28.9.4 Root guard

(1) Overview

Unintended topologies might occur if a switch is accidentally connected or a setting is changed somewhere where the network is not managed. When performance of the root bridge in an unintended topology is poor, a network fault might occur when traffic is congested. Root guard functionality avoids such network faults by identifying root bridge candidates for situations like this.

The figures below show problems that occur when switches are accidentally connected.

• Operation in which Switch A and Switch B run as root bridge candidates

  Figure 28-14 Operation of device A and device B as candidates for root bridge

• When Switch C, which has a higher bridge priority than Switch A or Switch B, is connected, it becomes the root bridge, and becomes congested with traffic

  Figure 28-15 Connect device C with higher bridge priority than device A and B.

Root guard functionality detects bridges with priorities higher than the current root bridge, and preserves the topology by discarding BPDUs. Loops can also be avoided by setting the corresponding port to be blocked. Root guard functionality cannot be used on ports for which loop guard functionality is set.