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Configuring the Switch
3-116
3
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 (i.e.,
around 1 to 3 seconds, compared to 30 seconds or more for 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 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 3-119), 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
x
Designated
Root
Designated
Port
Designated
Bridge
x x
x
Root
Port
x