IP Switching and Routing Essential Chapter 4 Link State Routing and OSPF

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IP Switching and Routing EssentialChapter 4Link
State Routing and OSPF

• Shuhei Tanigawa
• 2005/6/15

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Introduction

• The OSPF (Open Shortest Path First) protocol is
  TCP/IPs primary routing protocol.
• The OSPF protocol is an example of a link state
  protocol.
• All link state protocols share the same basic
  principles.

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Link State Routing

• Link state routing has a reputation for
  complexity.
• Actually, however, it relies on a few simple
  principles.
• Routing protocols are the primary tool of
  routers.
• Routers just want a map of network.

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Link State Routing

• Link state protocols create the map in three
  distinct phases.
• Each router meets its neighbors.
• Routers share information of first phase with all
  other routers on the network.
• Routers combine the information about individual
  neighbors.

This combination describes the entire network,
and from it routers calculate routes.
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Link State Routing Meet Your Neighbors

• Systems that participate in link state routing
  begin learning routes by meeting their neighbor.
• Each system simply sends a hello packet on all
  its links.
• A hello packet introduces the sender.

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Link State Routing Meet Your Neighbors
Seattle
Salt Lake City
Kansas City
90
San Francisco
40
35
Phoenix
Dallas
This figure shows Kansas City introducing itself
and its neighbors introducing themselves to
Kansas City as well.
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Link State Routing Meet Your Neighbors

• Once Kansas City receive hello packets from its
  neighbors, it knows identity of its neighbors.
• All the other cities take same action as Kansas
  City.

Each city quickly meets its neighbors.
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Link State Routing Share the Information

• In this phase, the cities share the information
  they have learned.
• Each city constructs a message containing its
  neighbor list, and sends that packet to all other
  cities.

These packets are link state advertisements, or
LSAs
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Link State Routing Share the Information

• Most link state advertisement ,including OSPF,
  share neighbor information by flooding LSAs.
• Systems flood a packet by resending a copy of
  that packet to nearly every link.
• Systems consider every LSA it receives a
  candidate to be flooded.
• If systems receive old packet, they can simply
  discard it.

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Link State Routing Share the Information
Seattle
Salt Lake City
Kansas City
San Francisco
Phoenix
Dallas
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Link State Routing Share the Information

• The example illustrates the most important
  feature of flooding.
• It ensures that all systems receive a copy of the
  LSA packet.
• Every system soon learns the neighbors of every
  other system.
• Each system has the view of the network.

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Link State Routing Calculate Routes

• Nearly all link state implementations rely on
  Dijkstras algorithm.
• Dijkstras algorithm constructs a directed graph,
  or tree,for the network.
• Once Dijkstras calculation is complete, the
  system has a complete picture of routing in
  network.

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Link State Routing Calculate Routes
Kansas (0)
SaltLake (3150)
Dallas (844)
Phoenix (2066)
San Francisco (3212)
SaltLake (3149)
Seattle (4464)
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Link State Routing Network Changes

• The discussion so far has assumed a static
  network.
• But real networks are not that stable.
• Routers restart, and links fail, yet the network
  must continue to function.

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Link State Routing Network Changes

• All the principles of the previous three
  subsections apply equally well to dynamic
  networks.
• The only difference is that all three phases
  happen in parallel, and they all take place
  continuously.

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Link State Routing Network Changes
Seattle
Salt Lake City
Kansas City
San Francisco
Phoenix
Dallas
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Link State Routing Network Changes

• Link state protocols react quickly to network
  changes.
• The entire process typically takes a second or
  two.
• The network continues to function.

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To be continued

• OSPF and Network Organization
• Special Networks
• Multicast routing