436 Dell PowerConnect 55xx Systems User GuideFILE LOCATION: C:\Users\gina\Desktop\Checkout_new\Maintenance Projects\DellContax\sources\CxUGSwitching_STP.fmD E L L C O N F I D E N T I A L – P R E L I M I N A R Y 1 0 / 3 0 / 1 3 - F O R P R O O F O N LYSpanning Tree Protocol OverviewSpanning Tree Protocol (STP) provides tree topography for any bridgearrangement. STP eliminates loops by providing a unique path between endstations on a network.Loops occur when alternate routes exist between hosts. Loops, in an extendednetwork, can cause bridges to forward traffic indefinitely, resulting in packetsnot arriving at their destination, increased traffic, and reduced networkefficiency.The device supports the following Spanning Tree versions:• Classic STP — Provides a single path between end stations, avoiding andeliminating loops. For more information on configuring Classic STP, see"Global Settings" on page 438.• Rapid STP (RSTP) — Provides faster convergence of the spanning treethan Classic STP. RSTP is most effective when the network topology isnaturally tree-structured, and therefore faster convergence might bepossible. RSTP is enabled by default.Although Classic STP is guaranteed to prevent Layer 2 forwarding loops, ina general network topology, there might be an unacceptable delay beforeconvergence. This means that before convergence, each bridge or switch inthe network must decide if it should actively forward traffic or not, on eachof its ports.For more information on configuring Rapid STP, see "Rapid SpanningTree" on page 451.• Multiple STP (MSTP) — MSTP is based on RSTP. It detects Layer 2loops, and attempts to mitigate them by preventing the involved port fromtransmitting traffic.Since loops exist on a per-Layer 2-domain basis, a situation can occurwhere there is a loop in VLAN A and no loop in VLAN B. If both VLANsare on Port X, and STP wants to mitigate the loop, it stops traffic on theentire port, including VLAN B traffic, where there is no need to stoptraffic.Multiple Spanning Tree Protocol (MSTP) solves this problem by enablingseveral STP instances, so that it is possible to detect and mitigate loopsseparately in each instance. By associating instances to VLANs, each
