STP Flavors
Loops occur when alternate routes exist between hosts. Loops in an extended network can cause Layer 2 switches to forward traffic indefinitely, resulting in increased traffic and reduced network efficiency.
STP provides a tree topology for any arrangement of Layer 2 switches and interconnecting links, creating a unique path between end stations on a network, eliminating loops.
The switch supports the following Spanning Tree Protocol versions:
- Classic STP provides a single path between any two end stations, avoiding and eliminating loops.
- Rapid STP (RSTP) detects network topologies to provide faster convergence of the spanning tree. This 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 each bridge or switch in the network needs to decide, if it should actively forward traffic or not on each of its ports.
- Multiple STP (MSTP) 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 instance is associated with the Layer 2 domain on which it performs loop detection and mitigation. This enables a port to be stopped in one instance, such as traffic from VLAN A that is causing a loop, while traffic can remain active in another domain where no loop was seen, such as on VLAN B.
MSTP provides full connectivity for packets allocated to any VLAN. MSTP is based on RSTP. In addition, MSTP transmits packets assigned to various VLANs through different multiple spanning tree (MST) regions. MST regions act as a single bridge.