Understanding Routers and Routing Principles

Dec 13, 2024

Lecture Notes: Routing and Routers

Overview of Router Functions

  • Router's Role: Evaluates incoming traffic, determines the destination, and sends it out the appropriate interface.
  • Routing Table: Essential for decision-making, containing information about the network.
  • Commonality: Workstations, servers, and routers all maintain routing tables.

Routing Table Components

  • Routing Table Legend: Provides codes and explanations for each route.
  • Route Codes: Indicate how a route was learned, e.g., 'C' for directly connected, 'R' for RIP (Routing Information Protocol).

Routing Decision Process

  • Subnets and Prefix Lengths: Important for determining the best route.
  • Example Route Evaluation:
    • IP: 192.168.1.6
    • Possible Routes:
      • 192.168.1.0/24
      • 192.168.0.0/16
      • 192.168.1.6/32
    • Most Specific Route: /32 (exact match)

Administrative Distance

  • Purpose: Used to select the best route when different protocols suggest different routes.
  • Values:
    • Directly Connected: 0
    • Static Routes: 1
    • EIGRP (Enhanced Interior Gateway Routing Protocol): 90
    • OSPF (Open Shortest Path First): 110
    • RIP: 120

Routing Metrics

  • Role: Used by routing protocols to choose the best route internally.
  • Non-comparable: Metrics are protocol-specific (e.g., BGP, OSPF, EIGRP).
  • Protocol Example: RIP uses hop count, EIGRP uses a unique calculation.

Routing Table Analysis

  • Example with RIP v2:
    • Admin Distance: 120
    • Metric (Hop count): 1
    • Next Hop: 10.10.50.2
    • Outgoing Interface: Serial 0/3/1
  • Example with EIGRP:
    • Code: 'D'
    • Admin Distance: 90
    • Different Metric Calculation

Redundancy and Virtual IPs

  • Default Gateway Limitations: Only one IP can be listed.
  • Virtual IP Address (VIP): Provides redundancy.
  • FHRP (First Hop Redundancy Protocol): Allows seamless failover between routers.

Subinterfaces on Routers

  • Concept: Single physical interfaces can have multiple logical subinterfaces.
  • Use Case: VLAN separation and configuration
  • Example Configuration:
    • Physical Interface: Ethernet 1/1
    • Subinterfaces: Ethernet 1/1.10, Ethernet 1/1.20, Ethernet 1/1.100
    • Each subinterface has unique IP and routing configurations.

These notes are designed to provide a concise overview and aid in understanding the key concepts discussed in the lecture regarding routing and router functionalities, including the construction and evaluation of routing tables, administrative distances, redundancy through virtual IPs, and the configuration of subinterfaces.

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