User’s
Manual 109
Chapter 9: Configuring Spanning Tree
The Spanning Tree Algorithm (STA) can be used to detect and disable network loops, and to provide
backup links between switches, bridges or routers. This allows the switch to interact with other bridging
devices (that is, an STA-compliant switch, bridge or router) in your network to ensure that only one route
exists between any two stations on the network, and provide backup links that automatically take over when
a primary link goes down.
The spanning tree algorithms supported by this switch include these versions:
•
STP – Spanning Tree Protocol (IEEE 802.1D)
•
RSTP – Rapid Spanning Tree Protocol (IEEE 802.1w)
•
MSTP – Multiple Spanning Tree Protocol (IEEE 802.1s)
STA uses a distributed algorithm to select a bridging device (STA-compliant switch, bridge or router) that
serves as the root of the spanning tree network. It selects a root port on each bridging device (except for the
root device), which incurs the lowest path cost when forwarding a packet from that device to the root device.
Then it selects a designated bridging device from each LAN, which incurs the lowest path cost when
forwarding a packet from that LAN to the root device. All ports connected to designated bridging devices are
assigned as designated ports. After determining the lowest cost spanning tree, it enables all root ports and
designated ports, and disables all other ports. Network packets are therefore only forwarded between root
ports and designated ports, eliminating any possible network loops.
Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge Protocol
Data Units) transmitted from the Root Bridge. If a bridge does not get a Hello BPDU after a predefined
interval (Maximum Age), the bridge assumes that the link to the Root Bridge is down. This bridge will then
initiate negotiations with other bridges to reconfigure the network to reestablish a valid network topology.
RSTP is designed as a general replacement for the slower, legacy STP. RSTP is also incorporated into
MSTP. RSTP achieves must faster reconfiguration (For example, around one tenth of the time required by
STP) by reducing the number of state changes before active ports start learning, predefining an alternate
route that can be used when a node or port fails, and retaining the forwarding database for ports insensitive
to changes in the tree structure when reconfiguration occurs.
When using STP or RSTP, it may be difficult to maintain a stable path between all VLAN members.
Frequent changes in the tree structure can easily isolate some of the group members. MSTP (an extension
of RSTP) is designed to support independent spanning trees based on VLAN groups. Once you specify the
VLANs to include in a Multiple Spanning Tree Instance (MSTI), the protocol automatically builds an MSTI
tree to maintain connectivity between each of the VLANs. MSTP maintains contact with the global network
because each instance is treated as an RSTP node in the Common Spanning Tree (CST).
9.1.1 Displaying Global Settings
You can display a summary of the current bridge STA information that applies to the entire switch using the
STA Information screen.
Field Attributes
Spanning Tree State – Shows if the switch is enabled to participate in an STA-compliant network.
Bridge ID – A unique identifier for this bridge, consisting of the bridge priority and MAC address (where the
address is taken from the switch system).
Max Age – The maximum time (in seconds) a device can wait without receiving a configuration message
before attempting to reconfigure. All device ports (except for designated ports) should receive configuration
