Example of Distance Vector Routing 1 - Georgia Tech - Network Implementation

Summary

The script explains the concept of distance vector routing in a three-node network, illustrating how each node initially has a single distance vector for the shortest path to each other node. It demonstrates the process of nodes sharing their distance vectors and updating their routing tables using the Bellman-Ford equation, leading to a complete routing table for the network. The script also touches on the issue of slow convergence when network failures occur, highlighting the challenge of 'bad news' traveling slowly.

Takeaways

• 📐 The script describes a three-node network with edge costs and initial distance vectors representing the shortest path between nodes.
• 🔄 Each node starts with a single distance vector and updates it based on the Bellman-Ford equation when receiving vectors from adjacent nodes.
• 🔄 The Bellman-Ford equation is used to compute the shortest path cost by summing the distances through intermediate nodes.
• 👀 Node x initially doesn't know about the shorter path between x and z via y, which is a key point in understanding the network's initial state.
• 🔄 When node x learns y's distance vector, it updates its shortest path to z from five to three, demonstrating the routing update process.
• 🔄 Node x also receives a distance vector from z, which it uses to further refine its routing table.
• 🔄 The process of updating routing tables is iterative and involves all nodes in the network until each has a complete routing table.
• 📉 The network converges quickly when costs decrease, indicating efficient routing updates in response to cost changes.
• 🚨 The script highlights a problem with distance vector routing: bad news (failures) can travel slowly, affecting network reliability.
• 🔄 Distance vector routing relies on the exchange of distance vectors between adjacent nodes to update routing information.
• 🔄 The script emphasizes the dynamic nature of routing in a network, where nodes continuously update their routing tables based on new information.
• 🔄 The example provided illustrates the importance of accurate and timely information exchange in maintaining an efficient and reliable network.

Q & A

What is the initial shortest path cost between node x and node y?

The initial shortest path cost between node x and node y is one, which is the direct path between them.

What is the initial shortest path cost from node x to node z according to node x?

The initial shortest path cost from node x to node z is five, as x only knows about the direct path.

Why does node x not know about the shorter path between x and z via y?

Node x does not know about the shorter path between x and z via y because it has not yet received the updated distance vector from node y.

How does distance vector routing work in updating routing tables?

In distance vector routing, each node sends its distance vectors to every adjacent node, and then updates its routing table according to the Bellman-Ford equation by calculating the minimum sum of distances through any intermediate node.

What happens when node x learns of node y's distance vectors?

When node x learns of node y's distance vectors, it updates its shortest path cost to node z from five to three, by adding the cost between x and y (one) to the distance between y and z (two).

How does the Bellman-Ford equation help in updating the shortest path costs?

The Bellman-Ford equation is used to update the shortest path costs by comparing the current known cost to a node with the cost calculated by adding the current node's cost to the next node's cost to the destination.

What is the updated shortest path cost from node x to node z after receiving node y's distance vectors?

After receiving node y's distance vectors, the updated shortest path cost from node x to node z is three, computed as the sum of the cost between x and y (one) and the distance between y and z (two).

How does the network react when the costs decrease?

When costs decrease, the network converges quickly as nodes update their routing tables with the new, lower cost paths as they receive updated distance vectors from adjacent nodes.

What is the issue mentioned when failures occur in the network?

When failures occur, bad news can travel slowly because it takes time for the updated distance vectors reflecting the failure to propagate through the network, leading to potential routing inefficiencies or loops.

How does the network eventually achieve a complete routing table?

The network achieves a complete routing table as each node iteratively receives and processes distance vectors from other nodes, updating their routing tables until all nodes have the shortest path costs to all other nodes.

What is the significance of the Bellman-Ford equation in distance vector routing?

The Bellman-Ford equation is significant in distance vector routing as it allows nodes to dynamically update their routing tables with the shortest path costs, even in the presence of changing network conditions or failures.