Distance vector routing (RIP) in Computer Networks - Step-by-Step Execution

Concept Flow - Distance vector routing (RIP)

Start: Each router knows distance to neighbors

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Send distance vector to neighbors

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Receive distance vectors from neighbors

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Update own routing table if shorter path found

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Repeat periodically or on change

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Converged

Routers share their distance info with neighbors, update routes if better paths found, and repeat until all have best paths.

Execution Sample

Computer Networks

Router A distance vector: {B:1, C:4}
Router B distance vector: {A:1, C:2}
Router C distance vector: {A:4, B:2}

Router A receives B's vector and updates routes

Shows routers exchanging distance info and updating routing tables based on neighbors' data.

Analysis Table

Step	Router	Received From	Distance Vector Received	Routing Table Before	Routing Table After	Update Made?
1	A	B	{A:1, C:2}	{B:1, C:4}	{B:1, C:3}	Yes (C updated from 4 to 3 via B)
2	B	A	{B:1, C:4}	{A:1, C:2}	{A:1, C:2}	No (no shorter path)
3	C	B	{A:1, C:2}	{A:4, B:2}	{A:3, B:2}	Yes (A updated from 4 to 3 via B)
4	A	C	{A:4, B:2}	{B:1, C:3}	{B:1, C:3}	No (no shorter path)
5	B	C	{A:4, B:2}	{A:1, C:2}	{A:1, C:2}	No (no shorter path)
6	C	A	{B:1, C:4}	{A:3, B:2}	{A:3, B:2}	No (no shorter path)
7	-	-	-	-	-	Converged: no updates in last exchanges

💡 Routing tables converge when no updates occur after exchanges.

State Tracker

Router	Start	After Step 1	After Step 3	After Step 6	Final
A Routing Table	{B:1, C:4}	{B:1, C:3}	{B:1, C:3}	{B:1, C:3}	{B:1, C:3}
B Routing Table	{A:1, C:2}	{A:1, C:2}	{A:1, C:2}	{A:1, C:2}	{A:1, C:2}
C Routing Table	{A:4, B:2}	{A:4, B:2}	{A:3, B:2}	{A:3, B:2}	{A:3, B:2}

Key Insights - 3 Insights

Why does Router A update its distance to C from 4 to 3 after receiving B's vector?

Because Router B reports a distance to C as 2, and Router A knows distance to B is 1, so total 1+2=3 is shorter than 4. See execution_table step 1.

Why does Router B not update its routing table after receiving A's vector?

Router B already has the shortest distances to neighbors, so no shorter path is found. See execution_table step 2 where no update is made.

What causes the routing tables to stop updating?

When no router finds a shorter path after exchanging vectors, the tables converge and updates stop. See execution_table step 7.

Visual Quiz - 3 Questions

Look at the execution_table at step 1, what is Router A's distance to C after update?

A 4

B 2

C 3

D 1

💡 Hint

Check the 'Routing Table After' column for Router A at step 1.

At which step does Router C update its distance to A?

A Step 1

B Step 3

C Step 5

D Step 6

💡 Hint

Look for Router C's updates in the 'Update Made?' column in execution_table.

If Router B's distance to C changed from 2 to 1, how would Router A's distance to C change after step 1?

A Change to 2

B Stay 3

C Change to 1

D Change to 4

💡 Hint

Calculate new distance as distance to B (1) + B's distance to C (1) = 2, check execution_table step 1 logic.

Concept Snapshot

Distance Vector Routing (RIP):
- Each router shares its distance vector with neighbors.
- Routers update routes if a shorter path is found via neighbors.
- Updates repeat periodically until no changes occur (convergence).
- Uses hop count as distance metric, max 15 hops.
- Simple, but can be slow to converge in large networks.

Full Transcript

Distance vector routing works by routers sharing their known distances to all destinations with their neighbors. Each router starts knowing only the distance to its immediate neighbors. When a router receives distance vectors from neighbors, it checks if it can reach any destination with fewer hops by going through that neighbor. If yes, it updates its routing table with the shorter path. This process repeats periodically or when changes occur, until all routers have the shortest paths and no updates happen anymore, called convergence. For example, Router A initially thinks C is 4 hops away, but after hearing from Router B that B can reach C in 2 hops, and since A is 1 hop from B, A updates its distance to C to 3 hops. This exchange continues until no router can improve its routes. RIP uses hop count as the distance metric and limits max hops to 15 to avoid routing loops.

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