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Open Shortest Path First OSPF

Open Shortest Path First OSPF. OSPF Overview OSPF Operation By Grace Deng Oct.16.2003. OSPF Overview History. Development began 1987 by IETF Goal—a link state protocol more efficient and scaleable than RIP Latest revision is RFC 2328 April 1998. OSPF Link state

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Open Shortest Path First OSPF

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  1. Open Shortest Path FirstOSPF • OSPF Overview • OSPF Operation By Grace Deng Oct.16.2003

  2. OSPF OverviewHistory • Development began 1987 by IETF • Goal—a link state protocol more efficient and scaleable than RIP • Latest revision is RFC 2328April 1998

  3. OSPF Link state Efficient routing updates (sends changes only) No hop count limit Fast Convergence Supports VLSM Path selection based on bandwidth RIP Distance vector Copies entire routing table Hop count limit of 15 Hold-down timers to prevent routing loops Does not advertise sub-net masks Uses only hop count as metric OSPF OverviewOSPF versus RIP

  4. OSPF Overview Concepts • OSPF is a Link-State Routing Protocol • Uses IP as transport, IP protocol 89 • Uses multicast addresses in neighbor maintenance and flooding of LSAs • 224.0.0.5 – All OSPF Routers • 224.0.0.6 – All DRouters • Employs Dijkstra’s Shortest Path First (SPF) algorithm to calculate the path tree

  5. OSPF Overview Concepts – (cont.) • Uses Metrics—path cost • Typically faster convergence than DVRPs • Support for CIDR, VLSM, Authentication, Multi-path and IP unnumbered • Relatively low steady state bandwidth requirements

  6. OSPF Overview Terminology

  7. OSPF Overview Terminology • Link • Link state • Link State (LS) or topological database • Area • OSPF Metric Cost • Routing table • Adjacencies database

  8. OSPF Overview Topology/Link State Database • A router has a separate Link State (LS) or topological database for each area to which it belongs • All routers belonging to the samearea should have identical databases • SPF calculation is performed independently for each area • LSA flooding is bounded by area

  9. OSPF Overview Areas • OSPF uses a 2 level hierarchical model • Areas labeled with a 32-bit number • Can be defined using single decimal or IP address format value • (i.e. Area 0.0.0.0 or Area 0) • Area 0 reserved for the backbone area • All areas must connect to area 0

  10. OSPF Overview OSPF Metric • Cost applied on all router link paths • 16-bit positive number 1–65,535 • The lower the more desirable • Relevant going out an interface only • Route decisions made on total cost of path

  11. OSPF Overview OSPF Packet Types OSPF Packet format

  12. OSPF Packet Types (cont.)

  13. OSPF Overview Router ID • Routers are identified by a unique 32-bit ID • RID: highest IP address configured on any active loopback interface • RID: if no loopback exists, highest IP address configured on any active physical interface • RID can be configured with • router-id <ip address>

  14. OSPF Overview OSPF Hello Packets • Multicast 224.0.0.5 on all router interfaces • Hello interval 10 sec. LAN, 30 sec. NBMA • Used to form adjacencies between routers

  15. DD seq=x+1,M DD seq=x+1,S ••• DD seq=x+n,M DD seq=x+n,S OSPF Overview Database Descriptor Packets (DDP) • Contain link state database headers • Describe the current LS database • Exchange stage

  16. OSPF Overview Link State Request & Update Packets • Request for specific parts of database • Send only database updates requested • Loading Stage, labeled Full when complete Link State Request Link State Update Link State Request Link State Update

  17. OSPF Operation • Network changes generate link-state advertisements (LSA) • Cost change to an interface • Link being added or deleted from topology • All routers exchange LSAs to build and maintain a consistent database • The protocol remains relatively quiet during steady-state conditions.

  18. OSPF Operation Steps to OSPF Operation 1. Establishing router adjacencies 2. Electing DR and BDR 3. Discovering Routes 4. Choosing Routes 5. Maintaining Routing Information

  19. OSPF OperationOSPF States OSPF router interfaces can be in one of seven states: • Down State • Init State • Two-way State • ExStart State • Exchange State • Loading State • Full Adjacency State

  20. OSPF OperationSteps to OSPF Operation with OSPF States 1. Establishing router adjacencies • Down State • Init State • Two-way State • (ExStart State unless DR/BDR election needed) 2. Electing DR and BDR • ExStart State with DR and BDR • Two-way State with all other routers

  21. OSPF OperationSteps to OSPF Operation with OSPF States 3. Discovering Routes • ExStart State • Exchange State • Loading State • Full State 4. Choosing Routes 5. Maintaining Routing Information

  22. OSPF Operation1. Establishing Adjacencies (1) • Initially, an OSPF router interface is in the down state.not exchanged information with any neighbor.

  23. OSPF Operation1. Establishing Adjacencies (2) Init State • Init State - OSPF routers send Type 1 Hello packets at regular intervals (10 sec.) to establish neighbors. • When a router receives its first Hello packet, it enters the init state, meaning the router is ready to take the relationship to the next level.

  24. OSPF Operation1. Establishing Adjacencies (3) From init state to the two-way state • RTB receives Hello packets from RTA and RTC (its neighbors), and sees its own Router ID (10.6.0.1) in the Neighbor ID field. • RTB declares takes the relationship to a new level, and declares a two-way state between itself and RTA, and itself and RTC.

  25. OSPF Operation1. Establishing Adjacencies (4) Two-way state to ExStart state? • RTB now decides who to establish a full adjacency with depending upon the type of network that the particular interfaces resides on. • If the interface is on a point-to-point link, the routers becomes adjacent with its sole link partner (aka “soul mates”), and take the relationship to the next level by entering the ExStart state. • If the interface is on a multi-access link (Ethernet, Frame Relay, …) RTB must enter an election process to see who it will establish a full adjacency with, and remains in the two-way state. (Next!)

