1. © 1999, Cisco Systems, Inc. www.cisco.com 8-1 OSPF Overview RFC 2328, 2178, 1583

2. Has fast convergence Supports VLSM Has no hop count limitation Processes updates efficiently Selects paths based on bandwidth What Is OSPF?

3. OSPF Terminology Links

4. OSPF Terminology Links Cost=10 Cost=1785 Cost=6

5. OSPF Terminology Area 1 Links Cost=10 Cost=1785 Cost=6 Area 0

6. OSPF Terminology Area 1 Links Cost=10 Cost=1785 Cost=6 Area 0 DR BDR

7. OSPF Terminology Adjacencies Database Lists Neighbors Area 1 Links Cost=10 Cost=1785 Cost=6 Area 0 DR BDR

8. OSPF Terminology Topology Database Lists All Routes Area 1 Links Cost=10 Cost=1785 Cost=6 Area 0 DR BDR Adjacencies Database Lists Neighbors

9. Area 1 Links Cost=10 Cost=1785 Cost=6 Area 0 OSPF Terminology DR BDR Routing Table Lists Best Routes Topology Database Lists All Routes Adjacencies Database Lists Neighbors

10. © 1999, Cisco Systems, Inc. www.cisco.com 8-10 OSPF Operation within a Single Area

11. Establishing Adjacencies Hello A D E C B afadjfjorqpoeru 39547439070713 Router ID Hello/Dead Intervals Neighbors Area-ID Router Priority DR IP Address BDR IP Address Authentication Password Stub Area Flag * * * *

12. Establishing Adjacencies (cont.) 172.68.5.1/24 E0 172.68.5.2/24 E1 A B Down State

13. Establishing Adjacencies (cont.) 172.68.5.1/24 E0 172.68.5.2/24 E1 Router B Adjacencies Database 172.68.5.1/24, int E1 I am router ID 172.68.5.1 and I see no one. A B Down State Init State

14. Establishing Adjacencies (cont.) 172.68.5.1/24 E0 I am router ID 172.68.5.2, and I see 172.68.5.1 172.68.5.2/24 E1 Router B Adjacencies Database 172.68.5.1/24, int E1 I am router ID 172.68.5.1 and I see no one. A B Down State Init State

15. Establishing Adjacencies (cont.) 172.68.5.1/24 E0 I am router ID 172.68.5.2, and I see 172.68.5.1 Router A Adjacencies Database 172.68.5.2/24, int E0 172.68.5.2/24 E1 Router B Adjacencies Database 172.68.5.1/24, int E1 I am router ID 172.68.5.1 and I see no one. A B Down State Init State Two-Way State

16. Hellos elect DR and BDR Each router forms adjacency with DR and BDR Electing the DR and BDR DRBDR

17. Hello packets exchanged via IP multicast Router with highest OSPF priority elected P=1P=0P=1 Electing the DR and BDR (cont.) P=3P=2 Hello DR BDR

18. Discovering Routes E0 172.68.5.1 DR E0 172.68.5.3 No, I will start exchange because I have a higher router ID. I will start exchange because I have router ID 172.68.5.1. Hello afadjfjorqpoeru 39547439070713 Hello afadjfjorqpoeru 39547439070713 Exstart State

19. Discovering Routes E0 172.68.5.1 DR E0 172.68.5.3 No, I will start exchange because I have a higher router ID. I will start exchange because I have router ID 172.68.5.1. Here is a summary of my link-state database. Hello afadjfjorqpoeru 39547439070713 Hello afadjfjorqpoeru 39547439070713 DBD afadjfjorqpoeru 39547439070713 Exstart State Exchange State Here is a summary of my link-state database. DBD afadjfjorqpoeru 39547439070713

20. Discovering Routes (cont.) E0 172.68.5.1 E0 172.68.5.3 Thanks for the information! LSAck afadjfjorqpoeru 39547439070713 LSAck afadjfjorqpoeru 39547439070713 DR

21. Discovering Routes (cont.) E0 172.68.5.1 E0 172.68.5.3 I need the complete entry for network 172.68.6.0/24. Here is the entry for network 172.68.6.0/24. Thanks for the information! LSR afadjfjorqpoeru 39547439070713 LSAck afadjfjorqpoeru 39547439070713 LSU afadjfjorqpoeru 39547439070713 Loading State. Thanks for the information! LSAck afadjfjorqpoeru 39547439070713 LSAck afadjfjorqpoeru 39547439070713 DR

