What is Administrative Distance & Metric Distance?

I just had an email from a student asking, "what is administrative distance and Metric Distance?".  I will give a brief answer here, but if any of you want more detail, please let me know.

When a Cisco router has more than one route to a particular destination and those routes come from different route sources (directly connected, static, and/or dynamic routing protocol(s)), the router ios uses the administrative distance to decide which of those routes to place in the routing table (the best route(s)).

Whereas, the metric value is used when there is more than one way to get to a particular destination and all of the routes to that destination come from a single route source (i.e. RIP dynamic routing protocol).

As we discussed in class, the metric used by the routing protocol will be different, depending on the routing protocol. RIP uses hop count, Cisco’s OSPF uses bandwidth; EIGRP uses bandwidth and delay by default (but can be configured to include load and reliability).

The administrative distance value is set by Cisco and not an actual standard. However, other vendors do use it. The route that has the lower administrative distance will be preferred over the router with the higher administrative distance and will be added to the routing table. The term “trustworthy” is commonly used when defining administrative distance. The lower the administrative distance value, the more “trustworthy” the route.

I hope this helps - Joanne