Chapter 6. Link State Database Synchronization


I have said it many times already, and I will say it again: The key concept behind link state routing protocols is that each router in an area performs localized route calculations based on information in a common topological database. It is therefore essential that every router in an area have exactly the same topological database. This is the purpose of link state database synchronization. Whenever a router becomes active on an OSPF or IS-IS network, it must synchronize its database with its neighbors to guarantee that they are the same. If the router becomes active on a point-to-point link, it synchronizes with the neighbor at the other end of the link. If it becomes active on a multi-access network, it synchronizes with the DR or DIS. After the initial synchronization, the router must take steps to ensure that it remains synchronized with its neighbors.

Notice that routers do not synchronize with all other routers in an area, only with their neighbors. Those neighbors synchronize with their neighbors, and so on throughout the area, and this is good enough to ensure identical databases in every area router. An important assumption behind this procedure is that within an area there is a path from every router to every other router. If a path cannot be found from one or more routers to the other routers without leaving the area, there is no guarantee that all databases in the area are identical. This conditioncalled a partitioned areais discussed in Chapter 7. For now, it is enough to understand that LS database consistency depends on an unbroken series of adjacencies connecting all routers within an area. At a minimum, the physical topology of an area should be such that no single link or interface failure can isolate some portion of the area from the rest of the area.




OSPF and IS-IS(c) Choosing an IGP for Large-Scale Networks
OSPF and IS-IS: Choosing an IGP for Large-Scale Networks: Choosing an IGP for Large-Scale Networks
ISBN: 0321168798
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Jeff Doyle

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