22. Discovering Routes (cont.) E0 172.68.5.1 E0 172.68.5.3 I need the complete entry for network 172.68.6.0/24. Here is the entry for network 172.68.6.0/24. Thanks for the information! LSR afadjfjorqpoeru 39547439070713 LSAck afadjfjorqpoeru 39547439070713 LSU afadjfjorqpoeru 39547439070713 Full State Loading State. Thanks for the information! LSAck afadjfjorqpoeru 39547439070713 LSAck afadjfjorqpoeru 39547439070713 DR

23. Choosing Routes Routing Table Net Cost Out Interface 2.2.2.0 6 TR0 3.3.3.0 7 TR0 3.3.3.0 10 E0 Cost=10 Cost=6 Cost=1 A B C This is the best route to C. 1.1.1.0/242.2.2.0/243.3.3.0/24 4.4.4.0/24

24. Maintaining Routing Information New Router xx Link-State Change LSU 1 DR A B New router tells all OSPF DRs on 224.0.0.6

25. Maintaining Routing Information New Router xx Link-State Change LSU 1 2 DR A B New router tells all OSPF DRs on 224.0.0.6 DR tells others on 224.0.0.5

26. New router tells all OSPF DRs on 224.0.0.6 DR tells others on 224.0.0.5 New Router xx Link-State Change LSU 1 2 3 Maintaining Routing Information DR A B

27. New router tells all OSPF DRs on 224.0.0.6 DR tells others on 224.0.0.5 Maintaining Routing Information New Router xx Link-State Change LSU 1 2 3 I need to update my routing table. DR A B 4

28. Maintaining Routing Information (cont.) Is entry in link-state database? No Run SPF to calculate new routing table Add to database Flood LSA End LSA LSU A

29. Maintaining Routing Information (cont.) Is entry in link-state database? Yes No Run SPF to calculate new routing table Add to database Flood LSA Is seq. # the same? No Send LSU with newer information to source End LSA LSU Is seq. # newer? No

30. Maintaining Routing Information (cont.) Is entry in link-state database? Yes No Run SPF to calculate new routing table Add to database Flood LSA Is seq. # the same? Yes No Send LSU with newer information to source End LSA LSU Is seq. # newer? No Ignore LSA

31. Maintaining Routing Information (cont.) Is entry in link-state database? Yes No Run SPF to calculate new routing table Add to database Flood LSA Is seq. # the same? Yes No Yes Send LSU with newer information to source End LSA LSU Is seq. # newer? No A Ignore LSA Send LSAck to DR Go to A

32. Configuring OSPF on Internal Routers Can assign network or interface address. Broadcast NetworkPoint-to-Point Network E0 10.64.0.1 10.64.0.2 E0 S0 10.2.1.2 10. 2.1.1 S1 A BC interface Ethernet0 ip address 10.64.0.1 255.255.255.0 ! router ospf 1 network 10.0.0.0 0.255.255.255 area 0 interface Ethernet0 ip address 10.64.0.2 255.255.255.0 ! interface Serial0 ip address 10.2.1.2 255.255.255.0 router ospf 50 network 10.2.1.2 0.0.0.0 area 0 network 10.64.0.2 0.0.0.0 area 0

33. Assigns a cost to an outgoing interface May be required for interoperability Use default cost between Cisco devices Cisco Non-Cisco Traffic Router(config-if)# ip ospf cost cost Configuring Optional Commands (cont.)

34. © 1999, Cisco Systems, Inc. www.cisco.com 9-34 Creating Multiple OSPF Areas

35. Issues with Maintaining a Large OSPF Network OSPF My routing table is too big, I am running low on memory. The SPF is running too often for me to route. I am only receiving LSAs, no data.

