ip access-group num in
欢迎进入网络技术社区论坛，与200万技术人员互动交流 >>进入 一直以为自己对这个ip access-group num in | out 应用很理解,当自己做实验分析的时候分现很模糊,以下是认真的分析总结: 首先： 路由器每个接口应用ACL是针对每个数据包的过滤，三层是IP数据包，
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一直以为自己对这个ip access-group num in | out 应用很理解,当自己做实验分析的时候分现很模糊,以下是认真的分析总结:
首先：
路由器每个接口应用ACL是针对每个数据包的过滤，三层是IP数据包，二层是数据帧MAC
路由器对自己内部产生的数据不会起作用
in就是路由器接口收到的数据，out就是路由器接口离开的数据
站在路由器的角度，路由器只能是一个接口in，从另一个接口out(不会是从一个接口in，又从这个接口out)
（有一个很恶心的比喻：人是router吃是in大便是out）
如下图：一个纯LAN的网络,在R1的f0/0应该ip access-group /out
!
interface FastEthernet0/0
ip address .1 255.255.255.0
ip access-group 1 out
duplex auto
speed auto
!
access-list 1 deny .2
access-list 1 permit .3
如果：ACL在接口out方向不起数据包过滤的作用
R1#ping .2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to .2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/44/136 ms
R1#ping .3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to .3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/43/136 ms
分析为什么:在R1路由器产生ping .2的数据包，源地址12.1.1.1，目的地址12.1.1.2但由于是路由器自己产生的数据包在接口上不起作用，所以相对f0/0的out方向不起作用。看回的ping包,从R2路由器产生回包,源地址12.1.1.2,目的地址12.1.1.1相对R1的f0/0接口是in方向，但没做in方向的数据过滤,同理ping 12.1.1.3也通
!
interface FastEthernet0/0
ip address .1 255.255.255.0
ip access-group
duplex auto
speed auto
!
相反在f0/0接口上起in方向的数据过滤结果:
R1#
R1#ping .1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to .1, timeout is 2 seconds:
....
Success rate is 0 percent (0/4)
R1#ping .2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to .2, timeout is 2 seconds:
...
Success rate is 0 percent (0/3)
R1#ping .3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to .3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = ms
结果分析：R1路由器自己产生数据包ping .2 源地址：12.1.1.1，目的地址：12.1.1.2，相对f0/0接口数据发送出去,但是内部自己产生的数据包不起作用，所以发送成功，看回的数据包,R2产生回的数据包,源地址:12.1.1.2，目的地址:12.1.1.1到达R1的f0/0接口相对R1路由器f0/0就是为in方向这时作用ACL过滤，过滤掉源为12.1.1.2的地址，所以不成功
相反，源地址.3满足ACL,所以ping通.
总之，访问列表设计为过滤通过路由器的流量，但不过滤路由器产生的流量。
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