Redundant topologies are good! When we have a redundant topology and have an issue with one path, the second path ensures that data can still get from point A to point B. Take the following topology for example:
In this topology PC1 can communicate with PC2 via two different layer 2 paths. There is a path between SW1, SW2 and SW3, and a second path between SW1 and SW3. The issue we run into with layer 2 redundancy stems from how frames are handled in a switch. The three issues are:
- Broadcast Storms
- Multiple Frame Copies
- MAC Address Database instability.
Let’s briefly explore each of these.
A broadcast storm occurs because there is no TTL in a layer 2 header. Therefore a switch will continue to forward broadcast traffic around the loop until something else stops it, like a link going down.
The reason the frames keep getting forwarded is because the switch examines the destination MAC address to make forwarding/filtering decisions. A broadcast has a destination MAC which is all F’s, or FFFF.FFFF.FFFF. This address is never a source address and therefore it is never learned and entered into the MAC Address Database. Since the address is not in the table the switch floods the frame out of all ports except the one it received the frame on.
Multiple Frame Copies
Multiple frame copies can exist for the same reason as a broadcast storm. The switch doesn’t have the destination MAC address so it floods it. Other switches flood the frame and we end up with multiple copies.
MAC Address Table Instability
Remember that a frame has a source MAC address and this is how switches learn and populate a MAC Address Database. So assume that a frame is copied and sent in two different directions. In the image below SW2 learns the source of PC1 on port F0/2. Shortly after it hears a copy of the frame on F0/4 with the same source, PC1. What does the switch do? Well it rewrites it’s MAC Address Database to relate the most current information. Imagine a frame, or multiple frames are getting passed in both directions with the same source, and the switch keeps rewriting the MAC Address Database all day long. You can see how this could cause the MAC Address Table to become unstable.
How Do We Solve These Issues?
These issues are nothing new. We’ve known about these issues for many years now and we deal with them using a protocol known as the Spanning Tree Protocol (STP). STP dynamically blocks the look preventing the frames from wandering aimlessly. STP is not covered in this post, however as a CCNA student you need to understand it.