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Explain administrative distance and how it affects route selection.

Administrative Distance (AD) is a concept in networking — specifically in routing protocols — that determines the trustworthiness or priority of a route when multiple routing sources offer paths to the same destination.

Explain administrative distance and how it affects route selection.

🧠 What Is Administrative Distance?

It’s a numerical value (from 0 to 255) that routers use to rank routes from different sources.

  • Lower AD = Higher Trust

  • If multiple routes exist to the same destination, the router chooses the one with the lowest AD.

📊 Common Administrative Distance Values (Cisco Defaults)

Route SourceAD Value
Directly connected0
Static route1
EIGRP (internal)90
OSPF110
RIP120
External EIGRP170
Unknown / Untrusted255 (never used)

📦 Example Scenario

Let’s say a router learns about the same destination network 10.0.0.0/24 from:

  • OSPF (AD 110)

  • EIGRP (AD 90)

Even if the OSPF route has a shorter metric (e.g., fewer hops), the router will prefer the EIGRP route because it has a lower administrative distance.

🎯 Why It Matters

  • Prevents routing conflicts: Helps routers resolve which route to install in the routing table.

  • Useful in fallback scenarios: You can configure a backup static route with a higher AD so it only takes over if the preferred route fails.

  • Ensures stability: Avoids route flapping between protocols of different trust levels.

🔧 Can You Change AD?

Yes. Most routers allow you to manually adjust the administrative distance:

  • To influence route selection.

  • To prefer one protocol over another even if it’s not the default.

  • For policy-based routing, traffic engineering, or backup routing.

✅ Summary

TermDefinition
Administrative DistanceA numeric value representing the trust level of a routing source.
Lower ADMeans more trusted, gets installed in the routing table over higher AD routes.
PurposeHelps the router choose the best route when multiple protocols suggest routes to the same destination.

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