S
PANNING
T
REE
P
ROTOCOL
C
ONFIGURATION
2-45
The Spanning Tree Protocols supported by the swi tch include the
following standards:
•STP – Spanning Tree Protocol (IEEE 802.1D).
•RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w).
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 wh en forwa rding a packet f rom tha t LAN to the roo t device . All po rts
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 is designed as a general replacement for the slower, legacy STP.
RSTP achieves much faster reconfiguration (around one tenth of that
required by 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.