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Lantronix
SM24TBT2DPA and SM24TBT2DPB Web User Guide
2-8 Spanning Tree
The Spanning Tree Protocol (STP) can be used to detect and disable network
loops, and to provide backup links between switches, bridges or routers.
This allows the switch to interact with other bridging devices (an STP-compliant
switch, bridge or router) in your network to ensure that only one route exists
between any two stations on the network and provide backup links which
automatically take over when a primary link goes down.
STP uses a distributed algorithm to select a bridging device (STP-compliant
switch, bridge or router) that serves as the root of the spanning tree network. It selects a root port on each
bridging device (except for the root device) which incurs the lowest path cost when forwarding a packet
from that device to the root device. Then it selects a designated bridging device from each LAN which
incurs the lowest path cost when forwarding a packet from that LAN to the root device. All ports connected
to designated bridging devices are assigned as designated ports. After determining the lowest cost
spanning tree, it enables all root ports and designated ports, and disables all other ports. Network packets
are therefore only forwarded between root ports and designated ports, eliminating any possible network
loops.
Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge Protocol
Data Units) transmitted from the Root Bridge. If a bridge does not get a Hello BPDU after a predefined
interval (Maximum Age), the bridge assumes that the link to the Root Bridge is down. This bridge will then
initiate negotiations with other bridges to reconfigure the network to reestablish a valid network topology.
Protocol versions
:
STP
: Spanning Tree Protocol is an OSI layer-2 protocol which ensures a loop free topology for any
bridged LAN. The original STP protocol is now obsolete by RSTP.
RSTP
: In 1998, the IEEE with document 802.1w introduced an evolution of STP: the Rapid Spanning Tree
Protocol, which provides for faster spanning tree convergence after a topology change. Standard IEEE
802.1D-2004 now incorporates RSTP and obsoletes STP, while at the same time being backwards
compatible with STP.
MSTP
: In 2002, the IEEE introduced an evolution of RSTP: the Multiple Spanning Tree Protocol. The
MSTP protocol provides for multiple spanning tree instances, while ensuring RSTP and STP compatibility.
The standard was originally defined by IEEE 802.1s but was later incorporated in IEEE 802.1D-2005.