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OSPF Routing Protocol Principle

OSPF Routing Protocol Principle. V2.1. Outline. Through learning this course, you will: Master computing methods of OSPF routing protocol Master advanced applications of OSPF routing protocol. Contents. OSPF Overview OSPF Concept OSPF Working Process Area Allocation

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OSPF Routing Protocol Principle

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  1. OSPF Routing Protocol Principle V2.1

  2. Outline Through learning this course, you will: • Master computing methods of OSPF routing protocol • Master advanced applications of OSPF routing protocol

  3. Contents • OSPF Overview • OSPF Concept • OSPF Working Process • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  4. OSPF Overview • Open Shortest Path First (OSPF) • OSPF is Internal Gateway Protocol (IGP). • OSPF belongs to link state protocol, maintains complex network topology database, and adopts SPF algorithm to compute the optimal route. • The types of OSPF network are classified into multi-access network and point-to-point network.

  5. OSPF Advantages • No route loop • Adapt to large-scale network • High convergence rate of route • Support area allocation • Support equivalent route • Support verification • Support hierarchic management • Transmit protocol messages in multicast address

  6. Contents • OSPF Overview • OSPF Concept • OSPF Working Process • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  7. OSPF Concept (1) Router ID • Definition: 32-bit binary to identify each router on OSPF network. • Selection method: The IP address of the first interface that is activated is selected. If multiple interfaces are activated, the minimum IP address of the router is selected. If loopback interface is configured on the router, router ID is the minimum IP address of all loopback interfaces, in spite of IP address of other physical interfaces or even when they are activated. • Features: Globally unique; Once selected, it cannot be changed unless OSPF progress is restarted. Hi, I am router A and my name is 192.132.1.3. ZXR10# clear ip ospf

  8. OSPF Concept (2) • Interface Interfaces run OSPF protocol; transmit protocol messages periodically (Hello packet) to search and find neighbors. • Designated Router (DR) and Backup Designated Router (BDR) To reduce traffic of OSPF synchronous link state information, broadcast network can be automatically represented as DR or BDR according to interface priority. • Link State Database Contains link state of all routers on the network and indicates the topology structure of the whole network. • Neighboring Routers OSPF routers on the direct network become neighbors automatically according to hello packets. • Adjacency On the basis of neighbors, link state advertisement (LSA), is synchronized to form adjacency.

  9. OSPF Working Process (1) • Run OSPF protocol and find neighbors; • Interact LSA, synchronize link state database (LSDB), and form adjacency; • Use SPF algorithm to compute the shortest path tree and form route.

  10. Interfaces start OSPF OSPF Working Process (2) Autonomous system Neighbor table Topology database Route table

  11. Contents • OSPF Overview • OSPF Concept • OSPF Working Process Finding Neighbors Selecting DR or BDR Forming Adjacency Updating LSA Computing Optimal Route • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  12. D E Hello B A C Neighborhood Router ID Hello/dead intervals Neighbors Area-ID Router priority DR IP address BDR IP address Authentication password Stub area flag * afadjfjorqpoeru 39547439070713 * Hello * * Interfaces run OSPF send periodically Items with * must be consistent to set up neighbors

  13. Neighborhood—Hello Message

  14. RA Neighbors List RB Neighbors List Down State Process of Forming OSPF Neighbor List Lo: 10.75.0.1 Lo: 10.75.0.2 10.75.32.1/30 Int1 10.75.32.2/30 Int1 A B 10.75.0.2, int1 , 2-way 10.75.0.1, int1 , init 10.75.0.1, int1, 2-way Hello, my router ID is 10.75.0.1 and see no neighbors. Init State Hello, my router ID is 10.75.0.2 and my neighbor is 10.75.0.1. Two-way Hello, my router ID is 10.75.0.1 and my neighbor is 10.75.0.2.

