Specifications

Switching 8-33
Software Release 2.7.3
C613-03098-00 REV A
Once the spanning tree is established, it is maintained by:
1. Replacing a failed path with a redundant backup path, if one is available.
2. Detecting and removing loops by declaring ports redundant and removing
them from the logical spanning tree.
3. Maintaining timers that control the ageing of the forwarding database
entries.
The logical spanning tree, sometimes called the active topology, includes the root
bridge and all designated bridges, meaning all ports that are to be used for
communication within the STP. These ports are in the forwarding state. Ports
removed from the logical spanning tree are not in the forwarding state. To
implement the spanning tree algorithm, switches communicate with one
another using the Spanning Tree Protocol. The primary protocol data unit
(PDU) is the Hello message or Configuration Bridge Protocol Data Unit (BPDU). It
includes the following information:
■ The bridge ID of the root bridge.
■ The distance (or cost) from this switch to the root bridge.
■ The bridge ID of the designated bridge on this LAN.
■ Hello messages are initiated at regular intervals by the root bridge and
propagate through the extended LAN.
Electing a Root Bridge and Designated Bridge
Each spanning tree (in STP) has a Root Bridge , which initiates the propagation of
hello messages through the extended LAN, and sets the values of parameters
that control the spanning tree computation process. Whereas, in RSTP and MSTP
each bridge can control the transmission of their own periodic hello messages.
The root bridge is the switch with the lowest bridge ID and is elected by the
exchange of
hello packets. When a switch receives a hello packet it compares the
value of the root bridge ID in the message to the value of the root bridge ID
parameter in its own spanning tree database. If the value in the message is better,
the switch stores the new value in its database and sends Hello messages with
the new value out on its other ports. Otherwise, the switch continues to send
Hello messages with the value currently stored in its spanning tree database. By
this process, all switches in the extended LAN eventually learn the bridge ID of
the root bridge.
Each LAN has a single switch, called the
designated bridge , that logically
connects the LAN to the next LAN closer to the root bridge. The designated
bridge for a LAN is the switch on the LAN with the lowest root path cost and
bridge ID. The designated bridge is elected by the exchange of Hello messages,
in the same way that the root bridge is elected. The election of a new root bridge
or a switch becoming unavailable due to a fatal error condition, typically results
in the election of a new designated bridge in the next few rounds of Hello
messages.
Spanning Tree Modes
STP can run in standard mode or rapid mode. Rapid mode allows rapid
configuration of the spanning tree. The Rapid Spanning Tree Protocol (RSTP) is
specified in IEEE Standard 802.1w.