3Com 4210 PWR, Руководство по настройке

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MSTP C
ONFIGURATION
STP Overview
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.
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:
■
Configuration BPDUs, used to calculate spanning trees and maintain the
spanning tree topology.
■
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.
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