Administrative distance and metrics help routers determine the best path when multiple routes to the same destination are learned. Administrative distance is a number used to select the preferred route, with lower numbers preferred. Metrics are values used by routing protocols to compare paths, such as hop counts for RIP. Both values help routers resolve issues that can arise when multiple routes are available.

1. Administrative Distance & Metric

2. Administrative distance
• A network can use more than one routing protocol, and
routers on the network can learn about a route from
multiple sources.
• Routers need to find a way to select a better path.
• Administrative distance number is used by routers to find
out which route is better (lower number is better).
• For example, if the same route is learned from RIP and
EIGRP, a Cisco router chooses EIGRP route and stores the
route in the routing table.
• This is because EIGRP routes have, by default, an
administrative distance of 90, while RIP route have a
higher administrative distance of 120.

3. • You can display the administrative distance of all routes
on your router by typing the show ip route command:

4. In the case above, the router has only one route in its routing table
learned from a dynamic routing protocols , an EIGRP route.
The following table lists the administrative distance default values:

5. Metric
• If a router learns two different paths for the same network
from the same routing protocol, it has to decide which route
is better and will be placed in the routing table.
• Metric is a measure used to decide which route is better
(lower number is better).
• Each routing protocol uses its own metric. For example, RIP
uses hop counts as a metric, while OSPF uses cost.

6. The following example explains the way RIP calculates its metric and
why it chooses one path over another.

7. • RIP has been configured on all routers.
• Router 1 has two paths to reach the subnet 10.0.0.0/24. One path is
goes through Router 2, while the other path goes through Router 3
and then Router 4. Because RIP uses the hop count as its metric, the
path through Router 1 will be chosen, because the subnet is only one
router away.
• The other path will have a higher metric of 2, because the subnet is
two routers away.
• TIP – the example above can be used to illustrate a disadvantage of
using RIP as a routing protocol.
• Imagine if the first path through R2 was the 56k modem link, while the
other path is a high speed WAN link.
• Router R1 would still chose the path through R2 as the best route,
because RIP uses only the hop count as its metric.

8. The following table lists what various routing protocols use as a metric: