Distance vector routing (RIP) in Computer Networks - Full Explanation

Introduction

Imagine you want to send a letter to a friend through a network of post offices, but you only know the direction to send it next, not the full path. Distance vector routing solves this by letting each post office share its best directions with neighbors, so messages find the shortest path over time.

Explanation

Basic Mechanism

Each router keeps a table listing the best known distance to every destination and the next hop to get there. Periodically, routers share their tables with their immediate neighbors. When a router receives a neighbor's table, it updates its own if it finds a shorter path.

Routers share distance information with neighbors to gradually learn shortest paths.

Distance Metric

RIP uses hop count as the distance metric, meaning the number of routers a packet must pass through to reach the destination. The path with the fewest hops is preferred. However, RIP limits the maximum hop count to 15 to avoid endless loops.

Hop count measures distance, with a maximum of 15 hops to prevent routing loops.

Periodic Updates

Routers send their routing tables to neighbors every 30 seconds. This regular update helps routers learn about network changes, like new routes or broken links, and adjust their tables accordingly.

Regular updates keep routing information current and help detect changes.

Limitations and Loop Prevention

RIP can suffer from slow convergence and routing loops. To reduce loops, it uses techniques like split horizon, route poisoning, and hold-down timers. These methods prevent routers from advertising routes back to the source or mark unreachable routes clearly.

Special rules help prevent routing loops and speed up network stability.

Real World Analogy

Imagine a group of hikers in a forest who only tell their neighbors how far they are from the camp, not the full path. Each hiker updates their own map based on neighbors' info, slowly learning the shortest way to camp. Sometimes, they mark paths as blocked to avoid wasting time.

Basic Mechanism → Hikers sharing distance info only with nearby hikers to update their maps

Distance Metric → Counting how many hikers you must pass to reach the camp

Periodic Updates → Hikers regularly telling neighbors their current distance to camp

Limitations and Loop Prevention → Marking blocked paths and avoiding telling neighbors about routes that lead back to you

Diagram

Diagram

┌─────────┐       ┌─────────┐       ┌─────────┐
│ RouterA │──────▶│ RouterB │──────▶│ RouterC │
│  (0)    │       │  (1)    │       │  (2)    │
└─────────┘       └─────────┘       └─────────┘
     ▲                 │                 │    
     │                 ▼                 ▼    
┌─────────┐       ┌─────────┐       ┌─────────┐
│ RouterD │◀──────│ RouterE │◀──────│ RouterF │
│  (1)    │       │  (2)    │       │  (3)    │
└─────────┘       └─────────┘       └─────────┘

A simple network of routers showing hop counts and neighbor connections used in distance vector routing.

Key Facts

Distance Vector Routing → A routing method where routers share their routing tables with neighbors to find shortest paths.

Hop Count → The number of routers a packet passes through to reach its destination.

Routing Information Protocol (RIP) → A distance vector routing protocol that uses hop count as its metric and limits hops to 15.

Split Horizon → A technique to prevent routing loops by not sending route info back to the neighbor it came from.

Route Poisoning → Marking a route as unreachable by setting its hop count to 16 in RIP.

Common Confusions

Believing RIP instantly knows the best path to all destinations.

Believing RIP instantly knows the best path to all destinations. RIP learns routes gradually through periodic updates from neighbors, so it takes time to find the shortest paths.

Thinking hop count measures physical distance or speed.

Thinking hop count measures physical distance or speed. Hop count only counts the number of routers passed, not actual distance or link speed.

Assuming RIP can handle very large networks well.

Assuming RIP can handle very large networks well. RIP is limited to 15 hops, so it is not suitable for large networks with longer paths.

Summary

Distance vector routing lets routers share their best path info with neighbors to find shortest routes over time.

RIP uses hop count as a simple distance measure and limits it to 15 to avoid loops.

Techniques like split horizon and route poisoning help prevent routing loops and improve stability.

Practice

1. What is the main metric used by Distance Vector Routing Protocol (RIP) to determine the best path?

easy

A. Hop count

B. Bandwidth

C. Delay

D. Cost

Solution

1. Step 1: Understand RIP metric

   RIP uses hop count as the metric to measure the distance to a destination network.

2. Step 2: Compare with other metrics

   Other protocols may use bandwidth, delay, or cost, but RIP specifically uses hop count.

3. Final Answer:

   Hop count -> Option A

4. Quick Check:

   RIP metric = Hop count [OK]

Hint: RIP counts hops, not speed or delay [OK]

Common Mistakes:

• Confusing hop count with bandwidth
• Thinking RIP uses delay as metric
• Assuming cost is used in RIP

2. Which of the following is the correct way RIP routers share their routing information?

easy

A. Routers send their entire routing table to neighbors periodically

B. Routers send only changed routes immediately

C. Routers broadcast only the best route to each destination

D. Routers send routing updates only when a link fails

Solution

1. Step 1: Recall RIP update method

   RIP routers send their full routing tables to all neighbors at regular intervals.

2. Step 2: Contrast with other methods

   Other protocols may send only changes or triggered updates, but RIP sends full tables periodically.

3. Final Answer:

   Routers send their entire routing table to neighbors periodically -> Option A

4. Quick Check:

   RIP sends full tables periodically [OK]

Hint: RIP sends full tables regularly, not just changes [OK]

Common Mistakes:

• Thinking RIP sends only changed routes
• Assuming RIP uses triggered updates only
• Believing RIP sends partial route info

3. Consider a RIP network where Router A has routes: Network X (2 hops), Network Y (3 hops). Router B receives this info and has Network X (1 hop), Network Y (4 hops). After update, what is Router B's hop count to Network Y?

medium

A. 3 hops

B. 2 hops

C. 5 hops

D. 4 hops

Solution

1. Step 1: Understand RIP update rule

   Router B updates routes by adding 1 hop to neighbors' hop counts.

2. Step 2: Calculate new hop count for Network Y

   Router A reports 3 hops to Network Y, so Router B's new cost is 3 + 1 = 4 hops. But Router B already has 4 hops, so it keeps 4 hops.

3. Step 3: Check if better route exists

   Router B's own route is 4 hops, neighbor's route plus 1 is also 4 hops, so it keeps 4 hops.

4. Final Answer:

   4 hops -> Option D

5. Quick Check:

   Neighbor hops + 1 = 4 hops [OK]

Hint: Add 1 hop to neighbor's count, pick smallest [OK]

Common Mistakes:

• Forgetting to add 1 hop to neighbor's count
• Choosing neighbor's hop count without adding 1
• Ignoring existing route cost

4. A RIP router is not updating its routing table correctly and keeps old routes indefinitely. What is the most likely cause?

medium

A. The router has a maximum hop count set too high

B. The router is using link-state routing instead of distance vector

C. The router is not receiving periodic updates from neighbors

D. The router is sending triggered updates too frequently

Solution

1. Step 1: Identify update mechanism in RIP

   RIP relies on periodic updates from neighbors to refresh routing tables.

2. Step 2: Analyze symptom

   If the router keeps old routes indefinitely, it likely isn't receiving these updates.

3. Final Answer:

   The router is not receiving periodic updates from neighbors -> Option C

4. Quick Check:

   No updates received = stale routes [OK]

Hint: No updates received means routes never refresh [OK]

Common Mistakes:

• Confusing routing protocol type
• Thinking hop count setting causes stale routes
• Assuming triggered updates cause stale routes

5. In a network using RIP, a router receives two routes to the same destination: one with 3 hops via Router X and another with 3 hops via Router Y. Which route will RIP choose and why?

hard

A. The route learned first, because RIP prefers the earliest route

B. Either route, because RIP considers equal hop counts as equal cost

C. The route with the lowest IP address of the next hop router

D. The route with the highest bandwidth link

Solution

1. Step 1: Understand RIP route selection

   RIP selects routes based on hop count; if multiple routes have the same hop count, they are considered equal cost.

2. Step 2: Analyze tie-breaking in RIP

   RIP can load balance across equal-cost routes or pick any; it does not prefer based on IP or bandwidth.

3. Final Answer:

   Either route, because RIP considers equal hop counts as equal cost -> Option B

4. Quick Check:

   Equal hops = equal cost, any route chosen [OK]

Hint: Equal hops mean equal cost; RIP can use either route [OK]

Common Mistakes:

• Assuming RIP prefers first learned route
• Thinking RIP uses next hop IP to decide
• Believing RIP considers bandwidth in route choice