15. Virtual Links

algorithm then iterates again. It terminates when the candidate list becomes empty.

The following steps describe the algorithm in detail. Remember that we are computing the shortest path tree for Area A. All references to link state database lookup below are from Area A's database.

(1) Initialize the algorithm's data structures. Clear the list of candidate vertices. Initialize the shortest-path tree to only the root (which is the router doing the calculation). Set Area A's TransitCapability to FALSE.

(2) Call the vertex just added to the tree vertex V. Examine the LSA associated with vertex V. This is a lookup in the Area A's link state database based on the Vertex ID. If this is a router-LSA, and bit V of the router-LSA (see Section A.4.2) is set, set Area A's TransitCapability to TRUE. In any case, each link described by the LSA gives the cost to an adjacent vertex. For each described link, (say it joins vertex V to vertex W):

(a) If this is a link to a stub network, examine the next link in V's LSA. Links to stub networks will be considered in the second stage of the shortest path calculation.

(b) Otherwise, W is a transit vertex (router or transit network). Look up the vertex W's LSA (router-LSA or network-LSA) in Area A's link state database. If the LSA does not exist, or its LS age is equal to MaxAge, or it does not have a link back to vertex V, examine the next link in V's LSA.[23]

(d) Calculate the link state cost D of the resulting path from the root to vertex W. D is equal to the sum of the link state cost of the (already calculated) shortest path to vertex V and the advertised cost of the link between vertices V and W. If D is:

o Greater than the value that already appears for vertex W on the candidate list, then examine the next link.

o Equal to the value that appears for vertex W on the candidate list, calculate the set of next hops that result from using the advertised link. Input to this calculation is the destination (W), and its parent (V). This calculation is shown in Section 16.1.1. This set of hops should be added to the next hop values that appear for W on the candidate list.

o Less than the value that appears for vertex W on the candidate list, or if W does not yet appear on the candidate list, then set the entry for W on the candidate list to indicate a distance of D from the root. Also calculate the list of next hops that result from using the advertised link, setting the next hop values for W accordingly. The next hop calculation is described in Section 16.1.1; it takes as input the destination (W) and its parent (V).

(3) If at this step the candidate list is empty, the shortest- path tree (of transit vertices) has been completely built and this stage of the procedure terminates. Otherwise, choose the vertex belonging to the candidate list that is closest to the root, and add it to the shortest-path tree (removing it from the candidate list in the process). Note that when there is a choice of vertices closest to the root, network vertices must be chosen before router vertices in order to necessarily find all equal-cost paths. This is consistent with the tie-breakers that were introduced in the modified Dijkstra algorithm used by OSPF's Multicast routing extensions (MOSPF).

(4) Possibly modify the routing table. For those routing table entries modified, the associated area will be set to Area A, the path type will be set to intra-area, and the cost will be set to the newly discovered shortest path's calculated distance.

If the newly added vertex is an area border router or AS boundary router, a routing table entry is added whose destination type is "router". The Options field found in the associated router-LSA is copied into the routing table entry's Optional capabilities field. Call the newly added vertex Router X. If Router X is the endpoint of one of the calculating router's virtual links, and the virtual link uses Area A as Transit area: the virtual link is declared up, the IP address of the virtual interface is set to the IP address of the outgoing interface calculated above for Router X, and the virtual neighbor's IP address is set to Router X's interface address (contained in Router X's router-LSA) that points back to the root of the shortest- path tree; equivalently, this is the interface that points back to Router X's parent vertex on the shortest-path tree (similar to the calculation in Section 16.1.1).

If the newly added vertex is a transit network, the routing table entry for the network is located. The entry's Destination ID is the IP network number, which can be obtained by masking the Vertex ID (Link State ID) with its associated subnet mask (found in the body of the associated network-LSA). If the routing table entry already exists (i.e., there is already an intra-area route to the destination installed in the routing table), multiple vertices have mapped to the same IP network. For example, this can occur when a new Designated Router is being established. In this case, the current routing table entry should be overwritten if and only if the newly found path is just as short and the current routing table entry's Link State Origin has a smaller Link State ID than the newly added vertex' LSA.

If there is no routing table entry for the network (the usual case), a routing table entry for the IP network should be added. The routing table entry's Link State Origin should be set to the newly added vertex' LSA.

