OECG – Chapter 11


In the context of the OECG, the book has spoken mostly about a single routing protocol at a time. Much of the purpose of this chapter is to highlight the issues that arise when running multiple routing protocols on a router at the same time. That introduces a number of challenges, including how to determine what route to use if a router learns a route from more than one routing protocol.

If a router learns a route from a muliple EIGRP neighbors, the router would determine the best route via a metric computation. But what if the router learns the route not only from an EIGRP neighbor, but also from an OSPF neighbor? The metrics don’t directly compare.

This is where administrative distance (AD) come in. AD values are assigned to route sources, so that in the event of multiple sources for the same route, the router can determine which to place in the routing table. The assigned values are as follows, the lower value being preferred:

  • 0: Connected
  • 1: Static
  • 5: EIGRP summary route
  • 20: eBGP
  • 90: EIGRP internal
  • 100: IGRP
  • 110: OSPF
  • 115: IS-IS
  • 120: RIP
  • 170: EIGRP external
  • 200: iBGP external
  • 255: Unreachable

Using the “distance” command (syntax varies from protocol to protocol), you can change from the default AD’s listed above.

By Ethan Banks

Ethan Banks is a podcaster and writer with a BSCS and 20+ years in enterprise IT. He's operated data centers with a special focus on infrastructure — especially networking. He's been a CNE, MCSE, CEH, CCNA, CCNP, CCSP, and CCIE R&S #20655. He's the co-founder of Packet Pushers Interactive, LLC where he creates content for humans in the hot aisle.