SMC Networks SMC TigerStack III SMC6824MPE - Manual (Page 196)

C
ONFIGURING THE
S
WITCH
3-146
Spanning Tree Algorithm Configuration
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)
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.