Understand the significance of administrative distance and metrics

Administrative distance and metrics are two important factors when it comes to which routing protocols.

Being knowledgeable about these two Values can make all the difference in network performance, reliability, and path selection.

If you haven’t seen or heard about these two value and you have completed the course of CCNA means that you have missed something important in the class.

Let me give you an example.

When you enter the command “Show ip route” you will see the following output. Look at the two values in the bracket.

R    10.0.0.0/8 [120/1] via 1.1.1.1, 00:00:15, FastEthernet0/0

Here, 120 represent the Ad Value and 1 represents the metric. The R at the starting represents the route learned via RIP. So the Ad value of RIP is 120 and the metric it took to reach the network is 1.

You can also get the more details about the specific network by using the command show ip route followed by the network you want to know more.

Here’s an example,

***************************************************************************************************************************************************

R2#sh ip route 10.0.0.0

Routing entry for 10.0.0.0/8

  Known via "rip", distance 120, metric 1

  Redistributing via rip

  Last update from 1.1.1.1 on FastEthernet0/0, 00:00:09 ago

  Routing Descriptor Blocks:

  * 1.1.1.1, from 1.1.1.1, 00:00:09 ago, via FastEthernet0/0

      Route metric is 1, traffic share count is 1

******************************************************************************************************************************************************

ADMINISTRATIVE DISTANCE:  It is value used by Cisco routers to select the best path between source and destination when multiple protocols are used.

E.g. An OSPF route with an administrative distance of 110 will be chosen over a RIP route with an administrative distance of 120.

Protocol	Administrative Distance
Directly Connected Route	0
Static route out of an interface	1*
Static route to next-hop ip address	1
EIGRP summary Route	5
External BGP	20
Internal EIGRP	90
IGRP	100
OSPF	110
IS-IS	115
RIP	120
EGP	140
ODR	160
External EIGRP	170
Internal BGP	200
DHCP- learned	254
Unknown	255

·         A route with the value of AD 255 will not be installed in routing table because the router will not trust that route.

·         Since IOS 12.2, the administrative distance of a static route with an exit interface is 1. Prior to the release of 12.2 it was in fact 0.

* Actual administrative distance is recognized somewhere between 0 and 1. This AD is more trustworthy than 1 but less trustworthy than 0.

Metric:

It is the method or formula used by a protocol to find the best path between source and destination, when more than one path is available for the same destination.

So, a routing protocol uses its metric to find the best path to reach the destination when more than one path is available. The lower the metric the more preferred path that will be installed in the routing table.

Unlike Administrative distance metric is involved in with only one routing protocol. Every routing protocol has its own metric to find the best path.

e.g.  Look at the output of the command of show ip eigrp topology

P 10.55.102.0/24, 1 successors, FD is 6049256
        via 10.220.100.11 (6049256/5537506), Serial3/0
        via 10.55.100.114 (52825700/281610), Serial2/0

In the above output EIGRP has two routes to reach the net 10.55.102.0/24. But only with the route with lower metric will be installed in the routing table.

So the first route i.e. via 10.220.100.11 will be installed in the routing table as it has lower metric. The values in the bracket in the above output says, the first part is the Feasible distance (FD) to the destination and the second part is the advertise distance to the destination (AD). The output in the brackets means (FD/AD).

Different routing protocols calculate their metric in different ways. RIP uses hops, OSPF uses bandwidth, and EIGRP uses a combination of bandwidth, delay, load, and reliability.

For more details mail me at arahim.cisco@gmail.com