Dell PowerConnect 5548 User Guide - Page 429

Spanning Tree Protocol Overview Spanning Tree Protocol (STP) provides tree topography for any bridge arrangement. STP eliminates loops by providing a unique path between end stations on a network. Loops occur when alternate routes exist between hosts. Loops, in an extended network, can cause bridges to forward traffic indefinitely, resulting in packets not arriving at their destination, increased traffic, and reduced network efficiency. The device supports the following Spanning Tree versions: • Classic STP - Provides a single path between end stations, avoiding and eliminating loops. For more information on configuring Classic STP, see "Global Settings" on page 431. • Rapid STP (RSTP) - Provides faster convergence of the spanning tree than Classic STP. RSTP is most effective when the network topology is naturally tree-structured, and therefore faster convergence might be possible. RSTP is enabled by default. Although Classic STP is guaranteed to prevent Layer 2 forwarding loops, in a general network topology, there might be an unacceptable delay before convergence. This means that before convergence, each bridge or switch in the network must decide if it should actively forward traffic or not, on each of its ports. For more information on configuring Rapid STP, see "Rapid Spanning Tree" on page 444. • Multiple STP (MSTP) - MSTP is based on RSTP. It detects Layer 2 loops, and attempts to mitigate them by preventing the involved port from transmitting traffic. Since loops exist on a per-Layer 2-domain basis, a situation can occur where there is a loop in VLAN A and no loop in VLAN B. If both VLANs are on Port X, and STP wants to mitigate the loop, it stops traffic on the entire port, including VLAN B traffic, where there is no need to stop traffic. Multiple Spanning Tree Protocol (MSTP) solves this problem by enabling several STP instances, so that it is possible to detect and mitigate loops separately in each instance. By associating instances to VLANs, each Dell PowerConnect 55xx Systems User Guide 429