Tripp Lite NGI-M05-C1 Owner's Manual page 154

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 IEEEStandard802.1D. 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,
making STP obsolete. 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 Non-root-bridge to Root-

bridge.
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.
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
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