Loading…

Routing: Metrics and Administrative Distance

When checking a router’s routing table, you will get something like this:

Notice the values between the brackets (Cisco output): they differ. These values are the costs of the routes.

The first value is the administrative distance (AD). It indicates the priority of each routing protocol. The second value is the Metric. It is a value calculated by the routing protocol. In both cases the lowest wins. Firstly, the router chooses by the AD; it there is a tie, the breaker is the metric.

Administrative Distance (AD)

AD values indicate route reliability. The default values are listed below. However, they can be changed.

From the routing table at the beginning of this  post, we see the reliability and preference of the routes in the following order:

  1. Static – AD: 1
  2. eBGP – AD: 20
  3. OSPF – AD: 110
  4. iBGP – AD: 200

Metric

The metric is a value calculated by each routing protocol, and the router uses the metric as a tiebreaker when it has more the one route to the same destination. The protocols have different ways to calculate the metric, OSPF uses bandwidth, RIP used hop count, EIGRP combines delay, load, and reliability. Some costs can be changed as a way to influence the best route selection process.

References

Leave a Reply