Legacy STP has numerous drawbacks with respect to convergence time and potential loops. Features such as portfast, Root Guard, and UplinkFast help address some the shortcomings of legacy STP. RSTP, 802.1w, has recently become available in 7.5(1) Catalyst OS code. Prior to this code, it was not possible to run RSTP alone. MST, 802.1s, implementation in Cisco devices also requires 802.1w and 802.1t protocols. Essentially, MST runs on top of RSTP. RSTP addresses the issue with convergence time. It natively has features similar to UplinkFast and BackboneFast. Unlike legacy STP, RSTP BPDUs have been modified to include port designation and proposal/agreement mechanism between adjacent bridges. MST helps scale Layer 2 networks while at the same time provides load-balancing capability. MST is a compromise between PVST+ and CST. Finally, this chapter also addressed UDLD protocol and its benefits.