  26. OSPF Operation Designated Router • Reduce OSPF traffic on multiaccess links • Routers form FULL adjacencies with DR/BDR • Store and distribute neighbors LSDBs • Backup DR for redundancy • OSPF priority used in DR selection • Range 1–255 default 1, 0 for non-candidate. Priority carried in Hello packet • ip ospf priority <value>

  27. Flood Link change 224.0.0.5 AllOSPFRouters 224.0.0.6 AllDRrouters Link change OSPF Operation Function of DR/BDR DR BDR

  28. OSPF Operation2.Electing a DR and BDR (1) • On point-to-point links adjacencies are established with all neighbors, because there is only one neighbor. • On multi-access networks,OSPF elects a DR and BDR to limit the number of adjacencies. • Reduce routing update traffic

  29. OSPF Operation2.Electing a DR and BDR (2) • DR- Designated Router • BDR – Backup Designated Router • DR’s serve as collection points for Link State Advertisements (LSAs) • A BDR back ups the DR. • If the IP network is multi-access, the OSPF routers will elect 1 DR and 1 BDR (unless there is only 1 router on the network).

  30. OSPF Operation2.Electing a DR and BDR (3) • The formation of an adjacency between every attached router would create many unncessary LSA (Link State Advertisements), n(n-1)/2 adjacencies. • Flooding on the network itself would be chaotic. • To prevent this problem, a Designated Router is elected on multi-access networks.

  31. OSPF Operation2.Electing a DR and BDR (4) • All other routers, “DRother”, establish adjacencies with only the DR and BDR. • DRother routers multicast LSAs to only the DR and BDR • (224.0.0.6 - all DR routers) • DR sends LSA to all adjacent neighbors • (224.0.0.5 - all OSPF routers)

  32. OSPF Operation2.Electing a DR and BDR (5) • Once a DR is established, a new router that enters the network with a higher priority or router id will NOT become the DR or BDR. (Bug in early IOS 12.0) • If DR fails, BDR takes over as DR and selection process for new BDR begins. • State of the relationship • DRothers enterExStart statewith DR and BDR andtwo-way statewith all other routers

  33. OSPF Operation2.Electing a DR and BDR (6) DR - Summary DR Election • Router with the highest interface priority (priority = 0 cannot become DR or BDR) • Router with the highest router ID. • Loopback address used first • IP Address on active interface used second • BDR is the second highest

  34. OSPF Operation2.Electing a DR and BDR (7) DR - Summary Adjacencies and multicasting • All other routers, DRother, establish adjacencies with only the DR and BDR. • All routers continue to multicast Hello packets to AllSPFRouters (224.0.0.5) so they can track neighbors. • But updates (LSAs) are multicast to DR and BDR only (224.0.0.6 - AllDRrouters) and in turn • DR floods updates (LSAs) to all adjacent neighbors (224.0.0.5 - AllSPFRrouters)

  35. OSPF Operation2.Electing a DR and BDR (8) BDR-summary • Listens, but doesn’t act. • If LSA is sent, BDR sets a timer. • If timer expires before it sees the reply from the DR, it becomes the DR and takes over the update process. • The process for a new BDR begins.

  36. OSPF Operation3. Discovering Routes and reaching Full State

  37. OSPF Operation4. Choosing routes (1) Dijkstra - Shortest Path First (SPF) Algorithm • Link state database • Created with Link State Packets (LSPs) from each router • TENT database • Tentative triples (ID, path cost, direction) • PATH database • Best path triples (ID, path cost, direction) • Forwarding database • The Routing Table

  38. OSPF Operation4. Choosing routes (2) Dijkstra (SPF) Overview (Cont.) • All routers exchange Link State Packets (LSPs) • Each router starts with itself as root • Tent is built from LSPs • Path is created by examining and comparing TENT triples • Once path is final the forwarding table is populated

  39. 4 • C • 1 • B • 4 • A • 2 • 2 • D • 1 • E • 2 • F • G • 2 OSPF Operation4. Choosing routes (3) Link State Packet (LSP) Data A B C D E F G B/4 G/2 A/4 C/1 B/1 D/4 E/2 C/4 E/1 C/2 D/1 F/2 E/2 G/2 A/2 F/2 Lowest cost best

  40. OSPF Operation 5. Maintaining routes Router 2, Area 1 Router 1, Area 1 LSA • .. Link State Table ACK • Every router in area receives the new LSA via flooding • Each router computes shortest path routing table when a link changes State. Dijkstra Algorithm Old Routing Table New Routing Table

  41. Issues with large OSPF nets • Large routing table • Large link-state table • Frequent SPF calculations

  42. reference • RFC 1403, "BGP OSPF Interaction", K. Varadhan, 1993. • RFC 1584, "Multicast Extensions to OSPF", J. Moy, March 1994. • RFC 1850, "OSPF Version 2 Management Information Base", F. Baker and R. Coltun, Nov 1995. • RFC 2328, "OSPF Version 2", J. Moy, April 1998, also STD 54. • RFC 2370, "The OSPF Opaque LSA Option", R. Coltun, July 1998. • http://www2.rad.com/networks/1995/ospf/ospf.htm, “OSPF”, B. Daniel, B. Omer, R. Carmel. • Internetworking with TCP/IP (Vol I) - Comer • www.et.fnt.hvu.nl/docenten/cuiterwijk/ccnp/guides,“The Technology Innovation Centre Brimingham”.

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