Routing - Administrative Distance

Administrative Distance, or AD is a feature that routers use to determine which is the best path to install in the routing table when routing information to a particular destination is made available from multiple routing protocols.

For example, if a router is running both EIGRP and RIP simultaneously, and the router receives advertisements for a route to 1.1.1.0/24 from both routing protocols, which route will be installed in the routing table?

The answer is, the advertisement from the routing protocol that has a lower AD.

The default ADs used by Cisco IOS routing devices are as follows:

• 0 Connected interface
• 1 Static route
• 5 Enhanced Interior Gateway Routing Protocol (EIGRP) summary route
• 20 External Border Gateway Protocol (BGP)
• 90 Internal EIGRP
• 100 IGRP
• 110 Open Shortest Path First (OSPF)
• 115 Intermediate System-to-Intermediate System (IS-IS)
• 120 Routing Information Protocol (RIP)
• 140 Exterior Gateway Protocol (EGP)
• 160 On Demand Routing (ODR)
• 170 External EIGRP
• 200 Internal BGP
• 255 Unknown

If the AD is 255, this means that the router does not consider the source of that route to be reliable, and does not install it in the routing table.

Links:

https://networklessons.com/cisco/ccna-routing-switching-icnd1-100-105/introduction-to-administrative-distance

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/15986-admin-distance.html