Correct Route Selection Order

 

Correct Route Selection Order

  1. Longest Prefix Match (Most Specific Route)
    • The router first chooses the route with the longest subnet mask (highest prefix length).

    • Example:

      10.1.0.0/16
      10.1.1.0/24

      Destination: 10.1.1.50

      • /24 is chosen because it is more specific than /16.
  2. Administrative Distance (AD)
    • If multiple routes have the same prefix length, the router selects the route with the lowest AD.

    • Common AD values:

      Route SourceAD
      Connected0
      Static1
      EIGRP Summary5
      eBGP20
      EIGRP Internal90
      OSPF110
      IS-IS115
      RIP120
      EIGRP External170
      iBGP200
  3. Metric
    • If the routes come from the same routing protocol (same AD), the router chooses the route with the lowest metric.
    • Examples:
      • RIP → Hop Count
      • OSPF → Cost
      • EIGRP → Composite Metric
      • IS-IS → Cost

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