Web Management Guide

Chapter 7 | Spanning Tree Algorithm
Configuring Interface Settings for STA
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◆ Root Guard – STA allows a bridge with a lower bridge identifier (or same
identifier and lower MAC address) to take over as the root bridge at any time.
Root Guard can be used to ensure that the root bridge is not formed at a
suboptimal location. Root Guard should be enabled on any designated port
connected to low-speed bridges which could potentially overload a slower link
by taking over as the root port and forming a new spanning tree topology. It
could also
be used to form a border around part of the network where the root
bridge is allowed. (Default: Disabled)
◆ Admin Edge Port – Since end nodes cannot cause forwarding loops, they can
pass directly through to the spanning tree forwarding state. Specifying Edge
Ports provides quicker convergence for devices such as workstations or servers,
retains the current forwarding database to reduce the amount of frame
flooding required to rebuild address tables during reconfiguration events, does
not cause the spanning tree to initiate reconfiguration when the interface
changes state, and also overcomes other STA-related timeout problems.
However, remember that Edge Port should only be enabled for ports
connected to an end-node device. (Default: Auto)
■
Enabled – Manually configures a port as an Edge Port.
■
Disabled – Disables the Edge Port setting.
■
Auto – The port will be automatically configured as an edge port if the
edge delay time expires without receiving any RSTP or MSTP BPDUs. Note
that edge delay time (802.1D-2004 17.20.4) equals the protocol migration
time if a port's link type is point-to-point (which is 3 seconds as defined in
IEEE 802.3D-2004 17.20.4); otherwise it equals the spanning tree’s
maximum age for configuration messages (see maximum age under
“Configuring Global Settings for STA” on page 175).
An interface cannot function as an edge port under the following conditions:
■
If spanning tree mode is set to STP (page 175), edge-port mode cannot
automatically transition to operational edge-port state using the automatic
setting.
■
If an interface is in forwarding state and its role changes, the interface
cannot continue to function as an edge port even if the edge delay time
has expired.
■
If the port does not receive any BPDUs after the edge delay timer expires,
its role changes to designated port and it immediately enters forwarding
state (see “Displaying Interface Settings for STA” on page 186).
When edge port is set as auto, the operational state is determined
automatically by the Bridge Detection State Machine described in 802.1D-2004,
where the edge port state may change dynamically based on environment
changes (e.g., receiving a BPDU or not within the required interval).