  15. Contents • OSPF Overview • OSPF Concept • OSPF Working Process Finding Neighbors Selecting DR or BDR Forming Adjacency Updating LSA Computing Optimal Route • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  16. LSA Synchronization on Broadcast Network • On broadcast network, waste can be caused when each neighbor transmits LSA (network bandwidth and CPU resources). Neighbor, receive my LSA.

  17. Functions of DR and BDR • To reduce traffic of OSPF protocol messages, on local area network, each network segment uses DR or BDR to represent the network. • Each router can synchronize DR and BDR to form adjacency. DR BDR

  18. Hello Selection of DR and BDR P=3 P=2 DR BDR P=1 P=1 P=0 • The router that is started first on the network is selected as DR; • When started simultaneously or reselected, the router with the superior priority (0-255) is selected as DR; • When started simultaneously or with the same priority, the router with the largest ID is selected as DR; • DR selection is not preemptive unless OSPF progress is restarted.

  19. Selection of DR and BDR—Hello Message

  20. Contents • OSPF Overview • OSPF Concept • OSPF Working Process Finding Neighbors Selecting DR or BDR Forming Adjacency Updating LSA Computing Optimal Route • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  21. RB Neighbors List RA Neighbors List Forming Adjacency of OSPF (1) Lo: 10.75.0.1 Lo: 10.75.0.2 10.75.32.1/30 Int1 10.75.32.2/30 Int1 A DR B 10.75.0.2, int1 , 2-way 10.75.0.2, int1, Exstart 10.75.0.2, int1, Exchange 10.75.0.1, int1 , init 10.75.0.1, int1, Exchange 10.75.0.1, int1, 2-way Exstart afadjfjorqpoeru 39547439070713 DBD DR, I will initiate interaction of LSA and I am the master sender (MS=1). afadjfjorqpoeru 39547439070713 No, I am the master sender, because I have higher router ID. DBD Exchange State afadjfjorqpoeru 39547439070713 This is summary information about my LSDB. DBD afadjfjorqpoeru 39547439070713 DBD This is summary information about my LSDB.

  22. RB Neighbors List RA Neighbors List afadjfjorqpoeru 39547439070713 LSAck Forming Adjacency of OSPF (2) lo：10.75.0.1 lo：10.75.0.2 10.75.32.1/30 Int1 10.75.32.2/30 Int1 A DR B 10.75.0.2, int1, Full 10.75.0.2, int1, Loading 10.75.0.1, int1, Full 10.75.0.1, int1 , init 10.75.0.1, int1, Exchange 10.75.0.1, int1 , init 10.75.0.2, int1, Exchange Loading State afadjfjorqpoeru 39547439070713 I have no information about 172.16.6.0/24 and I need entire LSA. LSR afadjfjorqpoeru 39547439070713 This is LSA of 172.16.6.0/24. LSU Received, thanks! Full State My LSDB is complete. No query is needed.

  23. Contents • OSPF Overview • OSPF Concept • OSPF Working Process Finding Neighbors Selecting DR or BDR Forming Adjacency Updating LSA Computing Optimal Route • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  24. 2 1 x x 3 Flooding Process (1) • The router notifies changed topology information of neighbors in 224.0.0.5. Point-to-point link state changes Update route table LSU B A LSU

  25. 2 1 x x 3 Flooding Process (2) • Router A notifies DR in 224.0.0.6. • DR notifies other routers in 224.0.0.5. Broadcast link state changes DR LSU Update route table 4 LSU B A LSU

  26. Contents • OSPF Overview • OSPF Concept • OSPF Working Process Finding Neighbors Selecting DR or BDR Forming Adjacency Updating LSA Computing Optimal Route • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  27. Process of Computing OSPF Protocol RTA RTB 1 LSDB 1 A B LSA of RTA 2 5 2 5 C LSA of RTB RTC 3 LSA of RTC 3 D LSA of RTD RTD (1) Network topology structure (3) Weighted digraph created by LSDB (2) LSDB of each router 1 1 1 1 A B A B A B A B 2 2 2 2 C C C C 3 3 3 3 D D D D (4) Each router computes the minimal spanning tree with itself as the root node.

