module11-ospf

module11-ospf - Dynamic Routing Protocols II OSPF Relates...

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1 Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol. Dynamic Routing Protocols II OSPF
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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 makes poor routing decisions if directions are not completely correct (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 D E F
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3 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 D E F A B C D E F A B C D E F A B C D E F A B C D E F A B C D E F A B C D E F
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4 Link State Routing: Properties Each node requires complete topology information Link state information must be flooded to all nodes Guaranteed to converge
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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 has 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
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6 Operation of a Link State Routing protocol Received LSAs IP Routing Table Dijkstra’s Algorithm Link State Database LSAs are flooded to other interfaces
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7 Dijkstra’s Shortest Path Algorithm for a Graph Input: Graph (N,E) with N the set of nodes and E N N the set of edges d vw link cost (d vw = infinity if (v,w) E, d vv = 0) s source node. Output : D n cost of the least-cost path from node s to node n M = {s}; for each n M D n = d sn ; while (M all nodes) do Find w M for which D w = min{D j ; j M}; Add w to M; for each n M D n = min w [ D n , D w + d wn ]; Update route; enddo
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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
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This note was uploaded on 03/05/2012 for the course CS 458 taught by Professor Staff during the Fall '08 term at UVA.

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module11-ospf - Dynamic Routing Protocols II OSPF Relates...

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