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Dynamic Routing Protocols II OSPF

Dynamic Routing Protocols II OSPF. Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol. . Distance Vector vs. Link State Routing. With distance vector routing, each node has information only about the next hop:

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Dynamic Routing Protocols II OSPF

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  1. Dynamic Routing Protocols IIOSPF Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol.

  2. Distance Vector vs. Link State Routing • With distance vector routing, each node has information only about the next hop: • Node A: to reach F go to B • Node B: to reach F go to D • Node D: to reach F go to E • Node E: go directly to F • Distance vector routing makespoor routing decisions if directions are not completelycorrect (e.g., because a node is down). • If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged. A B C F D E

  3. A A A A A A B B B B B B C C C C C C F F F F F F D D D D D D E E E E E E Distance Vector vs. Link State Routing • In link state routing, each node has a complete map of the topology • If a node fails, each node can calculate the new route • Difficulty:All nodes need to have a consistent view of the network A B C F D E

  4. Link State Routing: Properties • Each node requires complete topology information • Link state information must be flooded to all nodes • Guaranteed to converge

  5. Link State Routing: Basic princples 1. Each router establishes a relationship (“adjacency”) with its neighbors 2.Each router generates link state advertisements(LSAs) which are distributed to all routers LSA = (link id, state of the link, cost, neighbors of the link) 3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network as a graph with weighted edges 4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network

  6. Operation of a Link State Routing protocol IP Routing Table Link StateDatabase Dijkstra’s Algorithm ReceivedLSAs LSAs are flooded to other interfaces

  7. Dijkstra’s Shortest Path Algorithm for a Graph Input:Graph(N,E) with N the set of nodes and E the set of edges dvwlink cost (dvw = infinity if (v,w)  E, dvv = 0) s source node. Output: Dncost of the least-cost path from node s to node n M = {s}; for each n  M Dn = dsn; while (M  all nodes) do Find w  M for which Dw = min{Dj ; j  M}; Add w to M; for each n  M Dn = minw [ Dn, Dw + dwn ]; Update route; enddo

  8. OSPF • OSPF = Open Shortest Path First • The OSPF routing protocol is the most important link state routing protocol on the Internet • The complexity of OSPF is significant • History: • 1989: RFC 1131 OSPF Version 1 • 1991: RFC 1247 OSPF Version 2 • 1994: RFC 1583 OSPF Version 2 (revised) • 1997: RFC 2178 OSPF Version 2 (revised) • 1998: RFC 2328 OSPF Version 2 (current version)

  9. Features of OSPF • Provides authentication of routing messages • Enables load balancing by allowing traffic to be split evenly across routes with equal cost • Type-of-Service routing allows to setup different routes dependent on the TOS field • Supports subnetting • Supports multicasting • Allows hierarchical routing

  10. 4 2 1 3 2 3 1 5 • Link costs are called Metric • Metric is in the range [0 , 216] • Metric can be asymmetric Example Network Router IDs are selected independent of interface addresses

  11. 4 3 2 Link State Advertisement (LSA) • The LSA of router 10.10.10.1 is as follows: • Link State ID:10.10.10.1= Router ID • Advertising Router:10.10.10.1= Router ID • Number of links:3= 2 links plus router itself • Description of Link 1:Link ID = 10.1.1.1, Metric = 4 • Description of Link 2:Link ID = 10.1.2.1, Metric = 3 • Description of Link 3:Link ID = 10.10.10.1, Metric = 0 Each router sends its LSA to all routers in the network(using a method called reliable flooding)

  12. Network and Link State Database Each router has a database which contains the LSAs from all other routers

  13. Link State Database • The collection of all LSAs is called the link-state database • Each router has an identical link-state database • Useful for debugging: Each router has a complete description of the network • If neighboring routers discover each other for the first time, they will exchange their link-state databases • The link-state databases are synchronized using reliable flooding

  14. OSPF Packet Format OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89 TTL: set to 1 (in most cases) Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

  15. OSPF Packet Format 2: current version is OSPF V2 ID of the Area from which the packet originated Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Standard IP checksum taken over entire packet Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) Prevents replay attacks

