faults. When configuring UplinkFast on IOS switches, there are no scary messages like there are in CatOS: -3550(config)#spanning-tree uplinkfast 3550(config)# That's it. Simple! But don't let the simplicity fool you-enabling UplinkFast changes priorities in IOS, too. Unlike in CatOS, however, disabling the feature in IOS (via no spanning-tree uplinkfast) automatically resets the priorities to their defaults. Again, this might not be what you want or expect, SO be careful. Chapter 8: Spanning Tree BackboneFast When a switch receives a BPDU advertising a root bridge that's less desirable than the root bridge it already knows about, the switch discards the BPDU. This is true as long as the switch knows about the better root bridge. If the switch stops receiving BPDUs for the better root bridge, it will continue to believe that bridge is the best bridge until the max_age timeout is exceeded. max age defaults to 20 seconds. Figure 8-7 shows a network with three switches. All of these switches are core or dis- tribution switches, though they could also be access-layer switches. Switch A is the root bridge. Through normal convergence, the F0/2 port on Switch C is blocking, while all the others are forwarding. Root bridge Switch A Link failure Switch B Forwarding Forwarding Forwarding Forwarding Forwarding Blocking BackboneFast configured on all switches Switch C Figure 8-7. BackboneFast example Say an outage occurs that brings down the F0/3 link between Switch A and Switch B. This link is not directly connected to Switch C. The result is an indirect link failure on Switch C. When Switch B recognizes the link failure, it knows that it has no path to the root and starts advertising itself as the root. Until this point, Switch B had been adver- tising BPDUs showing the more desirable Switch A as the root. Switch C still has that information in memory, and refuses to 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 Swi