23. Routing Calculations
• Link costs configurable by administrator
• Smaller values for more preferred links
• Different costs for each link direction possible
• Dijkstra’s shortest path algorithm
• incrementally calculates tree of shortest paths
• each link in the network examined once
• computes multiple shortest paths (equal-cost multipath)
25. OSPF Areas
• Two-level hierarchical routing scheme through the use of
areas
• Areas identified by 32-bit id
• Each area has its own link state database which is a
collection of network-LSAs and router-LSAs
• Area’s topology hidden from all other areas
• All Areas connected through Backbone Area (Area 0)
• Interconnection of areas through area border routers
(ABRs)
• ABR leaks IP addressing information to other areas through
summary LSAs
27. OSPF Areas (Cont…)
• Reduction in link state databases of an area
• Reduction in amount of flooding traffic needed for
synchronization
• Reduction in the cost of the shortest path
calculations
• Increased robustness
• Routing protection
• Hidden prefixes
28. Incorporating external routing information
• Special routers called AS boundary routers at the
edge of OSPF domain
• ASBRs originate AS-External LSAs
• only routes for which the choice of an ASBR makes
sense are imported
• otherwise default routes are used
29. Interaction with areas
• Inter Area communication
• LSA type 1. Router LSA. All routers in an area
• LSA type 2. Network LSA. DR generates it.
• Intra Area communication
• LSA type 3. Summary ABR LSA.
• LSA type 5. Summary ASBR LSA.
30. OSPF Area Types
• Restrict the amount of external routing
information within an area
• Used when resources especially router memory is
very limited
• Many types of restricted areas
• Stub Areas
• Others: not covered on this training
31. OSPF Area Types (cont…)
• Stub Areas
• Don’t support ASBRs and hence no AS-External-LSAs
• Routing to external destinations based on default routes
originated by the area’s border routers
• Summary LSAs also made optional
• Must lie on the edge of OSPF routing domain
• Inter-area routing may also be based on default routes
• Improved scaling
• But not preferred due to the possibility of suboptimal
routes
33. OSPF configuration
Normal area configuration
R1#configure terminal
R1(config)#router ospf 1
R1(config-router)# router-id 150.1.1.1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 0
Stub area configuration
R1#configure terminal
R1(config)#router ospf 1
R1(config-router)# router-id 150.1.1.1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 1
R1(config-router)# area 1 stub
34. OSPF verification
• R1# show ip ospf neighbors
• R1# show ip ospf interfaces
• R1# show ip ospf database
• R1# show ip route
• R1# show ip interfaces brief
35. Issues not covered
• OSPF Area Types
• NSSA
• Totally stubby
• Totally NSSA
• OSPF Network Types
• Broadcast subnets
• NBMA Subnets
• OSPF in the WAN
• OSPF Management
• Virtual Links
• OSPF and IPv6
• Many others