Introduction

Ethernet has been a large success due to it being standards-based, low-cost due to widespread deployment, and because it is the most flexible Layer-2 technology. Ethernet supports countless numbers of topologies such as stars, rings, and ladders.

While this flexibility makes it ideal for a dynamic environment, it also can create situations where additional care is needed. When an Ethernet device receives traffic with a destination Media Access Control (MAC) address that is unknown to the device, the traffic will be broadcast to all the device’s neighbors. This mechanism allows the intended destination device to receive the traffic and reply to the source device. On the reply, the device sending back traffic will be able to locate that destination and create a forwarding entry for it. The next time that traffic is encountered with that destination MAC address, the device can forward it directly to the correct port without having to broadcast the frame to all egress ports.

Figure 1. Ethernet Topology Examples

Broadcast Storms

This broadcast mechanism is very effective. However, it can become an issue in network topologies that include physical loops.

In the diagram below, when device 1 receives packets addressed to an unknown destination, it will broadcast that packet on link A in an effort to learn which of its ports the destination device is connected to. When device 2 receives the packet, it will also attempt to learn which of its ports the destination device is connected to. In doing so it will broadcast the packet on link B. This pattern continues such that the packet is broadcast on links C, D and E. Device 1 then receives the original packet once again on link E as a result of device 5 broadcasting the packet. Device 1 then broadcasts the packet again, attempting to learn which port the destination device is connected to. For a network with a meshed topology, this will result in the creation of thousands of packets, or what is known as a broadcast storm. Broadcast storms result in diminished link capacity for application traffic, and in severe cases can render a network completely inoperable. Spanning Tree Protocol (STP) was created to prevent broadcast storms.

Figure 2. Broadcast Storms

Continue

• Part 1:
  Introduction/Broadcast Storms
• Part 2:
  Spanning Tree Protocol
• Part 3:
  LightningEdge Enhancements
• Part 4:
  Conclusion