16.2. Calculating the inter-area routes

The inter-area routes are calculated by examining summary link advertisements. If the router has active attachments to multiple areas, only backbone summary link advertisements are examined. Routers attached to a single area examine that area's summary links. In either case, the summary links examined below are all part of a single area's link state database (call it Area A).

Summary link advertisements are originated by the area border routers. Each summary link advertisement in Area A is considered in turn. Remember that the destination described by a summary link advertisement is either a network (Type 3 summary link advertisements) or an AS boundary router (Type 4 summary link advertisements). For each summary link advertisement:

1. If the cost specified by the advertisement is LSInfinity, or if the advertisement's LS age is equal to MaxAge, then examine the the next advertisement.

2. If the advertisement was originated by the calculating router itself, examine the next advertisement.

3. If the collection of destinations described by the summary link advertisement falls into one of the router's configured area address ranges (see Section 3.5) and the particular area address range is active, the summary link advertisement should be ignored. Active means that there are one or more reachable (by intra-area paths) networks contained in the area range. In this case, all addresses in the area range are assumed to be either reachable via intra-area paths, or else to be unreachable by any other means.

4. Else, call the destination described by the advertisement N (for Type 3 summary links, N's address is obtained by masking the advertisement's Link State ID with the network/subnet mask contained in the body of the advertisement), and the area border originating the advertisement BR. Look up the routing table entry for BR having Area A as its associated area. If no such entry exists for router BR (i.e., BR is unreachable in Area A), do nothing with this advertisement and consider the next in the list. Else, this advertisement describes an inter-area path to destination N, whose cost is the distance to BR plus the cost specified in the advertisement. Call the cost of this inter-area path IAC.

5. Next, look up the routing table entry for the destination N. (The entry's Destination Type is either Network or AS boundary router.) If no entry exists for N or if the entry's path type is "type 1 external" or "type 2 external", then install the inter-area path to N, with associated area Area A, cost IAC, next hop equal to the list of next hops to router BR, and Advertising router equal to BR.

6. Else, if the paths present in the table are intra-area paths, do nothing with the advertisement (intra-area paths are always preferred).

7. Else, the paths present in the routing table are also inter-area paths. Install the new path through BR if it is cheaper, overriding the paths in the routing table. Otherwise, if the new path is the same cost, add it to the list of paths that appear in the routing table entry.