Like HowStuffWorks on Facebook!

How LAN Switches Work


Broadcast Storms

In the last section, you discovered how switches learn where the nodes are located. With all of the switches now connected in a loop, a packet from a node could quite possibly come to a switch from two different segments. For example, imagine that Node B is connected to Switch A, and needs to communicate with Node A on Segment B. Switch A does not know who Node A is, so it floods the packet.

The packet travels via Segment A or Segment C to the other two switches (B and C). Switch B will add Node B to the lookup table it maintains for Segment A, while Switch C will add it to the lookup table for Segment C. If neither switch has learned the address for Node A yet, they will flood Segment B looking for Node A. Each switch will take the packet sent by the other switch and flood it back out again immediately, since they still don't know who Node A is. Switch A will receive the packet from each segment and flood it back out on the other segment. This causes a broadcast storm as the packets are broadcast, received and rebroadcast by each switch, resulting in potentially severe network congestion.

Which brings us to spanning trees...