辛碌力成

windows的ping中-l（length）是指ICMP的净荷，不包括IP包头（一般20字节）和ICMP头（8字节），所以默认情况下MTU为1500字节，windows中：

ping 1.1.1.1 -f -l 1472  是通的，

ping 1.1.1.1 -f -l 1473  是不通的。

IOS的ping中Datagram size和sweep size都是指MTU，即

R1#ping
Protocol [ip]:
Target IP address: 1.1.1.1
Repeat count [5]: 1
Datagram size [100]: 1600 (在扩展ping中此选项好像失效，没起作用)
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface:
Type of service [0]:
Set DF bit in IP header? [no]: y
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]: y  
Sweep min size [36]: 1500
Sweep max size [18024]: 1501
Sweep interval [1]:
Type escape sequence to abort.
Sending 2, [1500..1501]-byte ICMP Echos to 34.1.1.4, timeout is 1 seconds:
Packet sent with the DF bit set
!.
Success rate is 50 percent (1/2), round-trip min/avg/max = 212/212/212 ms

 如图，所有路由器均为3620路由器，将R2的s0/0口MTU设为1480,

R1#ping   
Protocol [ip]:
Target IP address: 34.1.1.4
Repeat count [5]: 1
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface:
Type of service [0]:
Set DF bit in IP header? [no]: y
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]: y
Sweep min size [36]: 1475
Sweep max size [18024]: 1485
Sweep interval [1]:
Type escape sequence to abort.
Sending 11, [1475..1485]-byte ICMP Echos to 34.1.1.4, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!!…..
Success rate is 54 percent (6/11), round-trip min/avg/max = 100/127/200 ms
 

从图中可看出编号1002的包，也就是图中最后一行显示的Data值为1453的包（加上ICMP和IP包头共1453+8+20＝1481字节）开始因为超过R2的s0/0口MTU而回不到R1，于是向R4发送不可达的包。

但实际上从R1发出去的request包是可以通过R2的s0/0口的。也就是端口的MTU值只限制从该端口发出去的包MTU大小，不限制收到的包。

原文出自: http://blog.too2.net/?p=196
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