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- Intra-area routes: routes distributed within the same area using LSA types 1 and 2.
- Inter-area routes: routes received from neighboring areas using LSA type 3.
- Tyoe Type 1 external routes: routes to external networks received from the ASBR. The route metric for this type of routes is counted as the sum of the metric set by the ASBR during the announcement plus the metric of the path to the ASBR.
- External Type 2 external routes type 2: are similar to External routes the type 1 external routes, with a different method for the metric calculation. The metric is equal to the value set by the ASBR during the announcement and does not include the path to the ASBR.
- Metric value: for two routes to the same network received from sources of the same type, the metric values are compared. The route with the lower metric value will be added to the RIB.
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- Normal;
- Stub;
- Totally Stub;
- NSSA;
- Totally NSSA.
Let's look at the main features of different area types using the example in (Figure 6): routers R1, R2, R3 and R4 are connected in chain with each other, forming three OSPF areas. Routers R3 and R4 have external links. In each example, we will change the type of area 4 and analyze the LSA types associated with that area. In these examples, the LSA's not related with area 4 and the type 1 and 2 LSAs will be omitted because they are distributed within any area type.
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Normal areas do not change the LSA propagation of the LSAs and the processing logic described above (Figure 7a). This area type is used by default. The backbone area is a special case of the Normal area.
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Stub areas are used in LAN segments that have no connection with the external networks, but the routers in this area must receive full routing information from the neighboring areas. The Using Stub area usage areas allows to obtain a small increase in performance by reducing the LSA number and to protect the network from attacks from the external network segment.
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