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TRENDnet User’s Guide
Industrial Managed Switch Series
79
STP
STP/RSTP
(R)STP detects and breaks network loops and provides backup links between switches,
bridges or routers. It allows a Switch to interact with other (R)STP compliant switches in
your network to ensure that only one path exists between any two stations on the
network.
The Switch supports Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol
(RSTP) as defined in the following standards.

IEEE 802.1D Spanning Tree Protocol

IEEE 802.1w Rapid Spanning Tree Protocol
The Switch uses IEEE 802.1w RSTP (Rapid Spanning Tree Protocol) that allows faster
convergence of the spanning tree than STP (while also being backwards compatible
with STP-only aware bridges). In RSTP, topology change information is directly
propagated throughout the network from the device that generates the topology
change. In STP, a longer delay is required as the device that causes a topology change
first notifies the root bridge and then the root bridge notifies the network. Both RSTP
and STP flush unwanted learned addresses from the filtering database.
In STP, the port states are Blocking, Listening, Learning, Forwarding.
In RSTP, the port states are Discarding, Learning, and Forwarding.
Note: In this document, “STP” refers to both STP and RSTP.
STP Terminology

The root bridge is the base of the spanning tree.

Path cost is the cost of transmitting a frame onto a LAN through that port.
The recommended cost is assigned according to the speed of the link to
which a port is attached. The slower the media, the higher the cost.

On each bridge, the bridge communicates with the root through the root
port. The root port is the port on this Switch with the lowest path cost to the
root (the root path cost). If there is no root port, then this Switch has been
accepted as the root bridge of the spanning tree network.

For each LAN segment, a designated bridge is selected. This bridge has the
lowest cost to the root among the bridges connected to the LAN.
Forward Time (Forward Delay):
This is the maximum time (in seconds) the Switch will wait before changing
states. This delay is required because every switch must receive information
about topology changes before it starts to forward frames. In addition, each port
needs time to listen for conflicting information that would make it return to a
blocking state; otherwise, temporary data loops might result. The allowed range
is 4 to 30 seconds.
Max Age:
This is the maximum time (in seconds) the Switch can wait without receiving a
BPDU before attempting to reconfigure. All Switch ports (except for designated
ports) should receive BPDUs at regular intervals. Any port that age out STP
information (provided in the last BPDU) becomes the designated port for the
attached LAN. If it is a root port, a new root port is selected from among the
Switch ports attached to the network. The allowed range is 6 to 40 seconds.