QTECH
Software Configuration Manual
15-185
Chapter 15
STP Configuration
15.1 Brief introduction of STP Configuration
STP(Spanning Tree Protocl) is a part of IEEE 802.1D network bridge. The realization of standard STP can
eliminate network broadcast storm caused by network circle connection and the circle connection caused by
misplaying and accidence, and it also can provide the possibility of network backup connection.
STP protocol with IEEE 802.1D standard provides network dynamic redundancy transferring mechanism
and prevents circle connection in bridge network. It determines which interface of the network bridge can transmit
data packet. After executing STP matching, switch in the LAN will form a STP dynamic topology which prevents the
loop existing between any two working station to prevent broadcast storm in LAN. At the same time, STP matching
is responsible to detect the change of physical topology to establish new spanning tree after the changes of topology.
For example : when there is a break in the switch or a channel, it can provide certain error tolerance to re-configure
a new STP topology.
15.1.1
Introduction to STP
15.1.1.1
Why STP?
The Spanning Tree Protocol (STP) was established based on the 802.1D standard of IEEE to eliminate
physical loops at the data link layer in a local area network (LAN). Devices running this protocol detect loops in the
network by exchanging information with one another and eliminate loops by selectively blocking certain ports until
the loop structure is pruned into a loop-free network structure. This avoids proliferation and infinite recycling of
packets that would occur in a loop network and prevents deterioration of the packet processing capability of network
devices cause by duplicate packets received.
15.1.1.2
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.
BPDUs contain sufficient information for the network devices to complete the spanning tree computing.
In STP, BPDUs come in two types :
·
Configuration BPDUs, used to maintain the spanning tree topology.
·
Topology change notification (TCN) BPDUs, used to notify concerned devices of network topology changes,
if any.
15.1.1.3
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 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 at a certain interval a BPDU and other
devices just forward this BPDU. 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 non-root-bridge device has one and only one root port. The
root bridge has no root port.
3) Designated bridge and designated port
The following table describes a designated bridge and a designated port.
Description of designated bridge and designated port
Classification
Designated bridge
Designated port
