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TRENDnet User’s Guide
Industrial Managed Switch Series
80
Hello Time:
This is the time interval in seconds between BPDU (Bridge Protocol Data Units)
configuration message generations by the root switch. The allowed range is 1 to
10 seconds.
PathCost:
Path cost is the cost of transmitting a frame on to a LAN through that port. It is
recommended to assign this value according to the speed of the bridge, the
slower the media, the higher the cost.
How STP Works?
After a bridge determines the lowest cost-spanning tree with STP, it enables the root
port and the ports that are the designated ports for connected LANs, and disables all
other ports that participate in STP. Network packets are therefore only forwarded
between enabled ports, eliminating any possible network loops.
STP-aware switches exchange Bridge Protocol Data Units (BPDUs) periodically. When
the bridged LAN topology changes, a new spanning tree is constructed. 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 (Max Age), the bridge assumes that the link to the
root bridge is down. This bridge then initiates negotiations with other bridges to
reconfigure the network to re-establish a valid network topology.
802.1D STP
The Spanning Tree Protocol (STP) is a link layer network protocol that ensures a loop-
free topology for any bridged LAN. It is based on an algorithm invented by Radia
Perlman while working for Digital Equipment Corporation. In the OSI model for
computer networking, STP falls under the OSI layer-2. Spanning tree allows a network
design to include spare (redundant) links to provide automatic backup paths if an active
link fails, without the danger of bridge loops, or the need for manual enabling/disabling
of these backup links. Bridge loops must be avoided because they result in flooding the
network.
The Spanning Tree Protocol (STP) is defined in the IEEE Standard 802.1D. As the name
suggests, it creates a spanning tree within a mesh network of connected layer-2 bridges
(typically Ethernet switches), and disables those links that are not part of the tree,
leaving a single active path between any two network nodes.
STP switch port states

Blocking - A port that would cause a switching loop, no user data is sent or
received but it may go into forwarding mode if the other links in use were to
fail and the spanning tree algorithm determines the port may transition to
the forwarding state. BPDU data is still received in blocking state.

Listening - The switch processes BPDUs and awaits possible new information
that would cause it to return to the blocking state.

Learning - While the port does not yet forward frames (packets) it does learn
source addresses from frames received and adds them to the filtering
database (switching database)

Forwarding - A port receiving and sending data, normal operation. STP still
monitors incoming BPDUs that would indicate it should return to the blocking
state to prevent a loop.

Disabled - Not strictly part of STP, a network administrator can manually
disable a port
802.1w RSTP
In 1998, the IEEE with document 802.1w introduced an evolution of the Spanning Tree
Protocol: Rapid Spanning Tree Protocol (RSTP), which provides for faster spanning tree
convergence after a topology change. Standard IEEE 802.1D-2004 now incorporates
RSTP and obsoletes STP. While STP can take 30 to 50 seconds to respond to a topology
change, RSTP is typically able to respond to changes within a second.
RSTP bridge port roles:

Root - A forwarding port that is the best port from Nonroot-bridge to
Rootbridge

Designated - A forwarding port for every LAN segment

Alternate - An alternate path to the root bridge. This path is different than
using the root port.