This step is only performed by area border routers attached to one or more transit areas. Transit areas are those areas supporting one or more virtual links; their TransitCapability parameter has been set to TRUE in Step 2 of the Dijkstra algorithm (see Section 16.1). They are the only non-backbone areas that can carry data traffic that neither originates nor terminates in the area itself.
The purpose of the calculation below is to examine the transit areas to see whether they provide any better (shorter) paths than the paths previously calculated in Sections 16.1 and 16.2. Any paths found that are better than or equal to previously discovered paths are installed in the routing table.
The calculation proceeds as follows. All the transit areas' summary link advertisements are examined in turn. Each such summary link advertisement describes a route through a transit area Area A to a Network N (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) or in the case of a Type 4 summary link advertisement, to an AS boundary router N. Suppose also that the summary link advertisement was originated by an area border router BR.
It is important to note that the above calculation never makes unreachable destinations reachable, but instead just potentially finds better paths to already reachable destinations. Also, unlike Section 16.3 of [RFC 1247], the above calculation installs any better cost found into the routing table entry, from which it may be readvertised in summary link advertisements to other areas.
As an example of the calculation, consider the Autonomous System pictured in Figure 17. There is a single non-backbone area (Area 1) that physically divides the backbone into two separate pieces. To maintain connectivity of the backbone, a virtual link has been configured between routers RT1 and RT4. On the right side of the figure, Network N1 belongs to the backbone. The dotted lines indicate that there is a much shorter intra-area backbone path between router RT5 and Network N1 (cost 20) than there is between Router RT4 and Network N1 (cost 100). Both Router RT4 and Router RT5 will inject summary link advertisements for Network N1 into Area 1.
........................ . Area 1 (transit) . + . . | . +---+1 1+---+100 | . |RT2|----------|RT4|=========| . 1/+---+********* +---+ | . /******* . | . 1/*Virtual . | 1+---+/* Link . Net|work =======|RT1|* . | N1 +---+\ . | . \ . | . \ . | . 1\+---+1 1+---+20 | . |RT3|----------|RT5|=========| . +---+ +---+ | . . | ........................ + Figure 17: Routing through transit areas
After the shortest-path tree has been calculated for the backbone in Section 16.1, Router RT1 (left end of the virtual link) will have calculated a path through Router RT4 for all data traffic destined for Network N1. However, since Router RT5 is so much closer to Network N1, all routers internal to Area 1 (e.g., Routers RT2 and RT3) will forward their Network N1 traffic towards Router RT5, instead of RT4. And indeed, after examining Area 1's summary link advertisements by the above calculation, Router RT1 will also forward Network N1 traffic towards RT5. Note that in this example the virtual link enables Network N1 traffic to be forwarded through the transit area Area 1, but the actual path the data traffic takes does not follow the virtual link. In other words, virtual links allow transit traffic to be forwarded through an area, but do not dictate the precise path that the traffic will take.