Routing Table

• The Routing Table identifies IP routes to networks
  • The most specific route (e.g., the route with the longest prefix) is the route that is chosen for any given traffic
• Routes can be Static route by the administrator, or dynamically added through routing protocols like RIP, EIGRP, or OSPF
• How Routes are selected, from creation to packet forwarding
  • Dynamic Routing Protocols choose routes to advertise based on Metric
    • The Metric process is different for each protocol
      • OSPF uses cost, and calculates reference bandwidth/interface bandwidth
      • RIP does a simple hop count, from 0-15
      • EIGRP boils down to slowest link + delay of all links combined
  • The router collects all routes that are advertised to it and decides which ones will appear on the table based on Administrative Distance
    • Administrative Distance is basically how much the router trusts the route, from 0-255, with lower being more trustworthy
      • All Unique routes from different sources are added to the table
      • If there are multiple identical routes, then the one with the lowest AD is added
        • Example: Two Three routes are solicited from different protocols
          • Routes:
            • An EIGRP route (AD 90) to 172.31.0.0/16, on Fa02
            • An OSPF route (AD 110) to 172.31.0.0/16, on Fa03
            • An OSPF route (AD 110) to 172.30.16.0/20, on Fa01
            • An EIGRP route (AD 90) to 172.30.0.0/16, on Fa04
          • The EIGRP route on Fa02 will be selected for the table instead of the OSPF route on FA03
          • The OSPF route on Fa01 AND the EIGRP route on Fa04 will be added to the table, because they are unique
        • Floating Static Routes are identical routes that are manually configured with a slightly higher AD than what's been added to the table, so if the existing route fails, the floating static route will fail-over
    • AD does not impact route selection, just whether it is added to the table
  • The router receives a packet and sends the packet on the route with the most specific (longest prefix) matching network
    • Example: A router receives a packet for 172.31.20.232, and there are four routes to pick from
      • Routes:
        • A static route (AD 1) to 172.16.0.0/8, on Fa01
        • An EIGRP route (AD 90) to 172.31.0.0/16, on Fa02
        • An OSPF route (AD 110) to 172.31.16.0/20, on Fa03
        • An Internal BGP route (AD 170) to 172.31.20.0/24, on Fa04
      • The router will send the packet out of interface Fa04 because that route has the longest prefix.
  • If there is no matching network on the table, the packet be forwarded out of the Gateway of last resort
    • This the catch-all network, 0.0.0.0 0.0.0.0, and the router that sits between the LAN and the WAN

Configure a Static route

config# ip route <destination> <subnet mask> <next hop> <opt: AD>
config# ipv6 route <destination network and mask> <opt: exit interface> <opt: next hop address> <opt: AD>

Routing Metrics and AD

Practice Tables

Metadata

OSI or TCP/IP Layer

CCNA Exam Topic

#extop-3-1 #extop-3-2 #extop-3-3

Contributors

Sources