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S
PANNING
T
REE
A
LGORITHM
C
ONFIGURATION
3-157
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 will automatically build an MSTI tree to
maintain connectivity among 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).
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, the MST Instance ID 0 for the Common Spanning Tree when
spanning tree mode is set to MSTP (page 3-161), 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
messages at regular intervals. Any port that ages out STA information
(provided in the last configuration message) becomes the designated
port for the attached LAN. If it is a root port, a new root port is selected
from among the device ports attached to the network. (References to
“ports” in this section mean “interfaces,” which includes both ports and
trunks.)