Administrative Distance, Metric and Next hop

[huyan]

Hi,

I am confused with the 3 terms : Administrative Distance, Metric and Next hop.

As I searched online:

-AD is what Router check when it decide which is the best route to put in the routing table.

-Metric is when there are more than 2 routes with the same routing protocol and the router needs to decide the best route, it will base on the metric.

-Next hop is the closet router that a packet can go through.

That's what I can find , please correct if I am wrong.

The problem that makes me scratch my head is this, there is a question in 100-105 exam, it's like this:

Which of the following used to identify immediate destination?
A. Administrative distance
B. Metric
C. Next hop
D. Destination network

Answer: C

I don't understand why it's C. I chose B.

Please help me.

Thank you.

[Philip D'Ath]

The "immediate destination" is not really dependent on how the route is selected (such as AD or metric) but ultimately where it is next sent to.

[huyan]

I see, thank you very much for your help!

[Richard Burts · Accepted solution]

The concepts of Administrative Distance and Metric can be confusing to people who are learning about networking and I would offer this explanation in hope that it will be helpful.

Administrative Distance is a way to evaluate information about a route based on the source of the information. We might learn a route to a destination from a static route, from a connected subnet, directly from a routing protocol, or redistributed from a routing protocol. Administrative Distance assigns a value for each of these sources which establishes an order of preference. A connected subnet is most preferred, and generally a static route is the next preference. Among the routing protocols an EIGRP native route is preferred over an OSPF route which is preferred over an EIGRP redistributed route.

Metric is a way to evaluate information about a route based on some type of measurement of the path to the destination. Each routing protocol has its own way to calculate a metric for a route

So if the routing logic has learned two routes toward a destination it needs to know which one to use. The first thing it does is to compare AD. If one route was learned as a connected subnet and the other route was learned from OSPF then there is a difference in AD and that determines which route will be used (and metric does not come into play). If both routes have the same AD then the routing logic will compare metric and the route with the better metric will be chosen.

HTH

Rick 

[Richard Burts]

I am glad that my explanation was helpful. Thank you for marking this discussion as solved. This will help other readers in the forum to identify discussions that have helpful information.

HTH

Rick