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User’s Manual of SGSD-1022 / SGSD-1022P
SGSW-2840 / SGSW-2840P
149
4.7 Spanning Tree
Spanning Tree Protocol
The Spanning Tree Algorithm (STA) 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 (that is, an STA-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. The spanning tree algorithms supported by this
switch include these versions:
STP – Spanning Tree Protocol (IEEE 802.1D)
RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w)
MSTP – Multiple Spanning Tree Protocol (IEEE 802.1s)
STP – STA uses a distributed algorithm to select a bridging device (STA-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.
RSTP – RSTP is designed as a general replacement for the slower, legacy STP. RSTP is also incorporated into MSTP. RSTP
achieves must faster reconfiguration (i.e., around 1 to 3 seconds, compared to 30 seconds or more for STP) by reducing the
number of state changes before active ports start learning, predefining an alternate route that can be used when a node or port
fails, and retaining the forwarding database for ports insensitive to changes in the tree structure when reconfiguration occurs.
MSTP – When using STP or RSTP, it may be difficult to maintain a stable path between all VLAN members. Frequent changes
in the tree structure can easily isolate some of the group members. MSTP (which is based on RSTP for fast convergence) is