12-1

C
HAPTER
12
S
PANNING
T
REE
A
LGORITHM
The Spanning Tree Algorithm (STA) can be used to detect and di sable
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 t his 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 – 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 de vice
(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 forwar ding a p acket fr om that L AN to th e root d evice. A ll por ts
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 design ated ports,
eliminating any possible network loops.