Cisco Systems OL-4580-01 manual Link-State Databases

Chapter 3 Configuring Open Shortest Path First (OSPF)

OSPF Overview

ASBR routes can be advertised as type1 or type2 ASE. The difference between type1 and type2 is how the cost is calculated. For a type2 ASE, only the external cost (metric) is used when comparing multiple paths to the same destination. For type1 ASE, the combination of the external cost and the cost to reach the ASBR is used.

Link-State Databases

OSPF routers advertise routes using LSAs. The link-state database stores the LSAs from routers throughout the area. The advertisements depict the topology of the autonomous system. They could include:

•Router links that describe the state and cost of each router’s interface to an area

•Network links from the designated router (see the “Setting the Priority of the CSS” section) that describe all routes on a segment for multi-access segments with more than one attached router

•Summarized links from ABRs that describe networks in the AS but outside an area

•External links from ASBRs that describe destinations external to the AS

All routers that are connected to an area maintain identical routing databases about the area. Routers that are connected to multiple areas maintain a separate routing database for each attached area.

Instead of each router sending routing information to every other router on the network, OSPF routers establish adjacencies among neighboring routers. When the link-state databases of two neighboring routers are synchronized, they are considered adjacent.

OSPF routers collect raw topological data from the LSAs that they receive. Each router then prunes this data down to a tree of the shortest network paths centered on itself. The router examines the total cost to reach each router or network node in its domain. By discarding all but the lowest-cost path to each destination, the router builds a shortest-path tree to each destination, which it uses until the network topology changes. It is possible to have multiple lowest-cost paths to a destination.

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