Administrative Distances
The administrative distance is used to rate the accuracy of
routing protocol information received on a router from a neighbor router. An administrative
distance is an integer value ranging from 0 to 255, where 0 means most trusted
and 255 untrusted no traffic will be passed through this route or this route
will never be installed in the routing table.
What will happen if our router receives two updates for the same remote network?
The first task our route will do is to check the administrative
distance (AD). If one of the two updates has a lower AD than the other route. our router will install a lower AD route in the
routing table.
Now again, what will happen if both the advertise updates have the same AD for the same remote network?
In this case, our router will find the best path for the remote
network by comparing routing protocol metrics like hop count or the bandwidth
of the lines depending on the routing protocol. The lowest metric will win and
our router will install the route in the routing table.
Now one more time again, what if both advertise updates (route) have the same AD, and the same metric which route a router will install in its routing table?
The answer is router will do load balances to the remote
network.
The administrative distance default values
|
Route source (protocol) |
Default Administrative
distance (AD) |
|
Connected interface |
0 |
|
Static route |
1 |
|
Enhanced Interior Gateway
Routing Protocol (EIGRP) summary route |
5 |
|
External Border Gateway Protocol
(BGP) |
20 |
|
Internal EIGRP |
90 |
|
IGRP |
100 |
|
Open Shortest Path First OSPF
OSPF |
110 |
|
Intermediate
System-to-Intermediate System (IS-IS) |
115 |
|
Routing Information Protocol
(RIP) |
120 |
|
Exterior Gateway Protocol (EGP)
|
140 |
|
On-Demand Routing (ODR)
|
160 |
|
External EIGRP
|
170 |
|
Internal BGP
|
200 |
|
Unknown*
|
255 |
if you want to change the default administrative distance value yes you can change it, so let's configure the experiment here we are going to advertise the same remote network with 5 different protocols. let's see the router will install which route and then we will change the default AD and see the router action.
(If you are familiar with Internetworks blogs you know we come straight to the point and like LABS, I request you please visit our YouTube program and click here thank you)
Topology: -
- configure the topology as per the diagram
- assign the IP addresses to their ports as per the topology
- configure a static route between routers 1-2-7
- configure EIGRP routing between routers 1-3-7
- configure OSPF routing between routers 1-4-7
- configure RIPv2 routing between routers 1-5-7
- configure IBGP routing between routers 1-6-7
- advertise directly connected network
- now you will see all the routers (R2, R3, R4, R5 and R6) advertising the same network 20.1.1.0 with different administrative distance
- now ping from PC-1 to PC-2 and see which path router-1 will choose in order to reach the 20.0.0.0 network.
