# Class 24: Bridges and Switches

Back to Switching and Forwarding. On to IP.

Held Friday, March 17, 2000

Overview

Today we continue our discussion of packet switching by visiting some topological issues and ATM.

Notes

• Have a great break.
• No new assignment until after break.

Contents

Summary

• Distributed spanning tree algorithm
• Limitations of bridges
• Cell switching and ATM

• What if we want to broadcast to all the nodes in the extended LAN?
• In a datagtam model, if the addresses is clearly a broadcast address (depends on the protocol), forward it to all active output channgels.
• If it's so simple, why do we visit this problem? Because in other architectures, it's not so simple.

## Handling Loops

• What do we do when there's a loop in our LAN graph?
• We can design our protocols to be smart about loops
• Techniques?

### Spanning Trees

• We can temporarily ignore some of the connections, turning our tree into a graph.
• Some of you may have seen a spanning tree algorithm in CSC152 or CSC153 (or MAT218).
• The standard spanning tree algorithm assumes complete knowledge of the graph. What do you do with partial knowledge?
• You create a distributed spanning tree graph.
• One bridge gets designated as the root of the spanning tree.
• This is typically the bridge with the smallest value.
• When two bridges are connected to the same subnet, the one closest to the root forwards messages from that subnet.
• That is, the other one may still forward messages from other subnets and to other subnets.
• If two are the same distance, the one with the smaller number wins.
• How does everyone figure this out?
• At each step: consider the packets you've received.
• If you received the name of a smaller bridge, you know that you're not the root.
• If you received a smaller path to the root along one port, stop forwarding along that port.
• We'll do an example to make sure we understand.

## Bridge Limitations

• Why are bridges not enough?
• Bridges typically are between homogeneous networks.
• The algorithm above is linear in the number of bridges.
• Basically, bridges do not scale.

## History

Thursday, 20 January 2000

• Created as a blank outline.

Friday, 17 March 2000

• Filled in the details.

Monday, 3 April 2000

• Removed uncovered section on ATM.

Back to Switching and Forwarding. On to IP.

Disclaimer Often, these pages were created "on the fly" with little, if any, proofreading. Any or all of the information on the pages may be incorrect. Please contact me if you notice errors.