  28. Computing Minimal Spanning Tree • Sum of COST from external ports in data direction 10.1.1.0/24 10.2.2.0/24 10.3.3.0/24 Cost=1 Cost=1 Cost=1 A B C Cost=5 Cost=5 Topology Table Net Cost 10.2.2.0 2 10.3.3.0 3 Topology Table Net Cost 10.1.1.0 10

  29. Contents • OSPF Overview • OSPF Concept • OSPF Working Process • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  30. Problems Large-Scale OSPF Network Meets OSPF Always computing route table, boring! Only LSA is received, no data. OSPF OSPF OSPF OSPF My route table is too large and my memory is too small.

  31. Solution: Allocate Areas Area 0 Area 1 Area 2 Autonomous system Rules for allocating areas: • Each network segment must belong to an area, that is, each interface that runs OSPF protocol must be designated with an area; • Each area is identified by area ID, which is an integer ranging from 0 to 32; • Backbone area (area 0) cannot be separated by non-backbone areas; • Non-backbone areas (not area 0) must be connected to backbone area (virtual link is not suggested).

  32. Advantages for Allocating Areas • Only LSDB of routers in the same area can be synchronized. The changes of network topology structure are first updated within the area.. • After areas are allocated, route aggregation can be performed on the boundary router within the area to reduce the number of LSA notified to other areas and minimize the influence brought by changes of network topology.

  33. External AS Types of OSPF Routers Area 1 Backbone area 0 Area 2 ABR and backbone router Backbone/internal router Internal router Internal router ABR and backbone router ASBR and backbone router

  34. Contents • OSPF Overview • OSPF Concept • OSPF Working Process • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization

  35. Types of LSA

  36. Area 1 Area 0 DR External Network ExternalAS Router ABR ASBR Summary Types of LSA

  37. Flooding of LSU among Multiple Area Network RIP Internal Internal ABR1 ABR2 BBone afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 Type 1 Type 3 Type 3 Type 4 Type 5 Type 5 Area 50 Area 1 Area 0

  38. Contents • OSPF Overview • OSPF Concept • OSPF Working Process • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization Stub Area Route Summarization

  39. Constitution of OSPF Route Route table Route within areas Area 1 Route among areas Area 1 Area 0 External route (non-OSPF route) RIP Area 1

  40. Types of Areas Totally Stub Area 0 Stub Not receive redistributed route ASBR Summary LSA Autonomous system external LSA Receive all LSA Not receive route outside areas Network Summary LSA Not receive redistributed route ASBR Summary LSA Autonomous system external LSA

  41. RD route table 1.1.1.0 int1 direct 1.1.2.0 int1 ospf 1.1.3.0 int1 ospf 2.1.1.0 int2 direct 2.1.2.0 int2 ospf 3.1.1.0 int1 ospf 3.1.2.0 int1 ospf 4.1.1.0 int1 ospf 4.1.2.0 int1 ospf Stub and Totally Stubby Area (1) 4.1.1.0 4.1.2.0 RIP Area 0 Area 50 Area 1 2.1.2.0 ABR ABR 3.1.1.0 1.1.1.0 1.1.2.0 1.1.3.0 D A B E F C 2.1.1.0 ASBR 3.1.2.0 RC route table RE route table RF route table 1.1.1.0 int1 ospf 1.1.2.0 int1 ospf 1.1.3.0 int1 ospf 2.1.1.0 int1 direct 2.1.2.0 int2 direct 3.1.1.0 int1 ospf 3.1.2.0 int1 ospf 4.1.1.0 int1 ospf 4.1.2.0 int1 ospf 1.1.1.0 int1 ospf 1.1.2.0 int1 ospf 1.1.3.0 int1 ospf 2.1.1.0 int1 ospf 2.1.2.0 int1 ospf 3.1.1.0 int1 direct 3.1.2.0 int2 direct 4.1.1.0 int1 ospf 4.1.2.0 int1 ospf 1.1.1.0 int1 ospf 1.1.2.0 int1 ospf 1.1.3.0 int1 direct 2.1.1.0 int1 ospf 2.1.2.0 int1 ospf 3.1.1.0 int2 direct 3.1.2.0 int2 ospf 4.1.1.0 int1 ospf 4.1.2.0 int1 ospf

