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Understanding Routing vs. Forwarding: Protocols, Tables, and Loop Prevention Techniques

This lecture explores the critical concepts of routing and forwarding in networking. It differentiates between routing tables and forwarding tables, discussing why routing protocols are essential for handling link failures, new nodes, and congestion. The lecture covers two main approaches: Distance Vector-based and Link State-based routing. Key topics include routing loops, heuristics for loop breaking, and the mechanics of the Router Information Protocol (RIP) and Link State Routing, highlighting how nodes calculate optimal routes using protocols such as OSPF and the Dijkstra algorithm.

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Understanding Routing vs. Forwarding: Protocols, Tables, and Loop Prevention Techniques

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Presentation Transcript

  1. Lecture 17

  2. Routing versus Forwarding Routing Table: Forwarding Table:

  3. Why routing protocols? • Link failures • New nodes • Congestion • Two approaches: • Distance Vector-based on local information • Link State-based on global information

  4. Distance-vector routing

  5. Routing Loops • Example 1 • F detects that link to G has failed • F sets distance to G to infinity and sends update t o A • A sets distance to G to infinity since it uses F to reach G • A receives periodic update from C with 2-hop path to G • A sets distance to G to 3 and sends update to F • F decides it can reach G in 4 hops via A • Example 2 • link from A to E fails • A advertises distance of infinity to E • B and C advertise a distance of 2 to E • B decides it can reach E in 3 hops; advertises this to A • A decides it can read E in 4 hops; advertises this to C • C decides that it can reach E in 5 hops…

  6. Loop-Breaking Heuristics • Set infinity to 16 • Split horizon • Split horizon with poison reverse

  7. Router Information Protocol (RIP)

  8. RIP packet

  9. Link State Routing • Each node establishes a list of directly connected neighbors and cost of each link • Floods that information in a LSP to all neighbors • Retransmits LSPs from other nodes- but does not echo to sender

  10. Propagation of LSPs

  11. LSP Information • ID of sending node • Link-state of sending node • Sequence number • Time to live

  12. Route Calculation • Each node has enough information to map the network • Dijkstra’s shorted path algorithm used to compute the routes

  13. Example: Link State Routing

  14. Routing Table Calculation

  15. Routing Table Calculation

  16. Routing Table Calculation

  17. OSPF • Authentication • Hierarchy-Domains and Areas • Load Balancing

  18. OSPF Header Format

  19. OSPF link-state advertisement

  20. Metrics • Issues • Number of Hops • Latency • Bandwidth or Capacity • Congestion • Difficult to assign a scalar cost to such a complex and changing problem

  21. ARPANET 1 • Lowest Cost=Shortest Queue

  22. ARPANET 2 • Delay=(Depart time-Arrival Time)+Transmission Time+Latency • Reliability incorporated through the Depart Time parameter • Wide spread of weights- • Oscillations

  23. ARPANET 3 • Reduce dynamic range of metric • Averaging • Hard limit on changes in metric-like the stock market

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