Hello everyone,
When there are two ABRs connecting two areas, we should connect them via two links. For example, if there are two ABRs between area 0 and area 1, we need to connect them through two links, one in area 0 and other one in area 1. It may cause suboptimal routing if there is only one link between ABRs.
I am using the following topology for my presentation.
In this topology, R3 and R4 are ABRs for area 1 and area 0. The left link between them in area 1 and the right link in area 0. I increased the cost between R4 and R1 to 1000.
The result of traceroute shows that R5 takes the path with the lowest cost to reach to 99.99.99.99. In this case, the best path is R4, R3 , R2, R1(last octet in the picture) . The path between R4 and R1 was not used because of the high cost.
Lets shutdown the left link( in area 1) between two ABRs.
And then try traceroute again.
The link 4.4.4.0/24 is down now. First, you would think that the other link between R3 and R4 will be used to reach R1 because of the lowest cost, but surprisingly, the link between R4 and R1 with cost of 1000 is used as you see the result of traceroute from R5 to R1. The path is R5,R4, R1.
Lets make sure the other link between R3 and R4 can be taken. The link between R4 and R1 is down now. As you see in the following picture, the path between R3 and are R4 was taken.
When the link between R1 and R4 is down, R4 receives inter area route(IA) from R3.
If there is only one link between two ABRs, route received by R4 from R3 is an inter area route(IA) and route received from R1 is an intra area route so route received from R1 is preferred even with higher cost. When there are two links( in area 1 and 0), R4 receives Intra area routes from R3 and R1 in area 1 and inter area routes from R3 in area 0, so the best path is intra area route coming from R3 since it has lower cost than route coming from R1.
Now, the link between ABRs in area 1 is down and link between R4 and R1 is up.As you see in the above picture, inter area route coming from R1 is replaced the inter area route coming from R3.
In short, we should connect two ABRs through two links since intra area routes are preferred over inter area routes so suboptimal path may be taken
Thank you all and wish you the best.