ospf - IntroductiontoOSPF NishalGoburdhan...

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    Introduction to OSPF Nishal Goburdhan
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    Routing and Forwarding Routing is not the same as Forwarding Routing is the building of maps Each routing protocol usually has its own routing  database Routing protocols populate the forwarding table Forwarding is passing the packet to the next  hop device Forwarding table contains the best path to the next  hop for each prefix There is only ONE forwarding table
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    OSPF Background Developed by IETF – RFC1247 Designed for Internet TCP/IP environment OSPF v2 described in RFC2328/STD54 OSPF v3 described in RFC2740 - IPv6  Link state/Shortest Path First  Technology Dynamic Routing Fast Convergence Route authentication
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    Link State Algorithm Each router contains a database containing a  map of the whole topology Links Their state (including cost) All routers have the same information All routers calculate the best path to every  destination Any link state changes are flooded across the  network “Global spread of local knowledge”
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    Link State Routing Automatic neighbour discovery Neighbours are physically connected routers Each router constructs a Link State Packet  (LSP) Distributes the LSP to neighbours… …using an LSA (Link State Announcement) Each router computes its best path to every  destination On network failure New LSPs are flooded All routers recompute routing table
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    Low Bandwidth Requirements Only changes are propagated Multicast used on multi-access broadcast  networks 224.0.0.5 used for all OSPF speakers 224.0.0.6 used for DR and BDR routers FDDI Dual Ring R1 LSA X LSA
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    “Shortest Path First” The optimal path is determined by the sum of the  interface costs FDDI Dual Ring FDDI Dual Ring N1 N2 N3 N4 N5 R1 R2 R3 R4 Cost = 1 Cost = 1 Cost = 10 Cost = 10 Cost = 10
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ospf - IntroductiontoOSPF NishalGoburdhan...

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