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Understanding Link-State Routing Protocols and Their Mechanisms

This chapter provides a comprehensive overview of link-state routing protocols, including Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS). It delves into the fundamental concepts of link-state packets, adjacency formation among routers, and the steps involved in the link-state routing process, culminating in the creation of a routing table using Dijkstra’s shortest path first (SPF) algorithm. Key advantages, memory and processing requirements, as well as methods for ensuring efficient bandwidth utilization during convergence, are also discussed.

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Understanding Link-State Routing Protocols and Their Mechanisms

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Presentation Transcript

  1. Link-State Routing Protocols Routing Protocols and Concepts – Chapter 10

  2. Introduction • Link-State Routing Introduction • Link-State Packets • Steps

  3. Link-State Routing Introduction • Link state routing protocols • Also known as shortest path first algorithms • These protocols built around Dijkstra’s SPF

  4. Link-State Routing Introduction • 2 link state routing protocols used for routing IP • Open Shortest Path First (OSPF) • Intermediate System-Intermediate System (IS-IS)

  5. Link-State Routing Introduction • Link • An interface on the router • Link State • Information about an interface such as • IP address • Subnet mask • Type of network • Cost associated with link • Neighboring routers on the link

  6. Link-State Introduction • Advantages of a Link-State Routing Protocol

  7. Link-State Introduction • Requirements for using a link state routing protocol • Memory requirements • Typically link state routing protocols use more memory • Processing Requirements • More CPU processing is required of link state routing protocols • Bandwidth Requirements • Initial startup of link state routing protocols can consume lots of bandwidth

  8. Link-State Packets • Sending Hello Packets to Neighbors • Connected interfaces that are using the same link state routing protocols will exchange hello packets • Once routers learn it has neighbors they form an adjacency • 2 adjacent neighbors will exchange hello packets • These packets will serve as a keep alive function

  9. Link-State Packets • LSPs are sent out under the following conditions: • Initial router start up or routing process • When there is a change in topology

  10. Steps • Dikjstra’s algorithm also known as the shortest path first (SPF) algorithm

  11. Steps - Link-State Routing • Link-State Routing Process • How routers using Link State Routing Protocols reach convergence • Step 1 - Learns of directly connected links • Step 2 – Says Hello • Step 3 – Creates LSP • Step 4 – Tells it’s neighbors • Step 5a – Created it’s database • Step 5b – Creates SPF Tree

  12. Step 1 – Discovering it’s links • Directly Connected Networks • Link • This is an interface on a router • Link state • This is the information about the state of the links

  13. Step 2 – Saying Hello • Sending Hello Packets to Neighbors • Link state routing protocols use a hello protocol • Purpose of a hello protocol: • To discover neighbors (that use the same link state routing protocol) on its link

  14. Step 3 – Creating the LSP

  15. Step 4 – Telling it’s Neighbors • Flooding LSPs to All Neighbors • Once LSP are created they are forwarded out to neighbors • After receiving the LSP the neighbor continues to forward it throughout routing area

  16. Step 5a – Creating a Link-State Database • R1 now has the topology

  17. Step 5b – Creating a SPF Tree

  18. Step 5b SPF Tree

  19. Steps – Routing Table • Once the SPF algorithm has determined the shortest path routes, these routes are placed in the routing table

  20. Updates • Triggered updates can be small • This requires less bandwidth • After convereged, LSP use less bandwidth • Routing loops are difficult since each router has the whole topology • Each router independently determines route to each network

  21. Areas • Similar to Process ID’s or Autonomous Systems • Splits routing topology into smaller areas • Saves • # of LSP Packets • Bandwidth for convergence • Processing time

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