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TRENDnet User's Guide
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.
⚫
Backup - A backup/redundant path to a segment where another bridge port
already connects.
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⚫
Disabled - Not strictly part of STP, a network administrator can manually
disable a port
Edge Port:
They are attached to a LAN that has no other bridges attached. These edge ports
transition directly to the forwarding state. RSTP still continues to monitor the port
for BPDUs in case a bridge is connected. RSTP can also be configured to
automatically detect edge ports. As soon as the bridge detects a BPDU coming to
an edge port, the port becomes a non-edge port.
Forward Delay:
The range is from 4 to 30 seconds. This is the maximum time (in seconds) the root
device will wait before changing states (i.e., listening to learning to forwarding).
Transmission Limit:
This is used to configure the minimum interval between the transmissions of
consecutive RSTP BPDUs. This function can only be enabled in RSTP mode. The
range is from 1 to 10 seconds.
Hello Time:
Set the time at which the root switch transmits a configuration message. The
range is from 1 to 10 seconds.
Bridge priority:
Bridge priority is used in selecting the root device, root port, and designated port.
The device with the highest priority becomes the STA root device. However, if all
devices have the same priority, the device with the lowest MAC address will
become the root device.
Managed Industrial L2 Switch
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