Both STP and RSTP completely disable select links to remove loops from the network. However, disabling a link in a carrier network would lead to inefficient use of bandwidth, since a complete link is idle. The Multiple Spanning Tree Protocol (MSTP), originally defined in IEEE 802.1s and later merged into IEEE 802.1Q-2003, defines an extension to the RSTP protocol to further develop the usefulness of virtual LANs (VLANs). This "Per-VLAN" Multiple Spanning Tree Protocol configures a separate Spanning Tree for each VLAN group and blocks the links that are redundant within each Spanning Tree. As a result, while some links can be disabled for a particular set of VLANs, they could be used in an instance of RSTP running for another set of VLANs. Thus, MSTP leads to a more efficient use of all the network links.
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