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GT ECE 4110 - Routing OSPF-Revised

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PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12OSPF Hello ProtocolNeighbors and AdjacencyOSPF Designated RouterLink State AdvertisementsInter-Area Routing (between areas)Autonomous System External RoutesSlide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 59Slide 60Slide 61Slide 62Slide 63Slide 64Slide 65Slide 66Slide 67Slide 68Slide 69Slide 70Slide 711OSPF:Open ShortestPath First13.313.32Figure 13-18Areas in an autonomous system3Figure 13-19Types of links4Figure 13-20Point-to-point link5Figure 13-21Transient link6Figure 13-22Stub link7Figure 13-23Example of an internet8Figure 13-24Graphical representation of an internet910Figure 13-32-Part 1Shortest path calculationBook uses cost only in inbound arrow, thus assigns a cost on input to an interface, can be different in different directions.Dijkstra’s Algorithm shortest path tree is created by each router.11Figure 13-32-Part 2Shortest path calculation12Figure 13-32 Part 3Shortest path calculation13OSPF Hello Protocol•OSPF routers use the Hello Protocol to learn about other routers with interfaces on the same network (“neighbors”)•On all multi-access networks (broadcast and nonbroadcast) the Hello Protocol also elects a Designated RouterHELLOHELLOHELLO14Neighbors and Adjacency•OSPF creates adjacencies between neighbors to facilitate exchange of routing information•Neighbors on point-to-point networks and virtual links always become adjacent•On multi-access networks, all routers become adjacent to the Designated Router•Adjacencies control the distribution of routing protocol packets15OSPF Designated Router•Each multi-access network has a Designated Router, elected by the Hello Protocol•The Designated Router generates as Link State Advertisements (LSA) for the multi-access network and has other responsibilities in the running of the protocol•The Designated Router concept enables a reduction in the number of adjacencies required•The reduction in the number of adjacencies reduces the amount of routing traffic and the size of the topological database16Link State Advertisements•Link State Advertisements are flooded throughout an area ensuring all routers in the area have exactly the same topological database•The topological database consists of the LSAs from each router belonging to the area•From the topological database, each router calculates a shortest-path tree, with itself as root•This shortest-path tree in turn yields a routing table for intra-area routing (inside area)17Inter-Area Routing (between areas)•Each Area Border Router summarizes the topology of its attached areas for transmission to all other Area Border Routers via the backbone•This process allows all ABRs to calculate paths to destinations not contained in its attached areas•ABRs then advertise these paths to its attached areas to enable routing to other areas18Autonomous System External Routes•Routers that have information regarding other ASs flood this information throughout the AS•To utilize external routing information: the path to all routers advertising external information must be known throughout the AS•The location of AS Boundary Routers are summarized by Area Border Routers19Figure 13-33Types of OSPF packets20Open Shortest Path First First OSPF RFC Oct 1989Final RFC 2328 April 1998OSPF Components 1) Hello Protocol - Detect neighbors and confirm still there 2) “Reliable Flooding” Mechanism - ensures all routers in an OSPF area have a consistent link state databaseOSPF is “Encapsulated” in IP and the IP protocol field is set to 0X59. Immediately following the IP header is a common 24 - Byte packet header. 1 2 30 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Version Type Packet LengthRouter IDArea IDChecksum Au TypeAuthenticationAuthenticationOnly neighborsin same areaform adjacencies Version : 2 Type Description 1 Hello 2 Database Description 3 Link State Request 4 Link State Update 5 Link State Acknowledgment OSPS's common header.Authentication type 0=none 1=password21OSPF packets are OSPF packets are encapsulated in IP datagrams.encapsulated in IP datagrams.22Figure 13-34OSPF packet header23Hello ProtocolDestination address is always 224.0.0.5 which is multicast and is represented in Ethernet as 0X01-00-5E-00-00-05This is a MAC layer multicast address. OSPF hellos are transmitted every 10 seconds ( By Default ).If 4 hello intervals pass (40 seconds ) without hearing a hello from a neighbor that neighbor is declared to be down Hello Protocol: 1. Advertises a router’s “Aliveness” 2. Verifies two - way connectivity 3. Avoids one - way connectivity 4. Elects designated and backup designated routers 5. Maintains OSPF adjacencies


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GT ECE 4110 - Routing OSPF-Revised

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