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Routing Protocols- Distance Vector and Link State

Routing Protocols: Distance Vector vs. Link-State

Routing protocols are essential for determining the best path for data packets in a network. Two primary types of routing protocols are Distance Vector and Link-State. These protocols differ in how they calculate routes, share information, and react to changes in the network.

1. Distance Vector Routing Protocols

Overview:

  • Concept: The router determines the best path by counting hops (distance) to the destination.
  • Routing Table Exchange: Routers periodically exchange their entire routing tables with their directly connected neighbors.
  • Algorithm: Bellman-Ford algorithm.
  • Metric: Hop count (number of routers to reach the destination).
  • Network View: Routers have a limited view — they only know the distance and direction to a destination, not the full network topology.

How It Works:

  1. Each router starts with knowledge of its directly connected networks.
  2. Periodically, routers share their routing tables with neighbors.
  3. If a neighbor advertises a shorter path to a destination, the router updates its table.
  4. The process continues until all routers converge (agree on the best paths).

Key Characteristics:

  • Simple to implement.
  • Slow convergence – takes time to detect network changes.
  • Looping problem – routers can enter routing loops.
  • Count to Infinity – Routing loops continue until hop count exceeds a limit (e.g., RIP limits at 15 hops).
  • Periodic Updates – Entire routing table is broadcast at regular intervals.

Examples of Distance Vector Protocols:

  • RIP (Routing Information Protocol)
    • Hop count as the metric.
    • Maximum of 15 hops (16 is considered unreachable).
    • Suitable for small networks.
  • IGRP (Interior Gateway Routing Protocol)
    • Cisco proprietary.
    • Uses multiple metrics (bandwidth, delay, load).
    • More scalable than RIP.

2. Link-State Routing Protocols

Overview:

  • Concept: Routers build a complete map of the network by learning about the state of each link (connection).
  • Routing Table Exchange: Routers exchange small updates (link-state advertisements) only when there is a network change.
  • Algorithm: Dijkstra’s Shortest Path First (SPF) algorithm.
  • Metric: Cost, usually based on bandwidth.
  • Network View: Routers have a complete view of the network topology.

How It Works:

  1. Each router discovers its neighbors by sending "hello" packets.
  2. Routers send link-state advertisements (LSAs) to inform others about the state of their links.
  3. LSAs are flooded throughout the network, allowing every router to build an identical network map.
  4. Each router independently calculates the shortest path to each destination.

Key Characteristics:

  • Faster convergence – quickly adapts to network changes.
  • No looping issues – full network map prevents routing loops.
  • Resource-intensive – requires more CPU and memory to maintain the topology.
  • Efficient updates – only sends updates when changes occur, reducing bandwidth usage.

Examples of Link-State Protocols:

  • OSPF (Open Shortest Path First)
    • Open standard.
    • Supports hierarchical design (areas).
    • Scalable for large networks.
  • IS-IS (Intermediate System to Intermediate System)
    • Common in ISPs and large networks.
    • Supports multi-area design like OSPF.

Comparison: Distance Vector vs. Link-State

FeatureDistance VectorLink-State
AlgorithmBellman-FordDijkstra’s SPF
Convergence SpeedSlowFast
ScalabilitySmall to medium networksLarge networks
Network ViewLimited (next hop info)Complete network topology
Routing LoopsPossible (count to infinity)Not possible
Resource UsageLow (simple computation)High (memory, CPU intensive)
Update TypePeriodic (entire table)Event-driven (LSA updates)
ExamplesRIP, IGRPOSPF, IS-IS

Key Takeaways:

  • Distance Vector protocols are simple and suitable for smaller networks but are prone to slower convergence and routing loops.
  • Link-State protocols are more efficient, scalable, and loop-free but require higher resources and are better suited for large networks.

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