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12 - Day 24 - Dynamic Routing.md

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Day 24 - Dynamic Routing

Reviewed: No

Introduction:

  • Dynamic routing contrasts with static routing by automating the route discovery and path selection processes.
  • Instead of manually configuring routes, dynamic routing protocols allow routers to discover and update routes based on network changes.
  • This session covers a general overview of dynamic routing protocols, their types, metrics, and administrative distance.

Key Concepts:

  1. Dynamic Routing Basics:
    • Dynamic Routing Protocols: Allow routers to automatically discover and maintain optimal paths to destination networks.
    • Benefits: Automatic updates to routing tables in response to network changes (e.g., link failures).
  2. Network Topology Example:
    • Four routers (R1, R2, R3, R4) are connected in a network.
    • Without dynamic routing, each router only knows about its directly connected networks.
  3. Dynamic Routing Process:
    • Routers exchange information about their known routes with neighbors.
    • For example, R4 advertises its connected network (192.168.4.0/24) to R2, which then shares it with R1, and so on.
  4. Routing Table Updates:
    • When a network change occurs (e.g., a link goes down), dynamic routing protocols automatically update the routing tables to reflect the best available paths.

Types of Dynamic Routing Protocols:

  1. Interior Gateway Protocols (IGPs):
    • Used within a single autonomous system (AS).
    • Examples: OSPF, EIGRP, RIP.
  2. Exterior Gateway Protocols (EGPs):
    • Used between different autonomous systems (AS).
    • Example: BGP (Border Gateway Protocol), the only EGP in use today.
  3. Algorithm Types:
    • Distance Vector Protocols: Use simple metrics like hop count. Examples: RIP, EIGRP.
    • Link State Protocols: Develop a complete network map and calculate the best paths. Examples: OSPF, IS-IS.
    • Path Vector Protocols: Used by EGPs like BGP.

Distance Vector vs. Link State Protocols:

  1. Distance Vector Protocols:
    • Operate by sharing route information with directly connected neighbors.
    • Known as “routing by rumor” because routers only know about routes their neighbors tell them about.
    • Examples: RIP, EIGRP.
  2. Link State Protocols:
    • Routers create a complete map of the network.
    • Each router calculates the best routes independently.
    • Requires more CPU and memory resources but provides faster convergence.
    • Examples: OSPF, IS-IS.

Routing Metrics:

  1. Metric:
    • A value used by routers to determine the best route to a destination.
    • Lower metrics are preferred.
    • Different protocols use different metrics:
      • RIP: Hop count.
      • EIGRP: Composite metric based on bandwidth and delay (can include other factors).
      • OSPF: Cost based on link bandwidth.
      • IS-IS: Cost (default is 10 for all links unless manually configured).
  2. Equal Cost MultiPath (ECMP):
    • If multiple routes to the same destination have the same metric, both can be added to the routing table, and traffic will be load-balanced between them.

Administrative Distance (AD):

  1. AD Definition:
    • A value used to rank the trustworthiness of routes learned via different routing protocols.
    • Lower AD values are preferred.
  2. Common AD Values:
    • Connected routes: 0
    • Static routes: 1
    • eBGP: 20
    • EIGRP: 90
    • IGRP: 100
    • OSPF: 110
    • IS-IS: 115
    • RIP: 120
    • Unusable routes: 255
  3. Floating Static Routes:
    • Static routes with a higher AD than dynamic routes, used as backups if the dynamic route fails.

Quiz Review:

  1. AD Comparison:
    • When routes from different protocols compete, the route with the lower AD is chosen.
    • Example: EIGRP (AD 90) is preferred over OSPF (AD 110).
  2. Longest Prefix Match:
    • When multiple routes to the same destination exist, the route with the longest prefix match (most specific) is chosen.

Summary of Key Commands:

  • **SHOW IP ROUTE**: Displays the current routing table, including metrics and AD.
  • **IP ROUTE <destination> <mask> <next-hop>**: Configures a static route.
  • **IP ROUTE <destination> <mask> <next-hop> [AD]**: Configures a floating static route with a specified AD.