Administrative Distance

Unanswered Question
May 29th, 2007

It has recently become unclear to me as to when AD is used for path determination. Care to help me solidify this concept? Read on...

Suppose we have RouterA which is only running EIGRP. It receives two routes: route1 and route2. Route1 is received as an internal EIGRP route (AD of 90) with a cost of 100 . Route2 is received as an external EIGRP route (AD 170) with a cost 10. Which of these routes will the routing engine install into the routing table and why?

Thank you in advance! :)

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jeffbriones Tue, 05/29/2007 - 21:23

Hi,

If you see EIGRP having AD of 170, its External EIGRP, it was redistributed. You can use show ip route eigrp to determine which routes are installed on your RT.

HTH

jeff

pavansuraj Tue, 05/29/2007 - 21:26

Hi Jermy,

The router always follows the below order of priority in installing a route in its routing table

1. The subnet with longest mask(the most specific subnet. eg. 192.168.1.0/25 is preffered to 192.168.1.0/24)

2. Administrative distance

3. Metrics

So Route 1 is installed in the routing table as it has less AD of 90 irrespective of the cost.

HTH, Please rate if it does

Pavan

royalblues Tue, 05/29/2007 - 21:43

Pawan is correct

Route1 would be installed in the routing table in your case

Narayan

jdevoll Wed, 05/30/2007 - 06:14

Thank you Pav, your answer was exactly what I needed. All of the books I've found seem to indicate that AD is only used on a router running multiple routing protocols. It appears that the use of AD is not limited to such a narrow scope.

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