1.2. The Routing Decision

Think of routing like a GPS system: given a destination, the router must decide which road to take. Unlike a GPS with one goal (shortest time), network routing must balance multiple factors—and sometimes the "obvious" path isn't the one packets actually take.

💡 First Principle: Routing follows the longest prefix match rule. When multiple routes match a destination, the most specific route (longest subnet mask) wins. Imagine searching for "123 Main Street, Apartment 4B"—you'd prioritize the exact apartment match over just "Main Street" or "that city."

What breaks without this understanding:

• Traffic goes to the wrong destination because a more specific route exists that you forgot about
• Your firewall is bypassed because a system route takes precedence
• VPN traffic leaks to the internet instead of the tunnel

Consider this scenario: you configure a route sending 10.0.0.0/8 through a firewall, but Azure has a system route for 10.1.0.0/16 going directly to a peered VNet. Which wins? The /16—it's more specific. The exam loves these "why isn't traffic flowing through my firewall?" questions.