Operation Manual – MSTP
H3C S3100 Series Ethernet Switches
Chapter 1 MSTP Configuration
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Chapter 1 MSTP Configuration
1.1 STP Overview
I. Functions 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 tree topology. As a network with tree topology is loop-free, it prevents packets
in it from being duplicated and forwarded endlessly and prevents device performance
degradation.
Currently, in addition to the protocol conforming to IEEE 802.1d, STP also refers to the
protocols based on IEEE 802.1d, such as RSTP, and MSTP.
II. 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. BPDUs contain sufficient information for the network devices to
complete the spanning tree calculation.
In STP, BPDUs come in two types:
z
Configuration BPDUs, used to calculate spanning trees and maintain the spanning
tree topology.
z
Topology change notification (TCN) BPDUs, used to notify concerned devices of
network topology changes, if any.
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 the entire network, and the root bridge can
change alone with changes of the network topology. Therefore, the root bridge is not
fixed.
Upon network convergence, the root bridge generates and sends out configuration
BPDUs periodically. Other devices just forward the configuration BPDUs received. This
mechanism ensures the topological stability.
