The ACL filters out odd or even routes.

In a multi-protocol network with an odd or even number of routes, the customer Address Allocation is very fine, many of which are: www.2cto.com 192.168.1.0/24192.168.2.0/24192.168.3.0/24192.168.4.0/24192.168.5.0/24 .......................... Some networks need to filter and analyze all the network segments with even digits: Because R1 is required to be able to learn only the network numbers of odd digits or even numbers of R3. split each subnet into a binary system to observe the rule 192.168.1.0/24192.168.00000001.0/24192.168.2.0/24192.168.00000010.0/24192.168.3.0/24 192.168.4.0/latest/24192.168.5.0/latest/24192.168.6.0/latest/24192.168.7.0/24 192.168.8.0/24192.168.00001000.0/24 we can find that, if the x of 192.168.x.0/24 is an even number, the 24th bits are 0. If it is odd, the 24th bits are 1, therefore, according to this rule, the 0 of the ACL is completely matched. 1 indicates that the matching rule is ignored. The following two ACLs can be written to match the 24th bits. www.2cto.com: access-list 2 permit 192.168.1.0 0.0.254.0192.168.00000001.0000.000.111110.0 network number used for even (even) bits: access-list 1 permit 192.168.0.0 complete basic configuration Verification
Directly in R2 redistribution, R1 learned all the networks. R1 # sh ip route 1.0.0.0/32 is subnetted, 1 subnetsC 1.1.1.1 is directly connected, Loopback0 23.0.0.0/24 is subnetted, 1 subnetsO E2 23.1.1.0 [110/20] via 12.1.1.2, 00:00:05, fastEthernet0/0O E2 192.168.4.0/24 [110/20] via 12.1.1.2, 00:00:05, FastEthernet0/0 12.0.0.0/24 is subnetted, 1 subnetsC 12.1.1.0 is directly connected, fastEthernet0/0O E2 192.168.1.0/24 [110/20] via 12.1.1.2, 00:00:05, FastEt Hernet0/0O E2 192.168.2.0/24 [110/20] via 12.1.1.2, 00:00:05, FastEthernet0/0O E2 192.168.3.0/24 [110/20] via 12.1.1.2, 00:00:05, FastEthernet0/0 R2 (config) # ip access-list standard permit_oddR2 (config-std-nacl) # per 192.168.1.0 0.0.254.255 # Only odd numbers are allowed (the Anti-mask is written as 0.0.254.0) R2 (config) # route-map permit_odd per 10R2 (config-route-map) # match ip add permit_oddR2 (config-router) # redistribute rip subnets route-map pe Rmit_odd: Check the R1 route table and find that only the odd route is left. The experiment is successful. The same applies to even network segments. R1 # sh ip route 1.0.0.0/32 is subnetted, 1 subnetsC 1.1.1.1 is directly connected, Loopback0 12.0.0.0/24 is subnetted, 1 subnetsC 12.1.1.0 is directly connected, fastEthernet0/0O E2 192.168.1.0/24 [110/20] via 12.1.1.2, 00:04:24, FastEthernet0/0O E2 192.168.3.0/24 [110/20] via 12.1.1.2, 00:04:24, FastEthernet0/0