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📘 12-03: Routing

Network Systems

Module 12: Wide Area Networks

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🧭 Static vs. Dynamic Routing

Static Dynamic
Configured Manually Protocols share routes
Adapts to change No Yes (auto-reroutes)
Overhead None CPU/bandwidth
Best for Small/stub networks, default route Large/changing networks

A default route (0.0.0.0/0) is the "gateway of last resort" for destinations not in the table.

🧭 Dynamic Routing Protocols

Protocol Type Metric Scope
RIP Distance-vector Hop count (max 15) Small
OSPF Link-state Cost (bandwidth) Enterprise IGP
EIGRP Advanced DV (Cisco) Bandwidth + delay Enterprise IGP
BGP Path-vector Path attributes Between autonomous systems (Internet)
  • IGP (RIP/OSPF/EIGRP) = within one organization.
  • EGP (BGP) = between organizations — the routing protocol of the Internet.

📌 Administrative Distance (AD)

When multiple sources offer a route, the router prefers the lowest AD (most trusted):

Source AD
Connected 0
Static 1
EIGRP 90
OSPF 110
RIP 120

📌 Convergence

Convergence is when all routers agree on the topology after a change. Faster convergence (OSPF/EIGRP > RIP) means less disruption during failures.

🧭 Scenario: Which Route Wins?

Routers choose routes by using a consistent order:

  1. Longest prefix match wins first. A /24 route beats a /16 route for a matching destination.
  2. If prefix length ties, the route with the lowest administrative distance is preferred.
  3. If AD ties, the protocol metric decides, such as OSPF cost or RIP hop count.

Example: a router has 10.10.0.0/16 via OSPF and 10.10.5.0/24 as a static route. Traffic to 10.10.5.25 uses the /24 static route because it is more specific, even before comparing AD.

This is why default routes (0.0.0.0/0) are used only when no more-specific route exists.

Tip

Key idea — Static for small/stub; dynamic for scale. OSPF/EIGRP = interior; BGP = between autonomous systems (Internet). Ties are broken by administrative distance (lower = preferred).

See also: WAN Fundamentals, Subnetting Basics