|  | 11.4 | Link-State Routing
| 11.4.5 |
Unsynchronized link-state advertisements (LSAs) leading to inconsistent path decisions amongst routers
The most complex and important aspect of link-state routing is making sure that all routers get all necessary LSA packets. Routers with different sets of LSAs calculate routes based on different topological data. Then, networks become unreachable as a result of a disagreement among routers about a link. Following is an example of inconsistent path information: Between Routers C and D, Network 1 goes down. Both routers construct an LSA packet to reflect this unreachable status. Soon afterward, Network 1 comes back up; another LSA packet reflecting this next topology change is needed. If the original "Network 1, Unreachable" message from Router C uses a slow path for its update, that update comes later. This LSA packet can arrive at Router A after Router D's "Network 1, Back Up Now" LSA. With unsynchronized LSAs, Router A can face a dilemma about which SPF tree to construct. Should it use paths that include Network 1, or paths without Network 1, which was most recently reported as unreachable? If LSA distribution to all routers is not done correctly, link-state routing can result in invalid routes. Scaling up with link-state protocols on very large internetworks can expand the problem of faulty LSA packet distribution. If one part of the network comes up first with other parts coming up later, the order for sending and receiving LSA packets will vary. This variation can alter and impair convergence. Routers might learn about different versions of the topology before they construct their SPF trees and routing tables. On a large internetwork, parts that update more quickly can cause problems for parts that update more slowly.