Link state routing (OSPF) in Computer Networks - Mini Project: Build & Apply

Understanding Link State Routing with OSPF

📖 Scenario: You are a network engineer learning how routers use the OSPF (Open Shortest Path First) protocol to find the best path in a network. OSPF uses link state routing, where each router knows the state of its links and shares this information with others.Imagine a small network of routers connected by links with different costs. Your task is to understand how OSPF builds a map of the network and calculates the shortest path.

🎯 Goal: Build a simple representation of a network's link states, configure a cost threshold, apply the OSPF link state routing logic to find reachable routers within the cost limit, and finalize the routing table.

📋 What You'll Learn

Create a dictionary representing routers and their link costs to neighbors

Add a cost threshold variable to limit path costs

Implement logic to find routers reachable within the cost threshold using link state routing principles

Complete the routing table with the final reachable routers and their costs

💡 Why This Matters

🌍 Real World

Network engineers use OSPF to efficiently route data in large networks by sharing link state information and calculating shortest paths.

💼 Career

Understanding OSPF and link state routing is essential for roles in network administration, network engineering, and IT infrastructure management.

1

Create the network link state data

Create a dictionary called link_states that represents the network. Use these exact entries: 'R1': {'R2': 10, 'R3': 20}, 'R2': {'R1': 10, 'R4': 15}, 'R3': {'R1': 20, 'R4': 30}, 'R4': {'R2': 15, 'R3': 30}.

Computer Networks

# Create the link_states dictionary with the given router connections and costs
# Your code here

Need a hint?

Think of link_states as a map where each router lists its neighbors and the cost to reach them.

2

Set the cost threshold for reachable routers

Create a variable called cost_threshold and set it to 25. This will limit the maximum cost to reach other routers.

Computer Networks

link_states = {
    'R1': {'R2': 10, 'R3': 20},
    'R2': {'R1': 10, 'R4': 15},
    'R3': {'R1': 20, 'R4': 30},
    'R4': {'R2': 15, 'R3': 30}
}
# Set the cost_threshold variable to 25
# Your code here

Need a hint?

The cost_threshold helps decide which routers are reachable within a certain cost limit.

3

Find reachable routers within the cost threshold

Create a dictionary called reachable_routers. Use a for loop with variables neighbor and cost to iterate over link_states['R1'].items(). Add neighbors to reachable_routers only if their cost is less than or equal to cost_threshold.

Computer Networks

link_states = {
    'R1': {'R2': 10, 'R3': 20},
    'R2': {'R1': 10, 'R4': 15},
    'R3': {'R1': 20, 'R4': 30},
    'R4': {'R2': 15, 'R3': 30}
}
cost_threshold = 25
# Create reachable_routers dictionary and add neighbors within cost_threshold
# Your code here

Need a hint?

Use a loop to check each neighbor's cost and add only those within the threshold to reachable_routers.

4

Complete the routing table with reachable routers

Create a dictionary called routing_table and set it equal to reachable_routers. Then add an entry for 'R1' with cost 0 to represent the current router.

Computer Networks

link_states = {
    'R1': {'R2': 10, 'R3': 20},
    'R2': {'R1': 10, 'R4': 15},
    'R3': {'R1': 20, 'R4': 30},
    'R4': {'R2': 15, 'R3': 30}
}
cost_threshold = 25
reachable_routers = {}
for neighbor, cost in link_states['R1'].items():
    if cost <= cost_threshold:
        reachable_routers[neighbor] = cost
# Create routing_table from reachable_routers and add 'R1' with cost 0
# Your code here

Need a hint?

The routing table shows the cost to reach each router including the current one with cost zero.