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| 关键词： nbsp size 字节 图中 包头 ping Timeout seconds address ICMP |
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#pingProtocol [ip]:Target IP address: 1.1.1.1Repeat count [5]: 1Datagram 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]: ySource address or interface:Type of service [0]:Set DF bit in IP header? [no]: yValidate reply data? [no]:Data pattern [0xABCD]:Loose, Strict, Record, Timestamp, Verbose[none]:Sweep range of sizes [n]: y Sweep min size [36]: 1500Sweep max size [18024]: 1501Sweep 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: 34.1.1.4Repeat count [5]: 1Datagram size [100]:Timeout in seconds [2]:Extended commands [n]: ySource address or interface:Type of service [0]:Set DF bit in IP header? [no]: yValidate reply data? [no]:Data pattern [0xABCD]:Loose, Strict, Record, Timestamp, Verbose[none]:Sweep range of sizes [n]: ySweep min size [36]: 1475Sweep max size [18024]: 1485Sweep 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。 本文出自 “辛－碌－力－成” 博客，请务必保留此出处http://orzorz.blog.51cto.com/4228156/756847 |
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