16.7. Events generated as a result of routing table changes

Changes to routing table entries sometimes cause the OSPF area border routers to take additional actions. These routers need to act on the following routing table changes:

• The cost or path type of a routing table entry has changed. If the destination described by this entry is a Network or AS boundary router, and this is not simply a change of AS external routes, new summary link advertisements may have to be generated (potentially one for each attached area, including the backbone). See Section 12.4.3 for more information. If a previously advertised entry has been deleted, or is no longer advertisable to a particular area, the advertisement must be flushed from the routing domain by setting its LS age to MaxAge and reflooding (see Section 14.1).

• A routing table entry associated with a configured virtual link has changed. The destination of such a routing table entry is an area border router. The change indicates a modification to the virtual link's cost or viability.

  If the entry indicates that the area border router is newly reachable (via TOS 0), the corresponding virtual link is now operational. An InterfaceUp event should be generated for the virtual link, which will cause a virtual adjacency to begin to form (see Section 10.3). At this time the virtual link's IP interface address and the virtual neighbor's Neighbor IP address are also calculated.

  If the entry indicates that the area border router is no longer reachable (via TOS 0), the virtual link and its associated adjacency should be destroyed. This means an InterfaceDown event should be generated for the associated virtual link.

  If the cost of the entry has changed, and there is a fully established virtual adjacency, a new router links advertisement for the backbone must be originated. This in turn may cause further routing table changes.