1. Link State Routing protocol

2. Link State Concepts

3. Link State Topology Changes

4. Link State Routing (LSR)
SPF
Routing
Table
LSP:
„My links to
R2 and R4 are up“
LSP:
„My links to R1 and R3 are up.
My link to R2 is down.“
Router 1
Router 4
Router 2
Router 3
LSP: „My links to
R1 and R3 are up,
my link to R4 is down.“
LSP: „My links to
R2 and R4 are up.“
LSP....link state packet
SPF... shortest path first

5. Link State Concerns

6. Link State Routing (LSR)1
Router A
Router C 4
Router E 2 2 1 4
Router B
Router D
Link State Database
B - 2
C - 1
A - 2
D - 4
A - 1
D - 2
E - 4
C - 2
B - 4
E - 1
C - 4
D - 1
Router A
Router B
Router C
Router D
Router E A C B D E A D E C B D A E B C

7. Link State Routing Features
Link-state algorithms are also known as Dijkstras algorithm or as SPF (shortest path first) algorithms.
Link-state routing algorithms maintain a complex database of topology information.
The distance vector algorithm are also known as Bellman-Ford algorithms. They have nonspecific information about distant networks and no knowledge of distant routers.
A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. Link-state routing uses:
Link-state advertisements (LSAs)
A link-state advertisement (LSA) is a small packet of routing information
that is sent between routers.
Topological database
A topological database is a collection of information gathered from LSAs.
SPF algorithm
The shortest path first (SPF) algorithm is a calculation performed on the
database resulting in the SPF tree.
Routing tables – A list of the known paths and interfaces.

8. Link State Routing

9. Comparing Routing Methods

10. OSPF (Open Shortest Path First) Protocol
© 2003, Cisco Systems, Inc. All rights reserved. 10

11. OSPF is a Link-State Routing Protocols
Link-state (LS) routers recognize much more information about the network than their distance-vector counterparts,Consequently LS routers tend to make more accurate decisions.
Link-state routers keep track of the following:
Their neighbours
All routers within the same area
Best paths toward a destination

12. Link-State Data Structures
Neighbor table:
Also known as the adjacency database (list of recognized neighbors)
Topology table:
Typically referred to as LSDB (routers and links in the area or network)
All routers within an area have an identical LSDB
Routing table:
Commonly named a forwarding database (list of best paths to destinations)

13. OSPF vs. RIP
RIP is limited to 15 hops, it converges slowly, and it sometimes chooses slow routes because it ignores critical factors such as bandwidth in route determination. OSPF overcomes these limitations and proves to be a robust and scalable routing protocol suitable for the networks of today.

14. OSPF Terminology
The next several slides explain various OSPF terms -one per slide.

15. OSPF Term: Link

16. OSPF Term: Link State

17. OSPF Term: Area

18. OSPF Term: Link Cost

19. OSPF Term: Forwarding Database

20. OSPF Term: Adjacencies Database

21. OSPF Terms: DR & BDR

22. Link-State Data Structure: Network Hierarchy
Link-state routing requires a hierachical network structure that is enforced by OSPF.
This two-level hierarchy consists of the following:
Transit area (backbone or area 0)
Regular areas (nonbackbone areas)

23. OSPF Areas

24. Area Terminology

25. LS Data Structures: Adjacency Database
Routers discover neighbors by exchanging hello packets.
Routers declare neighbors to be up after checking certain parameters or options in the hello packet.
Point-to-point WAN links:
Both neighbors become fully adjacent.
LAN links:
Neighbors form an adjacency with the DR and BDR.
Maintain two-way state with the other routers (DROTHERs).
Routing updates and topology information are only passed between adjacent routers.

26. OSPF Adjacencies
Routers build logical adjacencies between each other using the Hello Protocol. Once an adjacency is formed:
LS database packets are exchanged to synchronize each other’s LS databases.
LSAs are flooded reliably throughout the area or network using these adjacencies.

27. Open Shortest Path First Calculation
Routers find the best paths to destinations by applying Dijkstra’s SPF algorithm to the link-state database as follows:
Every router in an area has the identical link-state database.
Each router in the area places itself into the root of the tree that is built.
The best path is calculated with respect to the lowest total cost of links to a specific destination.
Best routes are put into the forwarding database.

28. OSPF Packet Types

29. OSPF Packet Header Format

30. Neighborship

31. Establishing Bidirectional Communication

32. Establishing Bidirectional Communication (Cont.)

33. Establishing Bidirectional Communication (Cont.)

34. Establishing Bidirectional Communication

35. Discovering the Network Routes

36. Discovering the Network Routes

37. Adding the Link-State Entries

38. Adding the Link-State Entries (Cont.)

39. Adding the Link-State Entries

40. Maintaining Routing Information
Router A notifies all OSPF DRs on

41. Maintaining Routing Information (Cont.)
Router A notifies all OSPF DRs on
DR notifies others on

42. Maintaining Routing Information (Cont.)
Router A notifies all OSPF DRs on
DR notifies others on

43. Maintaining Routing Information
Router A notifies all OSPF DRs on
DR notifies others on

44. Configuring Basic OSPF: Single Area
Router(config)#
router ospf process-id
Turns on one or more OSPF routing processes in the IOS software.
Router(config-router)#
network address inverse-mask area [area-id]
Router OSPF subordinate command that defines the interfaces (by network number) that OSPF will run on. Each network number must be defined to a specific area.

45. Configuring OSPF on Internal Routers of a Single Area

46. Verifying OSPF Operation
Router#
show ip protocols
Verifies the configured IP routing protocol processes, parameters and statistics
Router#
show ip route ospf
Displays all OSPF routes learned by the router
Router#
show ip ospf interface
Displays the OSPF router ID, area ID and adjacency information

47. Verifying OSPF Operation (Cont.)
Router#
show ip ospf
Displays the OSPF router ID, timers, and statistics
Router#
show ip ospf neighbor [detail]
Displays information about the OSPF neighbors, including Designated Router (DR) and Backup Designated Router (BDR) information on broadcast networks

48. The show ip route ospf Command
RouterA# show ip route ospf
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP, D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area, E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP, i - IS-IS, L1 - IS-IS
level-1, L2 - IS-IS level-2, * - candidate default
Gateway of last resort is not set
is subnetted, 2 subnets
O [110/10] via , 00:00:50, Ethernet0

49. The show ip ospf interface Command
RouterA# show ip ospf interface e0
Ethernet0 is up, line protocol is up
Internet Address /24, Area 0
Process ID 1, Router ID , Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DROTHER, Priority 1
Designated Router (ID) , Interface address
Backup Designated router (ID) , Interface address
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor (Designated Router)
Suppress hello for 0 neighbor(s)

50. The show ip ospf neighbor Command
RouterB# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
FULL/BDR 00:00: Ethernet0
FULL/ :00: Serial0

51. show ip protocol
show ip route

52. show ip ospf neighbor detail
show ip ospf database