Routing Concepts

• Router


  A network device that is used to move packets between logical networks
• Primary functions of a router

  • Determine the best path to forward packets based on the information in its routing table
  • Forward packets toward their destination
• Other beneficial functions of routers

  • Broadcast containment by limiting their propagation within the local network
  • Interconnect geographically separated locations
  • Group users logically by application or department within an organization
  • Provide enhanced security by filtering unwanted traffic through access control lists
• Routing table


  A data structure stored in RAM to track the list of known networks and how to reach them
• Contents of the routing table

  • Directly Connected Networks
  • Remote Networks
• Commands to display the routing table


  1. show ip route

  2. show ipv6 route
• Routing table entry contents

  • Route source
  • Destination network (prefix and prefix length)
  • Administrative distance
  • Metric
  • Next-hop
  • Route timestamp
  • Exit interface
• Next hop


  The IP address of the next router to which the packet would be forwarded
• Exit interface


  The egress interface to use for outgoing packets to reach their final destination
• Directly connected networks

  Networks that a router is a member of through one of its interfaces
• Remote networks


  Networks that a router must use a neighboring router to reach
• Route source codes


  • L - local
  • C - connected
  • S - static
  • O/R/D, etc - dynamically learned
  • * - candidate for default route
• Directly connected routes

  Automatically added when an interface is configured with an IP address and is activated
• Local route


  A route for each directly connected network, used by the router to efficiently differentiate when it receives a packet for its own interface instead of a packet that needs to be forwarded
• Static routes


  Manually configured paths between two networking devices
• Dynamic routes


  Routes learned automatically from other routers using routing protocols
• Default route


  Specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address
• Route metric


  A quantitative representation of the distance to the destination network, automatically calculated by the router based on the rules of the routing protocol
• Administrative distance (AD)


  Represents the "trustworthiness" of the route source, used to determine which route to install into the IP routing table when multiple routes to the same destination are learned
• Administrative distance values for different route sources

  • Directly connected - 0
  • Static route - 1
  • EIGRP summary route - 5
  • External BGP - 20
  • Internal EIGRP - 90
  • OSPF - 110
  • IS-IS - 115
  • RIP - 120
  • External EIGRP - 170
  • Internal BGP - 200
• When choosing among several route options to a destination, a router prioritizes the lowest AD followed by the lowest metric
• Administrative Distance (AD)


  Represents the "trustworthiness" of the route source. Lower AD = more trustworthy route source.
• Route Source


  • Directly connected
  • Static route
  • EIGRP summary route
  • External BGP
  • Internal EIGRP
  • OSPF
  • IS-IS
  • RIP
  • External EIGRP
  • Internal BGP
• Metric


  Used to choose among several route options to a destination, a router prioritizes the lowest AD followed by the lowest metric
• Every router makes its decision alone, based on the information it has in its own routing table
• The information in a routing table of one router does not necessarily match the routing table of another router
• Routing information about a path does not provide return routing information
• Child route


  A destination network that is a subnet of a classful network
• Parent route


  A classful network appearing above a child route, indicating which classful network the child route was subnetted from
• The concept of network classes was never part of IPv6, so the structure of an IPv6 routing table is very straight forward
• Static routing


  Manually configured by a network administrator to define an explicit path between two networking devices, not automatically updated
• Dynamic routing


  Uses routing protocols so that routers automatically share information among themselves about the reachability and status of remote networks
• Routing protocol


  Used to exchange routing information and populate the routing table with the choice of best paths
• Prefix


  Network address
• Prefix length


  Subnet mask
• To determine if a route is suitable for forwarding, a packet destination must match the route prefix by a number of far-left bits that is equal to or more than the prefix length
• When the routing table contains several routes that fulfill the prefix match requirement for a packet, the preferred route is the one with the longest matched prefix to the packet destination
• Prefix


  A number of far-left bits that is equal to or more than the prefix length
• Longest Match


  When the routing table contains several routes that fulfill the prefix match requirement for a packet, the preferred route is the one with the longest matched prefix to the packet destination
• Route Entry 3


  • 172.16.25.64/26