路由器情况二

 

R1和R2段是12.1.1.0/24

R2和R3段是23.1.1.0/24

R3和R4段是34.1.1.0/24

R2和R3之间运行EIGRP路由协议，发布的网段是0.0.0.0

R1设置网关地址是12.1.1.2

R4设置网关地址是34.1.13

 

R1的12.1.1.1要和R4和34.1.1.4发生通信，会丢一两个包，具体通信过程如下

1.R1先把对方的IP地址和自己的mask计算，然后把自己的IP地址和自己mask计算，发现不相同，就知道和自己不是同个网段的，发现自己有网关地址，

所以发送icmp包时先查询网关的mac地址，然后二层封装的源地址是自己的mac地址，目的地址是R2的F1/0的mac地址，把icmp请求包发给R2的F1/0

 

2.R2收到该数据包，发现是发给自己的，然后检查FCS域，没错的话，就去掉二层头部，裸露三层包头和ICMP数据，检查IP包头的校验和，没错的话，就查看目的地址

发现是34.1.1.4，然后查找FIB(FIB其实就是路由表的拷贝，是在控制层面把路由表拷贝进FIB这个硬件区域中的)，并行操作，符合最长掩码符合原则，最后查找到要去34.1.1.4

要从F1/1出去，下一跳地址是23.1.13，所以自己的三层源目地址不变，然后TTL减掉一，变为254，重新计算ip包头校验和，然后二层需要封装R2的F1/1的mac地址和

R3的F1/1的mac地址，通过adj表可以直接进行mac地址重写，然后计算FCS，从R2的F1/1发送出去

 

R2开启debug ip packets，发现R1去ping R4是CEF传送数据包

R2#

*Feb  8 11:06:15.363: CEF: Try to CEF switch 34.1.1.4 from FastEthernet1/0

*Feb  8 11:06:15.363: CEF: CEF switched to FastEthernet1/1

*Feb  8 11:06:15.451: CEF: Try to CEF switch 12.1.1.1 from FastEthernet1/1

*Feb  8 11:06:15.451: CEF: CEF switched to FastEthernet1/0

*Feb  8 11:06:15.483: CEF: Try to CEF switch 34.1.1.4 from FastEthernet1/0

*Feb  8 11:06:15.483: CEF: CEF switched to FastEthernet1/1

*Feb  8 11:06:15.527: CEF: Try to CEF switch 12.1.1.1 from FastEthernet1/1

*Feb  8 11:06:15.527: CEF: CEF switched to FastEthernet1/0

*Feb  8 11:06:15.547: CEF: Try to CEF switch 34.1.1.4 from FastEthernet1/0

*Feb  8 11:06:15.547: CEF: CEF switched to FastEthernet1/1

*Feb  8 11:06:15.567: CEF: Try to CEF switch 12.1.1.1 from FastEthernet1/1

*Feb  8 11:06:15.567: CEF: CEF switched to FastEthernet1/0

*Feb  8 11:06:15.599: CEF: Try to CEF switch 34.1.1.4 from FastEthernet1/0

*Feb  8 11:06:15.599: CEF: CEF switched to FastEthernet1/1

*Feb  8 11:06:15.619: CEF: Try to CEF switch 12.1.1.1 from FastEthernet1/1

*Feb  8 11:06:15.619: CEF: CEF switched to FastEthernet1/0

*Feb  8 11:06:15.679: CEF: Try to CEF switch 34.1.1.4 from FastEthernet1/0

*Feb  8 11:06:15.679: CEF: CEF switched to FastEthernet1/1

*Feb  8 11:06:15.699: CEF: Try to CEF switch 12.1.1.1 from FastEthernet1/1

*Feb  8 11:06:15.699: CEF: CEF switched to FastEthernet1/0

*Feb  8 11:06:16.447: CEF: Try to CEF switch 224.0.0.10 from FastEthernet1/1

 

通过show ip cef查看FIB区域里面的内容

R2#sh ip cef

Prefix              Next Hop             Interface

0.0.0.0/0           drop                 Null0 (default route handler entry)

0.0.0.0/32          receive

12.0.0.0/8          0.0.0.0              Null0

12.1.1.0/24         attached             FastEthernet1/0

12.1.1.0/32         receive

12.1.1.1/32         12.1.1.1             FastEthernet1/0

12.1.1.2/32         receive

12.1.1.255/32       receive

23.0.0.0/8          0.0.0.0              Null0

23.1.1.0/24         attached             FastEthernet1/1

23.1.1.0/32         receive

23.1.1.2/32         receive

23.1.1.3/32         23.1.1.3             FastEthernet1/1

23.1.1.255/32       receive

34.0.0.0/8          23.1.1.3             FastEthernet1/1

224.0.0.0/4         drop

224.0.0.0/24        receive

255.255.255.255/32  receive

 

R2#sh adjacency detail

Protocol Interface                 Address

IP       FastEthernet1/0           12.1.1.1(5)

                                   8 packets, 912 bytes

                                   CA0013FC001C

                                   CA0113FC001C0800

                                   ARP        never    

                                   Epoch: 0

IP       FastEthernet1/1           23.1.1.3(7)

                                   10 packets, 1140 bytes

                                   CA02030C001D

                                   CA0113FC001D0800

                                   ARP        never    

                                   Epoch: 0

 

3.R3收到该数据包，检查FCS，无错，拆掉二层，裸露ip包头和icmp数据，检查ip包头检验和，无错，就查看目的地址，然后通过CEF进行相关操作，把数据包从R3的F1/0发送出去

 

4.R3的F1/1收到该数据包，检查FCS，无错，拆掉二层，裸露ip包头和icmp数据，检查ip包头检验和，无错，查看目的地址，发现就是从自己这里出去，然后查看mac地址，发现自己没有34.1.1.4的mac地址，所以会丢包，同时进行arp查询

 

、

 

R1会收到R2的EIGRP的数据包，是因为没有配置被动接口

 

这里的回应包TTL为253是因为经过了R2和R3两个路由器