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Link State Overview, 1

Link State Overview, 1. Each router establishes an adjacency with each of its neighbors (Hello / reply) Each router sends Link State Advertisements (LSA) to each neighbor one LSA per link, with a sequence # link id, link state, metric, link neighbors

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Link State Overview, 1

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  1. Link State Overview, 1 • Each router establishes an adjacency with each of its neighbors (Hello / reply) • Each router sends Link State Advertisements (LSA) to each neighbor • one LSA per link, with a sequence # • link id, link state, metric, link neighbors • flood any LSA received to all neighbors (except the originator)

  2. Link State Overview, 2 • Each router stores all LSAs received • After all LSAs have flooded, each router has an identical topological database (Link State Database) • Each router uses Dijkstra algorithm to compute its own shortest path to each network • Hello packets now used for keep-alive

  3. Changes • What If: manual change of cost (claimed bandwidth) on one router’s link • That router will send a new LSA to every neighbor, then flooding • After all new LSAs have flooded, each router has a NEW identical topological database (Link State Database) • Each router uses Dijkstra algorithm to compute its own NEW shortest path to each network

  4. Dijkstra Algorithm • Go over Table 4.3, Figure 4.11, 4.12 • You should understand, but do not memorize the algorithm • The computation time is not a severe burden on modern routers (flooding and topological database maintenance can be issues)

  5. Neighbor Issue • What if there are 15 routers connected (perhaps at a central location) on a single broadcast network? • We don’t want to build 15*14 adjacencies and to flood LSAs about this LAN all over the network • Solution: the Designated Router (DR) and the Backup Designated Router (BDR)

  6. Designated Router • IMPORTANT • The designated router (DR) is NOT the gateway to the default route. • The DR and BDR exist to minimize traffic about link states. • See Doyle, page 418 and Figure 9.2

  7. OSPF Without Areas • So far, every router knows about every link that every other router has • Issue of growth of topology database • Areas are the OSPF solution • Must have Area 0 • May have other areas • Inter-area traffic must cross Area 0

  8. OSPF Areas, 2 • Every router knows about every link that every router has IN ITS AREA • ie, I only know the local topology • Router LSAs not flooded to other areas • Dijkstra algorithm only on local area • An Area Border Router (ABR) is in 2 or more areas, and must fully participate in each area • Network Summary LSAs between areas

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