
  • You can confirm whether an access VLAN exists.
  • You can explain why a device can have link but still be on the wrong network.

Hands-On Practice

Turn interface output into an evidence note about PC1, PRINTER1, and the upstream trunk.

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Run The Checks

Quick Check