How to See Whether a Junos Interface Is Working

If you just want to see whether an interface on a Junos device is operational, you can start with the Junos OS show interfaces brief command:

user@router> show interfaces brief
Physical interface: fe-0/0/0, Enabled, Physical link is Down
 Link-level type: Ethernet, MTU: 1514, Speed: 10m, Loopback: Disabled,
 Source filtering: Disabled, Flow control: Enabled
 Device flags  : Present Running Down
 Interface flags: Hardware-Down SNMP-Traps Internal: 0x4000
 Link flags   : None

This command shows all the interfaces on the router, though the preceding snippet shows only a single interface. For each interface, you see the name of the interface, whether it’s enabled, and the state of the physical link. For this Fast Ethernet interface, the physical link is down, which means that the interface isn’t operational (the link light is down).

The device flags confirm that the interface is present (that is, it’s on the box and the line card has been detected), running (meaning the line card is functioning), and down (the link light is down).

The output from the show interfaces brief command shows that the interface is Enabled. Remember that you can administratively disable an interface using the disable configuration statement within the interface configuration:

fe-0/0/0 {
  disable;
  unit 0 {
    family inet {
      address 10.0.24.2/32;
    }
  }
}
 

If you disable an interface, the output from the show interfaces command will indicate that the interface is administratively down:

user@router> show interfaces brief
Physical interface: fe-0/0/0, Administratively down, Physical link is Down
 Link-level type: Ethernet, MTU: 1514, Speed: 10m, Loopback: Disabled,
 Source filtering: Disabled, Flow control: Enabled
 Device flags  : Present Running Down
 Interface flags: Hardware-Down Down SNMP-Traps Internal: 0x4000

Many times, you may not want to see every interface on the device. In fact, more often than not, you’re checking the status of a particular interface. To see only the output for a specific interface, include that interface’s name as an argument in the command:

user@router> show interfaces brief fe-0/0/0
Physical interface: fe-0/0/0, Enabled, Physical link is Down
 Link-level type: Ethernet, MTU: 1514, Speed: 10m, Loopback: Disabled,
 Source filtering: Disabled, Flow control: Enabled
 Device flags  : Present Running Down
 Interface flags: Hardware-Down SNMP-Traps Internal: 0x4000
 Link flags   : None

This command limits the output to only the specific interface included in the command. Sometimes, you may want to see all interfaces of a certain type. Imagine, for example, that you want to monitor all your Fast Ethernet interfaces. You can use interface wildcards to view all your fe interfaces:

user@router> show interfaces fe* brief
Physical interface: fe-0/0/0, Enabled, Physical link is Down
 Link-level type: Ethernet, MTU: 1514, Speed: 10m, Loopback: Disabled,
 Source filtering: Disabled, Flow control: Enabled
 Device flags  : Present Running Down
 Interface flags: Hardware-Down SNMP-Traps Internal: 0x4000
 Link flags   : None
 
(output snipped)

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