?很多人知道MTU，也知道它全稱是最大傳輸單元，但是不知道MTU其實可以分成二層MTU和三層MTU。

二層MTU不能改變，三層MTU可以改！

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：有時候你覺得自己很了解一個東西，例如MTU，但是深入以后卻發覺這個東西不是你理解的這么淺，故，學習學習再學習，深入學習！

轉載于:https://blog.51cto.com/nocedric/515817

超強干貨來襲 云風專訪：近40年碼齡，通宵達旦的技術人生

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