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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.
Disabled - Not strictly part of STP, a network administrator can manually disable a
port.
Edage 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).
