In the realm of routing decisions within Cisco IOS, understanding how administrative distance influences route selection is fundamental. This internal metric helps routers determine the most trustworthy route when multiple sources provide paths to the same destination.
Administrative distance (AD) is essentially a ranking system. Routes learned via directly connected interfaces have the lowest AD (0), followed by static routes (1), and then dynamic routing protocols like EIGRP, OSPF, and RIP with increasing values.
Consider a scenario where a router learns about network 10.0.0.0/8 via both EIGRP (AD 90) and RIP (AD 120). The router installs the EIGRP-learned route because it has a lower administrative distance. This mechanism ensures that more reliable routing information takes precedence.
However, administrators can manipulate administrative distances to control route preference. For example, adjusting the AD of a static route to 250 makes it less likely to be selected over dynamic routes, useful in backup scenarios or routing policy enforcement.
Understanding these values and how they interact with the routing table allows network engineers to shape behavior more precisely, especially in complex enterprise environments with multiple routing protocols coexisting.
From a troubleshooting standpoint, verifying AD values is a go-to
step when expected routes are missing or incorrect routes appear in the
table. Tools like show ip route and show ip protocols can provide immediate insight.
As networks become increasingly hybrid, understanding administrative distance is vital for maintaining routing consistency and minimizing failover surprises. Whether deploying policy-based routing or preparing for future migrations, AD plays a quiet but critical role in route control.