believe that the less desirable Switch B is the root until the max_age timeout expires. After 20 seconds, Switch C will accept the BPDU advertisements from Switch B, and start sending its own BPDUs to Switch B. When Switch B receives the BPDUs from Switch C, it will understand that there is a path to Switch A (the more desirable root Additional Spanning Tree Features Root bridge Switch A Forwarding Link failure Switch B Forwarding Forwarding Forwarding Forwarding Blocking BackboneFast configured on all switches Switch C Figure 8-7. BackboneFast example bridge) through Switch C, and accept Switch A as the root bridge again. This pro- cess takes upwards of 50 seconds with the default spanning tree timers. BackboneFast adds functionality that detects indirect link failures. It actively discov- ers paths to the root by sending out root link query PDUs after a link failure. When it discovers a path, it sets the max_age timer to 0 SO that the port can cycle through the normal listening, learning, and forwarding states without waiting an additional 20 seconds. If BackboneFast is used, it must be enabled on every switch in the network. Enabling BackboneFast on an IOS switch is as simple as using the spanning-tree backbonefast global command: Cat-3550(config)# spanning-tree backbonefast Cat-3550(config)# Negating the command disables the feature. To enable BackboneFast on a CatOS switch, use the set spantree backbonefast enable command: Catos-6509 (enable) set spantree backbonefast enable Backbonefast enabled for all VLANs To disable this feature, change the enable keyword to disable. Common Spanning Tree Problems Spanning tree can be a bit of a challenge when it misbehaves. More to the point, spanning tree problems can be hard to diagnose if the network is not properly designed. Here are a couple of common problems and how to avoid them. Chapter 8: Spanning Tree Duplex Mismatch A bridge still receives and processes BPDUs on ports even when they are in a blocked state. This allows the bridge to know that a path to the roo