  16. OSPF LSA Format LSA Header Link 1 Link 2

  17. Discovery of Neighbors • Routers multicasts OSPFHello packets on all OSPF-enabled interfaces. • If two routers share a link, they can become neighbors, and establish an adjacency • After becoming a neighbor, routers exchange their link state databases Scenario:Router 10.1.10.2 restarts

  18. Discovery of adjacency Neighbor discovery and database synchronization Scenario:Router 10.1.10.2 restarts After neighbors are discovered the nodes exchange their databases Sends database description. (description only contains LSA headers) Sends empty database description Database description of 10.1.10.2 Acknowledges receipt of description

  19. Regular LSA exchanges 10.1.10.2 explicitly requests each LSA from 10.1.10.1 10.1.10.1 sends requested LSAs 10.1.10.2 has more recent value for 10.0.1.6 and sends it to 10.1.10.1(with higher sequence number)

  20. ACK ACK ACK Updatedatabase Updatedatabase ACK ACK ACK ACK ACK ACK ACK LSA ACK LSA LSA LSA LSA LSA LSA ACK LSA ACK LSA LSA Updatedatabase Updatedatabase Routing Data Distribution • LSA-Updates are distributed to all other routers via Reliable Flooding • Example: Flooding of LSA from 10.10.10.1 Updatedatabase

  21. Dissemination of LSA-Update • A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet) • Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes. • Acknowledgements of LSA-updates: • explicit ACK, or • implicit via reception of an LSA-Update • Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?

  22. Dynamic Routing Protocols IIIMore OSPF Relates to Lab 4. This module covers additional details on the Open Shortest Path First (OSPF) routing protocol.

  23. Functional Requirements of OSPF • Fast convergence and low consumption of network resources • A descriptive routing metric • Configurable • Value ranges between 1 and 65,535 • No restriction on network diameters (RIP has a limit of 15) • Equal-cost multipath • A way to do load balancing

  24. Functional Requirements of OSPF • Routing Hierarchy • Support large routing domains • Separate internal and external routes • Support of flexible subnetting schemes • Route to arbitrary [address,mask] combinations using variable length subnet masks (VLSMs) • Security • Type of Service Routing

  25. OSPF Basics(The Essence) • Distributed, replicated database model • Describes complete routing topology • Link State Advertisements (LSAs, sometimes called Link State Announcements) • Carry local piece of routing topology • Distribution of LSAs using reliable flooding • Link state database • Identical for all the routers

  26. OSPF Packet Format OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89 TTL: set to 1 (in most cases) Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

  27. OSPF Packet Format 2: current version is OSPF V2 ID of the Area from which the packet originated Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Standard IP checksum taken over entire packet Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) Prevents replay attacks

  28. OSPF LSA Format LSA Header Link 1 Link 2

  29. LSAs (1) • Identifying LSAs • LS type field • Link State ID field • Mostly carries addressing information • E.g. IP address of externally reachable network • Advertising Router field • Originating router’s OSPF router ID

  30. LSAs (2) • Identifying LSA instances • Needed to update self-originated LSAs • LS Sequence Number field • 32 bit values • Monotonically increasing until some max value • 600 years to roll over! • LSA checksum and LS Age guard against potential problems

  31. LSAs (3) • Verifying LSA contents • LS Checksum field • Computed by the originating router and left unchanged thereafter • LS age field not included in checksum • Removing LSAs from databases • LS Age field • Ranges from 0 to 30 min. • Max Age LSAs used to delete outdated LSAs

  32. LSAs (4) • Other LSA Header fields • Options field • Sometimes used to give special treatment during flooding or routing calculations • Length field • Includes LSA header and contents • Ranges from 20-65535 bytes

  33. Link State Database • Collection of all OSPF LSAs • Databases exchanged between neighbors • Synchronization thru reliable flooding • Gives the complete routing topology • Each OSPF router has identical link-state database

