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本帖最后由 ckyin 于 2019-3-26 16:05 编辑
如图所示配置R1和R2,R1上面有4个回环接口,其中Lo3的子网掩码长度是26,在R1上有一条默认路由指向R2的s0/1接口IP,R2上面有一条静态路由指向R1的s0/0接口IP
172.17.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.17.1.0/24 is directly connected, Loopback2
C 172.17.2.0/26 is directly connected, Loopback3
172.16.0.0/24 is subnetted, 2 subnets
C 172.16.1.0 is directly connected, Loopback0
C 172.16.2.0 is directly connected, Loopback1
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 12.1.1.2
172.17.0.0/24 is subnetted, 1 subnets
C 172.17.3.0 is directly connected, Loopback0
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.3.0 is directly connected, Loopback1
12.0.0.0/24 is subnetted, 1 subnets
C 12.1.1.0 is directly connected, FastEthernet0/0
S 172.16.0.0/15 [1/0] via 12.1.1.1
1.R2增加Lo 0接口172.17.3.1/24,然后R1pingR2 Lo0,结果在默认快速交换下都能ping通,切换成进程交换(no ip cef),有类(no ip classless)查找下ping不通,无类(ip classless)查找下可以ping通
分析:根据路由查找规则,首先查找级别1路由,找到匹配的父路由172.17.0.0/16,然后查找子路由没有匹配的最终路由,下一步进行有类和无类查找,有类终止查找所以ping不通,无类继续查找级别1的超网和默认路由,这边最终选择默认路由ping通
2.R2新增Lo1接口172.16.3.1/24,同样R1pingR2 Lo1,结果在默认快速交换下都能ping通,切换成进程交换(no ip cef),有类(no ip classless)查找下ping不通,无类(ip classless)查找下可以ping通
分析:根据路由查找规则,首先查找级别1路由,只有默认路由匹配进行转发,但问题是为什么没有进行到无类有类查找这一步,有类无类查找对结果有影响?
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