Open Shortest Path First (OSPF) is link-state protocol that uses the Djikstra Algorithm to determine a shortest path tree.  The OSPF algorithm uses cost defined on links to determine the best path to a network.  For OSPF to remain loop free, each and every router needs to know of all the routers and their respective links.  This information is passed through Link State Advertisements (LSA)

Link State Advertisements

• Type 1 Router LSA

The router announces it’s directly connected links.

• Type 2 Network LSA

The designated router (DR)  announces all the Type 1 Router LSA’s it’s gathered from it’s neighbors.

• Type 3 Summary LSA

The router straddles more than one area, and can manipulate routing updates by summarizing them. Reducing the OSPF calculation requirements.  Since Area 0 is the backbone area, all Type 3 LSAs come from routers in Area 0.

• Type 4 ASBR Summary LSA

Type 4 Autonomous System Border Router Summary LSA used to identify the ASBR that is publishing the Type 5 External LSA so next hop can be clearly defined.

• Type 5 External LSA

Type 5 External LSA is used to publish routes imported/ redistributed into OSPF.  These routes can carry a metric type

• Type 1 Metric the cost is incremented as it passes through routers, just like all other routes
• Type 2 Metric the cost is maintained as the original value learned on the ASBR, and is the default method of propagating the route

• Type 7 NSSA LSA

Type 7 Not So Stubby Area LSA are for routes in theNSSA to forward their external links to the other OSPF routers, but not receive Type 5 External LSAs.  By filtering out the Type 5, NSSA routers can only use the external path they have learned.

• Type 9 OPAQUE LSA

Type 9 OPAQUE LSA is used a provide additional information to OSPF routers, and used by OSPF Graceful Restart

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1. OSPF Wikipedia
2. OSPF Cisco
3. RFC 2328
4. OSPF HP Comware
5. Dijkstra’s algorithm

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