36. Area 0 Area 1Area 2 Autonomous System Consists of areas and autonomous systems Minimizes routing update traffic The Solution: OSPF Hierarchical Routing

37. OSPF Multiarea Components Area 0 I am a backbone. Area 2 I am a stub. Area 1 I am standard. AreasRoutersLSAs Internal ABR ASBR Backbone Type 1 afadjfjorqpoeru 39547439070713 Type 2 afadjfjorqpoeru 39547439070713 Type 3/4 afadjfjorqpoeru 39547439070713 Type 5 afadjfjorqpoeru 39547439070713

38. Types of OSPF Routers Internal Routers Area 1Area 2 ASBR and Backbone Router Backbone/ Internal Routers ABR and Backbone Router Backbone Area 0 External AS ABR and Backbone Router Internal Routers

39. Flooding LSUs to Multiple Areas (cont.) Routing Table Interarea routes Intra-area routes External (non-OSPF routes) Area 1 Area 0 Area 1RIP

40. OSPF Area Configuration Example 192.168.15.2 Area 0 Stub Area 2 192.168.14.1192.168.15.1 S0 E0 External AS R4 R3 R3# interface Ethernet 0 ip address 192.168.14.1 255.255.255.0 interface Serial 0 ip address 192.168.15.1 255.255.255.252 router ospf 100 network 192.168.14.0 0.0.0.255 area 0 network 192.168.15.0 0.0.0.255 area 2 R4# interface Serial 0 ip address 192.168.15.2 255.255.255.252 router ospf 15 network 192.168.15.0 0.0.0.255 area 2

41. Link-State Network Discovery Routers calculate the shortest path to destinations in parallel ABC X Y ZW Routing Table W W 0 0 X X 0 0 X X 0 0 Y Y 0 0 Y Y 0 0 Z Z 0 0 E0 S1S0 S1 E1 E0 S1 S0 S1E1

42. Link-State Network Discovery Routers calculate the shortest path to destinations in parallel SPF ABC X Y Z Topological Database SPF Topological Database SPF Topological Database W Routing Table W W 0 0 X X 0 0 X X 0 0 Y Y 0 0 Y Y 0 0 Z Z 0 0 E0 S1S0 S1 E1 E0 S1 S0 S1E1

43. Link-State Network Discovery Routers calculate the shortest path to destinations in parallel SPF SPF Tree ABC X Y Z Topological Database A Routing Table A Routing Table SPF Topological Database B Routing Table B Routing Table SPF Topological Database C Routing Table C Routing Table W Routing Table W W 0 0 X X 0 0 X X 0 0 Y Y 0 0 Y Y 0 0 Z Z 0 0 E0 S1S0 S1 E1 E0 S1 S0 S1E1

44. Link-State Topology Changes Update processes proceed using the same link-state update Process to update this routing table Process to update this routing table Process to update this routing table Process to update this routing table Topologychangeinlink-stateupdate Process to update this routing table Process to update this routing table

45. Link-State Concerns Processing and memory required for link-state routing SPF SPF Tree Topological Database A Routing Table A Routing Table

46. Link-State Concerns Processing and memory required for link-state routing Bandwidth consumed for initial link-state “flood” SPF SPF Tree Topological Database A Routing Table A Routing Table

47. Problem: Link-State Updates Unsynchronized updates, inconsistent path decisions D B A C Network 1, Unreachable Network 1 goes down then comes up X, ok Slow path update

48. Problem: Link-State Updates Unsynchronized updates, inconsistent path decisions D B A C Network 1, Unreachable Network 1, Back Up Now Network 1, Unreachable Network 1 goes down then comes up X, ok Fast path updates arrive first Slow path update arrives lastSlow path update Which SPF tree to use for routing?

49. Problem: Link-State Updates Unsynchronized updates, inconsistent path decisions D B A C Network 1, Unreachable Network 1, Back Up Now Network 1, Unreachable Network 1 goes down then comes up X, ok Fast path updates arrive first Slow path update arrives lastSlow path update Which SPF tree to use for routing?

50. Link-State Update Problems Synchronizing large networks—which network topology updates are correct?

51. Link-State Update Problems Synchronizing large networks—which network topology updates are correct? Router startup—order of start alters the topology learned

52. Link-State Update Problems Synchronizing large networks—which network topology updates are correct? Router startup—order of start alters the topology learned Partitioned regions—slow updating part separated from fast updating part

53. Reduce the need for resources –“Dampen” update frequency –Target link-state updates to multicast –Use link-state area hierarchy for topology –Exchange route summaries at area borders Solution: Link-State Mechanisms

54. Reduce the need for resources –“Dampen” update frequency –Target link-state updates to multicast –Use link-state area hierarchy for topology –Exchange route summaries at area borders Coordinate link-state updates –Use time stamps –Update numbering and counters –Manage partitioning using an area hierarchy Solution: Link-State Mechanisms