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Understanding Spanning Tree Protocol
STP Overview
Spanning-Tree Protocol (STP) is a Layer 2 link management protocol that prevents loops from being
formed when switches or bridges are interconnected via multiple paths. Spanning-Tree Protocol
implements the 802.1D IEEE algorithm by exchanging BPDU messages with other switches to detect
loops, and then removes the loop by shutting down selected bridge interfaces. This algorithm guarantees
that there is one and only one active path between two network devices.
Spanning-tree operation is transparent to end stations, which cannot detect whether they are connected
to a single LAN segment or to a LAN of multiple segments.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a
network. The spanning-tree algorithm calculates the best loop-free path throughout a Layer 2 network.
Infrastructure devices such as wireless bridges and switches send and receive spanning-tree frames,
called bridge protocol data units (BPDUs), at regular intervals. The devices do not forward these frames
but use them to construct a loop-free path.
Multiple active paths among end stations cause loops in the network. If a loop exists in the network, end
stations might receive duplicate messages. Infrastructure devices might also learn end-station MAC
addresses on multiple Layer 2 interfaces. These conditions result in an unstable network.
STP defines a tree with a root bridge and a loop-free path from the root to all infrastructure devices in
the Layer 2 network.
STP discussions use the term root to describe two concepts: the bridge on the network that serves as a
Note
central point in the spanning tree is called the root bridge, and the port on each bridge that provides the
most efficient path to the root bridge is called the root port. These meanings are separate from the role
in radio network setting that includes root and non-root options. A bridge whose role in radio network
setting is root bridge does not necessarily become the root bridge in the spanning tree. In this chapter,
the root bridge in the spanning tree is called the spanning-tree root.
STP forces redundant data paths into a standby (blocked) state. If a network segment in the spanning tree
fails and a redundant path exists, the spanning-tree algorithm recalculates the spanning-tree topology
and activates the standby path.
When two interfaces on a bridge are part of a loop, the spanning-tree port priority and path cost settings
determine which interface is put in the forwarding state and which is put in the blocking state. The port
priority value represents the location of an interface in the network topology and how well it is located
to pass traffic. The path cost value represents media speed.
STP Support
The bridge supports both per-VLAN spanning tree (PVST) and a single 802.1q spanning tree. The bridge
cannot run 802.1s MST or 802.1d Common Spanning Tree, which maps multiple VLANs into a
one-instance spanning tree.
Per-VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the
network. It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while
blocking for other VLANs. Since PVST treats each VLAN as a separate network, it has the ability to
load balance traffic (at layer-2) by forwarding some VLANs on one trunk and other Vlans on another
trunk without causing a Spanning Tree loop.
Book Title
2
Spanning Tree Protocol in a Wireless Environment
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