Showing posts with label Administrative Distances. Show all posts
Showing posts with label Administrative Distances. Show all posts

Thursday, 29 June 2023

What is Administrative Distance? How to change AD value of OSPF, EIGRP, RIPv2 and BGP?

 

 

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.

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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. 

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