Table of Contents
What is Administrative Distance?
Administrative Distance (abbreviation for AD) is a value that routers/Layer 3 switches use in order to select the best path when there are more than 1 routes to the same destination from two different routing protocols. Administrative Distance (AD) is a numeric value which can range from 0 to 255 and is referred as standard for route preference across vendors.
Default Administrative Distance for Routing Protocols
Below table gives the default Administrative Distance for all the Routing protocols:
| ROUTE TYPE | ADMINISTRATIVE DISTANCE | ROUTING TABLE ENTRY |
|---|---|---|
| Connected | 0 | C |
| Static Route out an interface | 0 | S |
| Static Route to Next-Hop IP | 1 | S |
| EIGRP Summary route | 5 | D |
| External BGP | 20 | B |
| Internal EIGRP | 90 | D |
| IGRP | 100 | I |
| OSPF | 110 | O |
| IS-IS | 115 | i |
| RIP v1 & v2 | 120 | R |
| Exterior Gateway Protocol (EGP) | 140 | E |
| ODR | 160 | O |
| External EIGRP | 170 | D EX |
| Internal BGP | 200 | B |
| Unknown | 255 |
Related FAQs
Q.1 Why is Administrative Distance important?
AD prevents routing conflicts when multiple routing protocols (like OSPF and EIGRP) advertise the same network.
It ensures the router selects the most reliable route source. Without AD, routers would struggle to prioritize routes from different protocols.
Q.2 How is Administrative Distance different from Metric?
Administrative Distance selects the best routing source. Whereas, Metric selects the best path within the same routing protocol. For example:
AD chooses between OSPF and RIP. And OSPF metric (cost) chooses the best OSPF path.
Q.3 Can Administrative Distance be changed?
Yes, AD can be changed on Cisco routers. You can manually adjust AD to influence route selection.
Router(config)# ip route 10.0.0.0 255.255.255.0 192.168.1.1 200
This sets the AD of the static route to 200.
Q.4 What happens if two routes have the same Administrative Distance?
If 2 routers have the same AD, the router compares the metric. In case, if both AD and metric are equal, the router may perform equal-cost load balancing (depending on configuration).
Q.5 Does Administrative Distance affect routers from different vendors?
Yes, but default values differ. For example: Juniper Networks uses the term Preference instead of Administrative Distance. While, Huawei Technologies and other vendors may assign different default values. Always verify vendor-specific documentation.
Watch Related Video
Watch the below given video for better understanding of Administrative distance:
ABOUT THE AUTHOR
You can learn more about her on her linkedin profile – Rashmi Bhardwaj