(5) Iterate the algorithm by returning to Step 2.

The stub networks are added to the tree in the procedure's second stage. In this stage, all router vertices are again examined. Those that have been determined to be unreachable in the above first phase are discarded. For each reachable router vertex (call it V), the associated router-LSA is found in the link state database. Each stub network link appearing in the LSA is then examined, and the following steps are executed:

(1) Calculate the distance D of stub network from the root. D is equal to the distance from the root to the router vertex (calculated in stage 1), plus the stub network link's advertised cost. Compare this distance to the current best cost to the stub network. This is done by looking up the stub network's current routing table entry. If the calculated distance D is larger, go on to examine the next stub network link in the LSA.

(2) If this step is reached, the stub network's routing table entry must be updated. Calculate the set of next hops that would result from using the stub network link. This calculation is shown in Section 16.1.1; input to this calculation is the destination (the stub network) and the parent vertex (the router vertex). If the distance D is the same as the current routing table cost, simply add this set of next hops to the routing table entry's list of next hops. In this case, the routing table already has a Link State Origin. If this Link State Origin is a router-LSA whose Link State ID is smaller than V's Router ID, reset the Link State Origin to V's router-LSA.

Otherwise D is smaller than the routing table cost. Overwrite the current routing table entry by setting the routing table entry's cost to D, and by setting the entry's list of next hops to the newly calculated set. Set the routing table entry's Link State Origin to V's router-LSA. Then go on to examine the next stub network link.

For all routing table entries added/modified in the second stage, the associated area will be set to Area A and the path type will be set to intra-area. When the list of reachable router-LSAs is exhausted, the second stage is completed. At

this time, all intra-area routes associated with Area A have been determined.

The specification does not require that the above two stage method be used to calculate the shortest path tree. However, if another algorithm is used, an identical tree must be produced. For this reason, it is important to note that links between transit vertices must be bidirectional in order to be included in the above tree. It should also be mentioned that more efficient algorithms exist for calculating the tree; for example, the incremental SPF algorithm described in [Ref1].

16.1.1. The next hop calculation

This section explains how to calculate the current set of next hops to use for a destination. Each next hop consists of the outgoing interface to use in forwarding packets to the destination together with the IP address of the next hop router (if any). The next hop calculation is invoked each time a shorter path to the destination is discovered. This can happen in either stage of the shortest-path tree calculation (see Section 16.1). In stage 1 of the shortest-path tree calculation a shorter path is found as the destination is added to the candidate list, or when the destination's entry on the candidate list is modified (Step 2d of Stage 1). In stage 2 a shorter path is discovered each time the destination's routing table entry is modified (Step 2 of Stage 2).

The set of next hops to use for the destination may be recalculated several times during the shortest-path tree calculation, as shorter and shorter paths are discovered. In the end, the destination's routing table entry will always reflect the next hops resulting from the absolute shortest path(s).

Input to the next hop calculation is a) the destination and b) its parent in the current shortest path between the root (the calculating router) and the destination. The parent is always a transit vertex (i.e., always a router or a transit network).

If there is at least one intervening router in the current shortest path between the destination and the root, the destination simply inherits the set of next hops from the parent. Otherwise, there are two cases. In the first case, the parent vertex is the root (the calculating router itself). This means that the destination is either a directly connected network or directly connected router. The outgoing interface in this case is simply the OSPF interface connecting to the destination network/router. If the destination is a router which connects to the calculating router via a Point-to-MultiPoint network, the destination's next hop IP address(es) can be determined by examining the destination's router-LSA: each link pointing back to the calculating router and having a Link Data field belonging to the Point-to-MultiPoint network provides an IP address of the next hop router. If the destination is a directly connected network, or a router which connects to the calculating router via a point-to-point interface, no next hop IP address is required. If the destination is a router connected to the calculating router via a virtual link, the setting of the next hop should be deferred until the calculation in Section 16.3.

In the second case, the parent vertex is a network that directly connects the calculating router to the destination router. The list of next hops is then determined by examining the destination's router-LSA. For each link in the router-LSA that points back to the parent network, the link's Link Data field provides the IP address of a next hop router. The outgoing interface to use can then be derived from the next hop IP address (or it can be inherited from the parent network).

16.2. Calculating the inter-area routes

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