Routing Tables

Routing table helps detemrin which route tenry is the best fit for the network.

Directly connected route - learned by physical connection between two routers.

Static Route - manually configured by administrator

default static route 0.0.0.0 “if you dont know where to go, jsut go here.

Dynamic Route - learned by exchanging information between routers.

split horizon

poision reverse

Routing Basics (N10-009)

Routing Table

• Holds all known routes to destinations.
• Router selects the best match (longest prefix match).
• Routes learned by:
  1. Directly Connected Routes – added automatically when an interface is configured and up.
  2. Static Routes – manually configured by an admin.
     • Example: ip route 192.168.2.0 255.255.255.0 10.0.0.1
     • Default Static Route: 0.0.0.0/0 → “if you don’t know where to send it, send it here.”
  3. Dynamic Routes – learned from other routers via routing protocols.

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Dynamic Routing Concepts

• Split Horizon: Prevents routing loops by disallowing a router to advertise a route back out the interface it learned it on.
• Poison Reverse: Instead of ignoring, router sends route update back with infinite metric (route cost = unreachable) to explicitly warn neighbors not to use that path.
• Both help prevent count-to-infinity loops in distance vector protocols (e.g., RIP).

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✅ Must-Know for Exam

• Directly connected = strongest (preferred first).
• Static = admin-defined, takes priority over dynamic if both exist.
• Default route (0.0.0.0/0) = catch-all.
• Split Horizon + Poison Reverse = loop prevention techniques.

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⚡ Flashcard Drill

• Q: Which route is automatically added when you configure an interface?

  A: Directly connected route.

• Q: What does 0.0.0.0/0 represent?

  A: Default route.

• Q: What prevents a router from advertising a route back the way it came?

  A: Split Horizon.

• Q: What sends back a route with infinite cost to avoid loops?

  A: Poison Reverse.