Two words for every interface
show ip interface brief lists two columns for each interface: Status is the physical layer (is there a working link?), and Protocol is the data-link layer (can frames get across?).
In NetLab
▶ R1 R1>enable R1#show ip interface brief Interface IP-Address OK? Method Status Protocol GigabitEthernet0/0 192.168.1.1 YES manual up up GigabitEthernet0/1 192.168.2.1 YES manual administratively down down GigabitEthernet0/2 192.168.3.1 YES manual down down
| Status / Protocol | Meaning | Usual causes |
|---|---|---|
| up / up | Working | - |
| administratively down / down | Switched off with shutdown | Someone shut it, or it's a new router port (they start shut) |
| down / down | No working physical link | No cable, cable broken or wrong type, far end shut down or powered off, speed mismatch |
| up / down | Physical link fine, but frames can't get through | Mismatched settings at layer 2 (for example on serial links) |
Here Gi0/1 is administratively down because of shutdown, and Gi0/2 is down/down: it's switched on, but no cable is plugged in.
Bringing a port up
In NetLab
▶ R1 R1>enable R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface g0/1 R1(config-if)#no shutdown *Mar 1 00:00:04.000: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up *Mar 1 00:00:04.000: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up R1(config-if)#end *Mar 1 00:00:05.000: %SYS-5-CONFIG_I: Configured from console by console R1#show ip interface brief | include 0/1 GigabitEthernet0/1 192.168.2.1 YES manual up up
Notice the log messages: %LINK-3-UPDOWN reports the physical link and %LINEPROTO-5-UPDOWN the protocol. The switch at the other end logs the same thing for its port.
The switch's view
On a switch, show interfaces status is the quickest check. connected means up, notconnect means no link, disabled means shut down:
In NetLab
▶ SW1 SW1>enable SW1#show interfaces status | include Port|Fa0/1_|Gi0/ Port Name Status Vlan Duplex Speed Type Fa0/1 connected 1 a-full a-100 10/100BaseTX Gi0/1 connected 1 a-full a-1000 10/100/1000BaseTX Gi0/2 notconnect 1 auto auto 10/100/1000BaseTX