The subnet 10.1.1.1/24 is redistributed into EIGRP domain so it will have the Administrative Distance (AD) of 170. Therefore R4 also learns about this subnet advertised from R2 with the same AD of 170.
In the other hand, subnet 10.1.1.0/24 is also redistributed into OSPF on R3 so R5 & R4 will learn about this subnet with AD of 110, which is better than the above AD of 170 so R4 will choose path R4 -> R5 -> R3 -> R2 -> R1.
In order to solve this problem, we can configure an outbound distribute list on R5 to prevent (filter out) this subnet from advertising to R4. Then R4 only has one way to reach R1, which is R4 -> R2
- > R1. But this method will remove the backup route so it is not the best solution.
Another solution is to reduce the AD of the external EIGRP to a value smaller than 110. This method reserves the backup route in case of the main route fails -> This is the best solution. To do this, we can use the following command on R4:
router eigrp 1
distance eigrp 90 91 //Changes the AD to 90 for internal EIGRP routes and changes the AD to 91 for EIGRP external routes We tested this lab in GNS3 and you can read this lab here. This is the result when we type the
"distance eigrp ..." command above on R4:
R4#show ip route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 6 subnets
D 10.1.12.0 [90/30720] via 10.1.24.2, 00:00:05, FastEthernet0/0 D EX 10.1.1.0 [91/33280] via
10.1.24.2, 00:00:05, FastEthernet0/0 C 10.1.24.0 is directly connected, FastEthernet0/0 D 10.1.23.0 [90/30720] via 10.1.24.2, 00:00:05, FastEthernet0/0 C 10.1.45.0 is directly connected, FastEthernet1/0 O 10.1.35.0 [110/2] via 10.1.45.5, 00:00:11, FastEthernet1/0 Note: We can change the AD of EIGRP routes via the "distance eigrp ..." command but the effect of this command is local only.