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OECG – Chapter 11

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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.

About the author

Ethan Banks

Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.

Ethan Banks On productivity.

Most people know me because I write & podcast about IT on the Packet Pushers network. I also co-authored "Computer Networks Problems & Solutions" with Russ White.

Find out more on my about page.