为什么当我执行show interface命令时我看到Last input总是显示never？

FastEthernet1/31 is up, line protocol is up (connected)
  Hardware is C6k 100Mb 802.3, address is 000d.bd5a.e94e (bia 000d.bd5a.e94e)
  Description: Server
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Full-duplex, 100Mb/s
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  **Last input never**, output 00:03:19, output hang never
  Last clearing of "show interface" counters 1y3w
  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 2000 bits/sec, 4 packets/sec
  5 minute output rate 13000 bits/sec, 20 packets/sec
     148174387 packets input, 16331478383 bytes, 0 no buffer
     Received 465 broadcasts (0 multicast)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     798328682 packets output, 102549789248 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out

如您所见，几分钟后我运行该命令时，其他计数器已增加。

FastEthernet1/31 is up, line protocol is up (connected)
  Last input never, output 00:01:39, output hang never
  5 minute input rate 2000 bits/sec, 4 packets/sec
  5 minute output rate 14000 bits/sec, 20 packets/sec
     148176042 packets input, 16331609502 bytes, 0 no buffer
     798337129 packets output, 102550523914 bytes, 0 underruns