The Link State protocol compute the best base on Link Stat Data Base (LSDB).
For OSPF, the Intra > Inter > External,
For ISIS,
1. Level 1 intra-area routes with an internal metric
2. Level 1 external routes with an internal metric
3. Level 2 intra-area routes with an internal metric
4. Level 2 external routes with an internal metric
5. Inter-area routes (Level 1 to Level 2) with an internal metric
6. Inter-area external routes (Level 1 to Level 2) with an internal metric
7. Inter-area routes (Level 2 to Level 1) with an internal metric
8. Inter-area external routes (Level 2 to Level 1) with an internal metric
9. Level 1 external routes with an external metric
10. Level 2 external routes with an external metric
11. Inter-area external routes (Level 1 to Level 2) with an external metric
12. Inter-area external routes (Level 2 to Level 1) with an external metric
If the same prefix in LSDB locate in different Level or Area, the Intra-area always the best even you cahnge the Inter-area AD.
Here is a smiple question for you :
R2,R3 link = Level-2
R2,R1 link = Level-1
R1,R3 link = Level-1
R2 Lo0 = Level-2
R3 Lo0 = Level-2
R1 Lo0 = Level-1
1, R1,R2,R3 in the same area.
2, R1 can't using 0/0 to reach either R2/R3.
3, All router Lo0 must reach each other in best way, you can't have suboptimal routing in your network.
Can you do it ?
