Special Rules Concerning Intra-area and Interarea Routes on ABRs
OSPF has a couple of rules concerning intra-area and interarea routes that take precedence over the simple comparison of the cost calculated for the various routes. The issue exists when more than one ABR connects to the same two areas. Many designs use two routers between the backbone and each nonbackbone area for redundancy, so this design occurs in many OSPF networks.
The issue relates to the fact that with two or more ABRs, the ABRsthemselves, when calculating their own routing tables, can calculate both an intra-area route and interarea route for subnets in the backbone area. For example, consider the perspective of Router R1 from the last several examples, as depicted in Figure 6-14.
Conceptually, R1 could calculate both the intra-area route and interarea route to 10.10.99.0/24. However, the OSPF cost settings could be set so that the lower cost route.
Figure 6-14 R1’s Choice: Intra-Area or Interarea Route to 10.10.99.0/24
for R1 actually goes through area 34, to ABR R2, and then on through Area 0 to 10.10.99.0/24. However, two OSPF rules prevent such a choice by R1:
Step 1. When choosing the best route, an intra-area route is always better than a competing interarea route, regardless of metric.
Step 2. If an ABR learns a Type 3 LSA inside a nonbackbone area, the ABR ignores that LSA when calculating its own routes.
Because of the first rule, R1 would never choose the interarea route if the intra-area route were available. The second rule goes further, stating that R1 could never choose the interarea route at all–R1 simply ignores that LSA for the purposes of choosing its own best IP routes.
