二層及三層MTU淺析

 不少人知道MTU，也知道它全稱是最大傳輸單元，可是不知道MTU其實能夠分紅二層MTU和三層MTU。

二層MTU不能改變，三層MTU能夠改！

Router#sh ip int f0/0 

FastEthernet0/0 is up, line protocol is up

  Internet address is 10.1.1.1/24

  Broadcast address is 255.255.255.255

  Address determined by setup command

  MTU is 1500 bytes

  Helper address is not set

  Directed broadcast forwarding is disabled

  Outgoing access list is not set

  Inbound  access list is not set

  Proxy ARP is enabled

  Security level is default

  Split horizon is enabled

  ICMP redirects are always sent

  ICMP unreachables are always sent

  ICMP mask replies are never sent

  IP fast switching is enabled

  IP fast switching on the same interface is disabled

  IP Flow switching is disabled

  IP Fast switching turbo vector

  IP multicast fast switching is enabled

  IP multicast distributed fast switching is disabled

  IP route-cache flags are Fast

  Router Discovery is disabled

以上這個是三層MTU！

 

 

Router#sh int f0/0 

FastEthernet0/0 is administratively down, line protocol is down 

  Hardware is AmdFE, address is c800.17d4.0000 (bia c800.17d4.0000)

  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, 

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

  Half-duplex, 100Mb/s, 100BaseTX/FX

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input never, output never, output hang never

  Last clearing of "show interface" counters never

  Queueing strategy: fifo

  Output queue 0/40, 0 drops; input queue 0/75, 0 drops

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 0 bits/sec, 0 packets/sec

     0 packets input, 0 bytes

     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog

     0 input packets with dribble condition detected

     0 packets output, 0 bytes, 0 underruns(0/0/0)

     0 output errors, 0 collisions, 0 interface resets

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier

     0 output buffer failures, 0 output buffers swapped out

以上這個是二層MTU！

 

Router(config-if)#mtu 666 

% Interface FastEthernet0/0 does not support user settable mtu.

二層MTU不能被改變！

Router(config-if)#ip mtu ?

  <68-1000000>  MTU (bytes)

 

Router(config-if)#ip mtu 666

Router(config-if)#end

 

Router#sh ip int f0/0 

FastEthernet0/0 is up, line protocol is up

  Internet address is 10.1.1.1/24

  Broadcast address is 255.255.255.255

  Address determined by setup command

  MTU is 666 bytes

  Helper address is not set

  Directed broadcast forwarding is disabled

  Outgoing access list is not set

  Inbound  access list is not set

  Proxy ARP is enabled

  Security level is default

  Split horizon is enabled

  ICMP redirects are always sent

  ICMP unreachables are always sent

  ICMP mask replies are never sent

  IP fast switching is enabled

  IP fast switching on the same interface is disabled

  IP Flow switching is disabled

  IP Fast switching turbo vector

  IP multicast fast switching is enabled

  IP multicast distributed fast switching is disabled

  IP route-cache flags are Fast

  Router Discovery is disabled

  IP output packet accounting is disabled

  IP access violation accounting is disabled

  TCP/IP header compression is disabled

  RTP/IP header compression is disabled

  Probe proxy name replies are disabled

  Policy routing is disabled

  Network address translation is disabled

  WCCP Redirect outbound is disabled

  WCCP Redirect inbound is disabled

  WCCP Redirect exclude is disabled

  BGP Policy Mapping is disabled

 

 

Router#sh int f0/0 

FastEthernet0/0 is up, line protocol is up 

  Hardware is AmdFE, address is c800.17d4.0000 (bia c800.17d4.0000)

  Internet address is 10.1.1.1/24

  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, 

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

  Half-duplex, 100Mb/s, 100BaseTX/FX

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input never, output 00:00:09, output hang never

  Last clearing of "show interface" counters never

  Queueing strategy: fifo

  Output queue 0/40, 0 drops; input queue 0/75, 0 drops

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 0 bits/sec, 0 packets/sec

     0 packets input, 0 bytes

     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog

     0 input packets with dribble condition detected

     9 packets output, 2075 bytes, 0 underruns(0/0/0)

     0 output errors, 0 collisions, 1 interface resets

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier

     0 output buffer failures, 0 output buffers swapped out

 

結論：有時候你以爲本身很瞭解一個東西，例如MTU，可是深刻之後卻發覺這個東西不是你理解的這麼淺，故，學習學習再學習，深刻學習！