OSPF Basics
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Open Shortest Path First (OSPF) is an interior gateway protocol that uses link state routing and the Dijkstra algorithm to calculate the shortest path to destinations within an autonomous system. It elects a Designated Router to generate network link advertisements and assist in database synchronization between routers. Routers run the Shortest Path First algorithm on their link state databases to determine the best routes and populate their routing tables.
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OSPF Basics
- 1. Open Shortest Path First Protocol (OSPF) Speaker: Martín Bratina. martin.bratina@att.com Date 22/03/2013
- 2. Agenda • What is OSPF? • Why is OSPF needed? • How OSPF works? • LAB 2 © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.
- 3. What is OSPF? • Open Shortest Path First • Internal routing protocol (IGP) • Open Standard. RFC 2328 • Link State Protocol • Designed for IP networks © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 3 AT&T Intellectual Property.
- 4. Why is OSPF needed? • Open standard • Link State Protocol • Scalability • Fast convergence • Supports authentication/security • Supports VLSM (Variable Length Subnet Masks) • Support IPv6 (OSPFv3) • Support load balancing • Consumes low bandwidth • Uses a hierarchical structure © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 4 AT&T Intellectual Property.
- 5. How OSPF works? © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 5 AT&T Intellectual Property.
- 6. How OSPF works? • Uses a Link State Logic. • Neighbor discovery • Topology database exchange • Route computation • Divides routers into areas for route optimization and segmentation © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 6 AT&T Intellectual Property.
- 7. Neighbor Discovery • Neighbor discovery • Each router discover its neighbors routers sending hello messages • Routers from adjacencies with its neighbors • Neighbor adjacency requirements • Interfaces in the same subnet • Different router ID between neighbors • Routers in the same area • Same authentication parameters © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 7 AT&T Intellectual Property.
- 8. Topology Database Exchange • Topology Database Exchange • Each router generates its own network topology information (LSA Message) • Each router flood its LSA message to its neighbors • Each router stores a LSA copy on its internal OSPF topology database (LSDB) • Each router has the same LDSB • Link State Advertisement (LSA) Message information • Router ID of the LSA • List of the router interfaces, IP address, subnet and mask • List of routers reachable (neighbors) on each interface © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 8 AT&T Intellectual Property.
- 9. Link State Advertisements(LSAs) LS Age Options LS Type Link State ID Advertising Router LS Sequence Number LS Checksum Length 0 16 LSA Header © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 9 AT&T Intellectual Property.
- 10. Route Computation • Route Computation • Each router analyzes its topology database and executes Shortest Path First (SPF) algorithm • Router choses the shortest (best) route to each destination from its perspective (SPF Tree) • Router generates entries in the routing table for those selected destinations © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 10 AT&T Intellectual Property.
- 11. SPF Tree BEFORE run SPF AFTER run SPF © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 11 AT&T Intellectual Property.
- 12. Communication between OSPF Routers • OSPF packets encapsulated in IP packets • IP protocol # 89 • Standard 24 byte header • OSPF packet type field • OSPF router ID of sender • Packet checksum • Authentication fields • OSPF Area ID • Uses Multicast IP address 224.0.0.5 for all OSPF routers • DR router use Multicast IP address 224.0.0.6 © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 12 AT&T Intellectual Property.
- 13. The Hello Protocol • Maintains neighbor relationships • Elects Designated Router (DR) in multi-access networks • The Hello Packet • Hello packets sent out every 10 seconds • RouterDead Interval (default 40 seconds) • Network Mask • List of neighbors • Ensures that link is bidirectional © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 13 AT&T Intellectual Property.
- 14. The Hello Protocol Hello Hello Hello Hello © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 14 AT&T Intellectual Property.
- 15. Designated Router (DR) • One per multi access network • Generates Network Link Advertisements (LSA type 2) • All routers form adjacency to it • Assist in DB synchronization • Elected by priority. It tie, it is elected by the highest router ID Adjacencies WITHOUT DR © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 15 AT&T Intellectual Property. Adjacencies WITH DR DR
- 16. Database Synchronization • Crucial to ensure correct and loop free routing • Must be done before 2 neighbors start communication • Whenever new LSAs are introduced • uses reliable flooding • Each router sends LSA headers (small LSA’s) to its neighbor when connection comes up • Requests only those LSAs which are recent © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 16 AT&T Intellectual Property.
- 17. Database Exchange • Neighboring routers first exchange hellos • A database description packet establishes the sequence number • The other router sends LSA headers • Sequence number incremented for every pair of database description packets • implicit acknowledgement for the previous pair • After examining LSA headers explicit request sent for complete LSA © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 17 AT&T Intellectual Property.
- 18. Reliable Flooding • Starts when a router wants to update self-originated LSAs • Neighbor installs more recent LSAs into its database • Floods out on all interfaces except the one on which it arrived • Reliability-retransmissions until acks received © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 18 AT&T Intellectual Property.
- 19. Reliable Flooding (cont..) 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 10.1.1.3 10.1.1.5 Time T1 u u u © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 19 AT&T Intellectual Property.
- 20. Reliable Flooding (cont..) 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 u u 10.1.1.3 10.1.1.5 Time T2 u u u 20 © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.
- 21. Reliable Flooding (cont..) 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 10.1.1.3 10.1.1.5 Time T3 u u © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 21 AT&T Intellectual Property.
- 22. Reliable Flooding (cont..) 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 10.1.1.3 10.1.1.5 Time T3+ ack ack ack ack ack © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 22 AT&T Intellectual Property.
- 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)
- 24. LAB © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 24 AT&T Intellectual Property.
- 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
- 26. Sample Area Configuration AREA 0 Backbone AREA 1 AREA 35 ASBR Internet ABR ABR
- 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
- 32. LAB © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of 32 AT&T Intellectual Property.
- 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
- 36. Thank You!