Troubleshooting the physical layer step by step

Start at the bottom

Most network faults are physical. Before you look at addresses or routing, prove the cable and the ports are healthy. A routine that works:

1. Is it up? show ip interface brief or show interfaces status. Administratively down means no shutdown. Down/down means look at the cable and the far end.
2. Do speed and duplex agree? show interfaces status on the switch. Watch out for a-half next to a forced port on the other side.
3. Are errors growing? show interfaces: CRC, runts, late collisions. Clear the counters and look again after some traffic.
4. Inspect the cable. Right type (straight, crossover, multimode, single-mode)? Under 100 m for copper? Is auto-MDIX on at either end?
5. Fix one thing, then test again, and save the configuration if you changed it.

Example: a port with auto-MDIX turned off

Here PC1 is plugged into the switch with a crossover cable, and someone has turned off auto-MDIX on that switch port. Neither end swaps its pins, so there is no link:

In NetLab

▶ SW1
SW1>enable
SW1#show interfaces status | include Port|Fa0/1_
Port      Name               Status       Vlan       Duplex  Speed Type
Fa0/1                        notconnect   1            auto   auto 10/100BaseTX
SW1#show running-config interface fa0/1
Building configuration...

Current configuration : 44 bytes
!
interface FastEthernet0/1
 no mdix auto
end
SW1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
SW1(config)#interface fa0/1
SW1(config-if)#mdix auto
*Mar  1 00:00:06.000: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up
*Mar  1 00:00:06.000: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
SW1(config-if)#end
*Mar  1 00:00:07.000: %SYS-5-CONFIG_I: Configured from console by console
SW1#show interfaces status | include Port|Fa0/1_
Port      Name               Status       Vlan       Duplex  Speed Type
Fa0/1                        connected    1          a-full  a-100 10/100BaseTX

Turning auto-MDIX back on fixed it from the switch side. Swapping in a straight-through cable would have fixed it too.