Spanning Tree Algorithm
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Instruction Manual - NXA-ENET8-POE+
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 the
Configuring Global Settings for
An interface cannot function as an edge port under the following conditions:
• If spanning tree mode is set to STP, edge-port mode cannot automatically transition to
operational edge-port state using the automatic setting.
• If loopback detection is enabled and a loopback BPDU is detected, the interface cannot function
as an edge port until the loopback state is released.
• 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 the
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).
BPDU Guard
This feature protects edge ports from receiving BPDUs. It prevents loops by shutting down an edge
port when a BPDU is received instead of putting it into the spanning tree discarding state. In a valid
configuration, configured edge ports should not receive BPDUs. If an edge port receives a BPDU an
invalid configuration exists, such as a connection to an unauthorized device. The BPDU guard
feature provides a secure response to invalid configurations because an administrator must
manually enable the port. (Default: Disabled)
BPDU guard can only be configured on an interface if the edge port attribute is not disabled (that
is, if edge port is set to enabled or auto).
BPDU Guard Auto Recovery
Automatically re-enables an interface after the specified interval. (Range: 30-86400 seconds;
Default: Disabled)
BPDU Guard Auto Recovery Interval
The time to wait before re-enabling an interface. (Range: 30-86400 seconds; Default: 300
seconds)
BPDU Filter
BPDU filtering allows you to avoid transmitting BPDUs on configured edge ports that are
connected to end nodes. By default, STA sends BPDUs to all ports regardless of whether
administrative edge is enabled on a port. BPDU filtering is configured on a per-port basis. (Default:
Disabled)
BPDU filter can only be configured on an interface if the edge port attribute is not disabled (that is,
if edge port is set to enabled or auto).
Migration
If at any time the switch detects STP BPDUs, including Configuration or Topology Change
Notification BPDUs, it will automatically set the selected interface to forced STP-compatible mode.
However, you can also use the Protocol Migration button to manually re-check the appropriate
BPDU format (RSTP or STP-compatible) to send on the selected interfaces. (Default: Disabled)
TC Propagate Stop
Stops the propagation of topology change notifications (TCN). (Default: Disabled)
Recommended STA Path Cost Range
Port Type
IEEE 802.1D-1998
IEEE 802.1w-2001
Ethernet
50-600
200,000-20,000,000
Fast Ethernet
10-60
20,000-2,000,000
Gigabit Ethernet
3-10
2,000-200,000
10G Ethernet
1-5
200-20,000
Spanning Tree - STA Options