When it comes to monitoring the network, the most essential CLI commands are show interfaces brief and show interface detail. For example, if you suspect that Gigabit Ethernet interface ge-0/0/0 is down, a simple show command reveals its status (which is indeed “Down”):

user@router>show interfaces ge-0/0/0 brief
Physical interface: ge-0/0/0, Enabled, Physical link is Down
  Link-level type: Ethernet, MTU 1514, Speed 1ge, Loopback: Disabled
  Source filtering: Disabled, Flow control: Enabled
  Device flags    : Present Running Down
  Interface flags : Hardware-Down SNMP-Traps Internal: 0x4000
  Link flags      : None

On the other hand, to see a lot more detail about the interface’s queues, input-output byte counts, and the same for packet counts, you can use the show interfaces ge-0/0/0 command to add this information:

user@router>show interfaces ge-0/0/0 detail
Physical interface: ge-0/0/0, Enabled, Physical link is Down
  Link-level type: Ethernet, MTU 1514, Speed 1ge, Loopback: Disabled
  Source filtering: Disabled, Flow control: Enabled
  Device flags    : Present Running Down
  Interface flags : Hardware-Down SNMP-Traps Internal: 0x4000
  Link flags      : None
  CoS queues      : 4 supported, 4 maximum usable queues
  Hold-times      : Up 0 ms, Down 0 ms
  Current address : 00:05:85:02:a4:00, Hardware address: 00:05:85:02:a4:00
  Last flapped    : 2010-12-15 14:30:58 PST (1w2d 23:03 ago)
  Statistics last cleared: Never
  Traffic statistics:
    Input bytes   :             0               0 bps
    Output bytes  :             0               0 bps 
    Input packets :             0               0 bps 
    Output packets:             0               0 bps
  IPv6 transit statistics:
    Input bytes   :             0               
    Output bytes  :             0               
    Input packets :             0               
    Output packets:             0               
  Egress queues: 4 supported, 4 in use
  Queue counters:    Queued packets  Transmitted packets   Dropped packets
    0 best-effort                 0                    0                 0
    1 expedited-fo                0                    0                 0
    2 assured-forw                0                    0                 0
    3 network-cont                0                    0                 0
  Active alarms   : LINK
  Active defects  : LINK