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Chapter 7 - Spanning Tree Management
Spanning Tree Protocol protects a Layer 2 Broadcast domain from Broadcast storms by
selectively setting links to standby mode to prevent loops. In standby mode, these links
temporarily stop transferring user data. After the topology changes so that the data transfer is
made possible, the links are automatically reactivated.
Loops occur when alternate routes exist between hosts. Loops in an extended network can cause
switches to forward traffic indefinitely, resulting in increased traffic load and reduced network
efficiency.
STP provides a tree topology for any arrangement of switches and interconnecting links, by
creating a unique path between end stations on a network, and thereby eliminating loops.
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.
•
Multiple STP (MSTP) – Classic STP and Rapid STP detect Layer 2 loops, and attempt to
mitigate them by preventing the involved port from transmitting traffic. Since loops exist
on a per-Layer 2-domain basis, there can be a loop in VLAN A and no loop in VLAN B. If
both VLANs are on Port X, Classic STP and Rapid STP will mitigate the loop by stopping
traffic on the entire port, including VLAN B traffic.
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.