Spanning Tree Algorithm
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Instruction Manual - NXA-ENET8-POE+
Configuring Global Settings for STA
Use the Spanning Tree > STA (Configure Global - Configure) page to configure global settings for the spanning tree that apply to
the entire switch.
Command Usage
Spanning Tree Protocol* - This option uses RSTP set to STP forced compatibility mode. It uses RSTP for the internal state
machine, but sends only 802.1D BPDUs. This creates one spanning tree instance for the entire network. If multiple VLANs
are implemented on a network, the path between specific VLAN members may be inadvertently disabled to prevent network
loops, thus isolating group members. When operating multiple VLANs, we recommend selecting the MSTP option.
Rapid Spanning Tree Protocol* - RSTP supports connections to either STP or RSTP nodes by monitoring the incoming
protocol messages and dynamically adjusting the type of protocol messages the RSTP node transmits, as described below:
STP Mode - If the switch receives an 802.1D BPDU (i.e., STP BPDU) after a port's migration delay timer expires, the
switch assumes it is connected to an 802.1D bridge and starts using only 802.1D BPDUs.
RSTP Mode - If RSTP is using 802.1D BPDUs on a port and receives an RSTP BPDU after the migration delay expires,
RSTP restarts the migration delay timer and begins using RSTP BPDUs on that port.
Multiple Spanning Tree Protocol - MSTP generates a unique spanning tree for each instance. This provides multiple
pathways across the network, thereby balancing the traffic load, preventing wide-scale disruption when a bridge node in a
single instance fails, and allowing for faster convergence of a new topology for the failed instance.
To allow multiple spanning trees to operate over the network, you must configure a related set of bridges with the same
MSTP configuration, allowing them to participate in a specific set of spanning tree instances.
A spanning tree instance can exist only on bridges that have compatible VLAN instance assignments.
Be careful when switching between spanning tree modes. Changing modes stops all spanning-tree instances for the
previous mode and restarts the system in the new mode, temporarily disrupting user traffic.
* - STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLAN boundaries.
The following table lists the options on this page:
Spanning Tree - STA Options
Basic Configuration of Global Settings
Spanning Tree Status
Enables/disables STA on this switch. (Default: Disabled)
When spanning tree is enabled globally or enabled on an interface, loopback detection is disabled.
Spanning Tree Type
Specifies the type of spanning tree used on this switch:
• STP: Spanning Tree Protocol (IEEE 802.1D); i.e., when this option is selected, the switch will use
RSTP set to STP forced compatibility mode).
• RSTP: Rapid Spanning Tree (IEEE 802.1w); RSTP is the default.
• MSTP: Multiple Spanning Tree (IEEE 802.1s)
Priority
Bridge priority is used in selecting the root device, root port, and designated port. The device with
the highest priority becomes the STA root device. However, if all devices have the same priority, the
device with the lowest MAC address will then become the root device. (Note that lower numeric
values indicate higher priority.)
• Default: 32768
• Range: 0-61440, in steps of 4096
• Options: 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056,
49152, 53248, 57344, 61440
BPDU Flooding
Configures the system to flood BPDUs to all other ports on the switch or just to all other ports in
the same VLAN when spanning tree is disabled globally on the switch or disabled on a specific port.
• To VLAN: Floods BPDUs to all other ports within the receiving port's native VLAN (i.e., as
determined by port's PVID). This is the default.
• To All: Floods BPDUs to all other ports on the switch.
The setting has no effect if BPDU
flooding is disabled on a port.
Cisco Prestandard Status
Configures spanning tree operation to be compatible with Cisco pre-standard versions. (Default:
Disabled)
Cisco pre-standard versions prior to Cisco IOS Release 12.2(25)SEC do not fully follow the IEEE
standard, causing some state machine procedures to function incorrectly. This command forces
the spanning tree protocol to function in a manner compatible with Cisco prestandard versions.
Advanced Configuration Settings
The following attributes are based on RSTP, but also apply to STP since the switch uses a backwards-compatible subset of RSTP to
implement STP, and also apply to MSTP which is based on RSTP according to the standard.
Path Cost Method
The path cost is used to determine the best path between devices. The path cost method is used to
determine the range of values that can be assigned to each interface.
• Long: Specifies 32-bit based values that range from 1-200,000,000. (This is the default.)
• Short: Specifies 16-bit based values that range from 1-65535.
Transmission Limit
The maximum transmission rate for BPDUs is specified by setting the minimum interval between
the transmission of consecutive protocol messages. (Range: 1-10; Default: 3)