Bridging Loop

In this basic example, there are redundant links without STP. This creates a bridged loop. In this case, the redundant links cause the Ethernet data frame to have more than one path. Because the bridges are transparent, a copy of the data frame is sent across both paths. Bridge1 and Bridge2 both receive a copy of the data frame that was sent by the other. Then, each bridge sees alternating data frames, assumes that the source host is on the wrong side of the bridge, and updates the bridge table. The data frames then start to be recopied on each side of the bridge again and again. Think about how many data frames are needed for a simple e-mail message. With this bridging loop, the frames would be copied over and over again until they timed out. However, because the upper layer protocols are generating many requests, the process keeps happening. The entire network gets overwhelmed and legitimate traffic cannot pass.

Figure 1-14 revisits the example again, this time adding STP, which blocks one of the redundant links, eliminating the bridging loop. If the first link or Bridge1 were to fail, STP would re-examine the network and enable the shutdown connection. This is how STP provides redundancy in a transparent bridging environment.