11 Spanning Tree
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11 Spanning Tree
1. Purpose of STP
Spanning tree protocol (STP) is a protocol conforming to IEEE 802.1d. It aims to eliminate
loops on data link layer in a local area network (LAN). Devices running this protocol detect
loops in the network by exchanging packets with one another and eliminate the loops
detected by blocking specific ports until the network is pruned into one with a tree topology.
As a network with a tree topology is loop-free, STP prevents packets in it from being
duplicated and forwarded endlessly and prevents device and network performance
degradation caused by data loops.
In the narrow sense, STP refers to IEEE 802.1d STP; in the broad sense, STP refers to the
IEEE 802.1d STP and various enhanced spanning tree protocols derived from that protocol.
2. Protocol Packets of STP
STP uses bridge protocol data units (BPDUs), also known as configuration messages, as its
protocol packets. STP identifies the network topology by transmitting BPDUs between STP
compliant network devices, typically switches and routers. BPDUs contain sufficient
information for the network devices to complete the spanning tree calculation.
3. Basic concepts in STP
(1) Root bridge
A tree network must have a root; hence the concept of
root bridge
has been introduced in
STP.
There is one and only one root bridge in an entire STP-based network at a given time. But the
root bridge can change because of with changes of the network topology. Therefore, the root
bridge is not fixed.
Upon initialization of a network, each device generates and sends out BPDUs periodically
with itself as the root bridge; after network convergence, only the root bridge generates and
sends out configuration BPDUs at a certain interval, and the other devices just forward the
BPDUs.
(2) Root port
On a non-root bridge device, the root port is the port with the lowest path cost to the root
bridge. The root port is used for communicating with the root bridge. A non-root-bridge device
has one and only one root port. The root bridge has no root port.
(3) Designated bridge and designated port
Refer to the following table for the description of designated bridge and designated port.
Classification Designated bridge
Designated port
For a device
A designated bridge is a device that is The port through which the
