The early version of the spanning-tree protocol compliant with IEEE 802.1D, here referred to as classic spanning tree, had the problem that changes to topology after a connection failure were implemented very slowly: Depending on the complexity of the network, the classic spanning tree takes between 20 seconds and a minute to establish new routes. For many network services a failure of this length of time is unacceptable.
The spanning tree protocol was improved and published as the "Rapid Spanning Tree Protocol" (RSTP), initially as the IEE 802.1t/w standard and later as a part of the newly published IEEE 802.1D. Even though the classic spanning tree protocol was thus withdrawn, it continues to be supported by LCOS.
