Class 24: Bridges and Switches

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

• Broadcast
• Handling Loops
  • Spanning Trees
• Bridge Limitations

Summary

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

Broadcast

• Most of our addressing discussions have had to do with single-destination addressing.
• 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?
• Initially: broadcast your bridge number and distance.
• 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.
• What are the advantages and disadvantages of this technique?

Bridge Limitations

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