User manual

APS User Manual
87
1.2.4 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 (that is, 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.
STP can run in one of three modes: STP, RSTP or MSTP. A device running RSTP is compatible with
other devices running STP; a device running MSTP is compatible with other devices running RSTP or
STP. By default, on a device in MSTP mode each port automatically detects the mode of the device
connected to it (MSTP, RSTP or STP), and responds in the appropriate mode by sending messages
(BPDUs) in the corresponding format. Ports on a device in RSTP mode can automatically detect and
respond to connected devices in RSTP and STP mode. Particular ports can also be forced to only
operate in a particular mode (spanning-tree force-version command).
STP
The Spanning Tree Protocol (STP) is the original protocol defined by IEEE standard 802.1D-1988. It
creates a single spanning tree over a network.