  34. Reliable Flooding • Robustness • Updates flooded over all the links , so failure of any link does not affect database synchronization • LSAs refreshed every 30 minutes • LSA checksum field detects corruption • Flooding loops avoided by LS Age field • MinLSInterval limits rate of LSA origination • Receivers can refuse to accept LSA updates if they received an update less than a second ago

  35. Routing Calculations • Link costs configurable by administrator • Smaller values for more preferred links • Must make sense to add link costs • 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)

  36. IP Multicast to Send/Receive Changes • Multi-Access networks • All routers must accept packets sent to the AllSPFRouters (224.0.0.5) address • All Designated Router (DR) and Backup Designated Router (BDR) must accept packets sent to the AllDRouters (224.0.0.6) address • Hello packets are sent to the AllSPFRouters address (Unicast for point-to-point and virtual links)

  37. Hierarchical Routing • Technique used to build large networks • Minimizes consumption of network resources: • Router memory • Router computing resources • Link bandwidth • Flat Routing: linear increase in routing table size • Hierarchical Routing: size increases logarithmically

  38. An Example of Hierarchical Routing (1) 10.0.3 10.3.0.0/16 10.0.0.0/8 10.3.2 10.3.1 10.1.3 10.2.3 10.2.0.0/16 10.1.0.0/16 10.1.1 10.1.2 10.2.2 10.2.1

  39. An Example of Hierarchical Routing (2) • Consider a router in 10.1.1 • Assume 16 entries in each of the first level partitions • With flat routing, 9*16 = 144 entries/router • With 3 level hierarchy, the router has 16 entries within 10.1.1.0/24 + entries for 10.1.2.0/24, 10.1.3.0/24,10.2.0.0/16 and 10.3.0.0/16 for a total of 20 entries. • Significant reduction in routing table size • But might lead to suboptimal routing

  40. 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 • Interconnection of areas through area border routers (ABRs) • ABR leaks IP addressing information to other areas through summary LSAs

  41. OSPF Areas • 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

  42. Area Organization • All the areas are connected to area 0.0.0.0 also called the Backbone Area • Need not have a direct physical connection though • Virtual links provide logical link to backbone • Summary LSAs tunneled across non backbone areas • Exchange of routing information between areas using Distance Vector Protocol • Absence of redundant paths between areas • Not subject to convergence problems

  43. Area 0 Backbone Area Area 2 Area 3 Area 4 Area 1 OSPF Areas • Group of nodes/networks • Per area topology DB • Invisible outside the area • Reduces routing traffic • Backbone Area is contiguous • All others areas must connect to the backbone • Virtual Links

  44. IR Area 3 Area 2 ABR/BR Area 0 ASBR To another AS Router Classification • Internal Router (IR) • Area Border Router (ABR) • Backbone Router (BR) • Autonomous System Border Router (ASBR) IR/BR Area 1

  45. OSPF Route Types Intra-Area Route • All routes within an area Inter-Area Route • Routes announced from area to another by an ABR External Route • Routes imported into OSPF from another protocol or Static routes Area 0 Area 2 Area 3 ABR ASBR To Another AS

  46. Inter-Area Route Summarization • Prefix or all subnets • Prefix or all networks R2 Backbone Area 0 FDDI Dual Ring Network 1 Next Hop R1 With Summarization R1 (ABR) Area 1 Network 1.A 1.B 1.C Next Hop R1 R1 R1 Without Summarization 1.A 1.B 1.C

  47. External Routes • Redistributed into OSPF • Flooded without changes throughout the AS • OSPF supports two type of external metrics • Type 1 • Type 2 (Default) RIP IGRP EIGRP BGP etc. OSPF Redistribute

  48. Topology/Links-State DB • A router has a separate DB for each area it belongs to • All routers within an area have an identical DB • SPF calculation is done separately for each area • LSA flooding is limited to the particular area

  49. Protocol Functionality • Bringing up adjacencies • LSA Types • Area Classification

  50. The Hello Protocol • Responsible to establish and maintain neighbor relationships • Elects designated router in multi-access networks Hello FDDI Dual Ring Hello Hello

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