4.9 Spanning Tree
For some networking services, always-on connections are required to ensure that end users’ online related activities are
not interrupted due to unexpected disconnections. In these circumstances, multiple active paths between network nodes
are established to prevent disconnections from happening. However, multiple paths interconnected with each other have
a high tendency to cause bridge loops that make networks unstable and in worst cases make networks unusable. For
example, the MAC address table used by the switch or bridge can fail, since the same MAC addresses (and hence the
same network hosts) are seen on multiple ports. Second, a broadcast storm occurs. This is caused by broadcast packets
being forwarded in an endless loop between switches. A broadcast storm can consume all available CPU resources and
bandwidth.
To solve problems causing by bridge loops, spanning tree allows a network design to include redundant links to provide
automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manually
enabling/disabling these backup links.
The Spanning Tree Protocol (STP), defined in the IEEE Standard 802.1s, can create a spanning tree within a mesh network
of connected layer-2 bridges (typically Ethernet switches) and disable the links which are not part of that tree, leaving a
single active path between any two network nodes.
To provide faster spanning tree convergence after a topology change, an evolution of the Spanning Tree Protocol “Rapid
Spanning Tree Protocol (RSTP)”, is introduced by IEEE 802.1w. RSTP is a refinement of STP; therefore, it shares most of its
basic operation characteristics. This essentially creates a cascading effect away from the root bridge where each
designated bridge proposes to its neighbors to determine if it can make a rapid transition. This is one of the major
elements which allows RSTP to achieve faster convergence times than STP.
The other extension of RSTP is IEEE 802.1s Multiple Spanning Tree protocol (MSTP) that allows different VLANs to travel
along separate instances of spanning tree. Unlike STP and RSTP, MSTP eliminates the needs for having different STP for
each VLAN. Therefore, in a large networking environment that employs many VLANs, MSTP can be more useful than
legacy STP.
4.9.1 Bridge Settings
Basic Settings
Protocol Version: Select the appropriate spanning tree protocol. Protocol versions provided include “STP”, “RSTP”, and
“MSTP”.
Bridge Priority: Each switch has a relative priority and cost that is used to decide what the shortest path is to forward a
packet. The lowest cost path (lowest numeric value) has a higher priority and is always used unless it is down. If you have
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