  42. afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 afadjfjorqpoeru 39547439070713 Stub and Totally Stubby Area (2) RIP Area 0 Area 50—Stub Area 1—Totally Stubby ABR1 BBone ABR2 Internal ASBR Internal Summary Summary Default Summary External Default External Default

  43. RD route table 1.1.1.0 int1 direct 1.1.2.0 int1 ospf 1.1.3.0 int1 ospf 2.1.1.0 int2 direct 2.1.2.0 int2 ospf 3.1.1.0 int1 ospf 3.1.2.0 int1 ospf 4.1.1.0 int1 ospf 4.1.2.0 int1 ospf Stub and Totally Stubby Area (3) 4.1.1.0 4.1.2.0 RIP Area 0 Area 50—Stub Area 1—Totally Stubby 2.1.2.0 ABR ABR 3.1.1.0 1.1.1.0 1.1.2.0 1.1.3.0 D A B E F C 2.1.1.0 ASBR 3.1.2.0 RC route table RE route table RF route table 1.1.1.0 int1 ospf 1.1.2.0 int1 ospf 1.1.3.0 int1 ospf 2.1.1.0 int1 direct 2.1.2.0 int2 direct 3.1.1.0 int1 ospf 3.1.2.0 int1 ospf 0.0.0.0 int1 ospf 3.1.1.0 int1 direct 3.1.2.0 int2 direct 0.0.0.0 int1 ospf 1.1.1.0 int1 ospf 1.1.2.0 int1 ospf 1.1.3.0 int1 direct 2.1.1.0 int1 ospf 2.1.2.0 int1 ospf 3.1.1.0 int2 direct 3.1.2.0 int2 ospf 4.1.1.0 int1 ospf 4.1.2.0 int1 ospf

  44. X X Limitation of Stub and Totally Stubby Area Area 2 0.0.0.0 0.0.0.0 R3 R4 Unique outlet ExternalAS • Only one outlet • No ASBR within the area • Can no be AREA 0 (Backbone) • No Virtual links

  45. NSSA • Not So Stubby Area Area 2 Area 0 R3 To Other Area Type 5 R4 Type 7 Type 7 RIP ExternalAS

  46. Contents • OSPF Overview • OSPF Concept • OSPF Working Process • Area Allocation • LSA Transmission among Multiple Areas • Route Optimization Stub Area Route Summarization

  47. x x Supporting Route Summarization Area 0 backbone area Summarizing route ABRs Area 1 • Reduce the size of route table • Limit the influence of topology changes on the local • Reduce the number of LSAs and save CPU resources

  48. Route Summarization ABR Area 1 Area 0 C B A Summarizing route Route table of router B LSAs transmitted to route C O 172.16.8.0 255.255.252.0O 172.16.12.0 255.255.252.0O 172.16.16.0 255.255.252.0O 172.16.20.0 255.255.252.0O 172.16.24.0 255.255.252.0O 172.16.28.0 255.255.252.0 IA 172.16.8.0 255.255.248.0 • A piece of route information can represent multiple sub network. IA 172.16.16.0 255.255.240.0

  49. Review • OSPF concept and features • OSPF working principle • OSPF area allocation • Stub and Totally Stubby area • Route summarization

  50. Questions • Can OSPF achieve load balance or equivalent load balance? • How is OSPF Metric computed? • How many types does OSPF LSA have? Who create theses types? What range are the types released in? Which content is described? • Can you describe the working process of OSPF?

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