Use the Spanning Tree > STA (Configure Global - Configure) page to configure global settings for the spanning tree that apply to the entire switch.
Basic Configuration of Global Settings
Enables/disables STA on this switch. (Default: Disabled)
Specifies the type of spanning tree used on this switch:
Spanning Tree Protocol (IEEE 802.1D); i.e., when this option is selected, the switch will use RSTP set to STP forced compatibility mode).
Rapid Spanning Tree (IEEE 802.1w); RSTP is the default.
Multiple Spanning Tree (IEEE 802.1s)
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
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.
This setting has no effect if BPDU flooding is disabled on a port.
Configures spanning tree operation to be compatible with Cisco prestandard versions. (Default: Disabled)
Cisco prestandard 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. The 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.
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.
Specifies 32-bit based values that range from 1-200,000,000. (This is the default.)
Specifies 16-bit based values that range from 1-65535.
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)
When the Switch Becomes Root
Interval (in seconds) at which the root device transmits a configuration message.
Default: 2
Minimum: 1
Maximum: The lower of 10 or [(Max. Message Age / 2) -1]
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.)
Default: 20
Minimum: The higher of 6 or [2 x (Hello Time + 1)]
Maximum: The lower of 40 or [2 x (Forward Delay - 1)]
The maximum time (in seconds) this device will wait before changing states (i.e., discarding to learning to forwarding). This delay is required because every device must receive information about topology changes before it starts to forward frames. In addition, each port needs time to listen for conflicting information that would make it return to a discarding state; otherwise, temporary data loops might result..)
Default: 15
Minimum: The higher of 4 or [(Max. Message Age / 2) + 1]
Maximum: 30
Configuration Settings for MSTP
The maximum number of MSTP instances to which this switch can be assigned.
An MD5 signature key that contains the VLAN ID to MST ID mapping table. In other words, this key is a mapping of all VLANs to the CIST.
The revision for this MSTI. (Range: 0-65535; Default: 0)
The name for this MSTI. (Maximum length: 32 characters; Default: switch's MAC address)
The maximum number of hops allowed in the MST region before a BPDU is discarded. (Range: 1-40; Default: 20)
Note: Region Revision and Region Name and are both required to uniquely identify an MST region.
Use the Spanning Tree > STA (Configure Global - Show Information) page to display a summary of the current bridge STA information that applies to the entire switch.
The parameters displayed in the web interface are described in the preceding section, except for the following items:
A unique identifier for this bridge, consisting of the bridge priority, the MST Instance ID 0 for the Common Spanning Tree when spanning tree type is set to MSTP, and MAC address (where the address is taken from the switch system).
The priority and MAC address of the device in the Spanning Tree that this switch has accepted as the root device.
The number of the port on this switch that is closest to the root. This switch communicates with the root device through this port. If there is no root port, then this switch has been accepted as the root device of the Spanning Tree network.
The path cost from the root port on this switch to the root device.
The number of times the Spanning Tree has been reconfigured.
Time since the Spanning Tree was last reconfigured.
Use the Spanning Tree > STA (Configure Interface - Configure) page to configure RSTP and MSTP attributes for specific interfaces, including port priority, path cost, link type, and edge port. You may use a different priority or path cost for ports of the same media type to indicate the preferred path, link type to indicate a point-to-point connection or shared-media connection, and edge port to indicate if the attached device can support fast forwarding. (References to "ports" in this section means "interfaces," which includes both ports and trunks.)
Displays a list of ports or trunks.
Enables/disables STA on this interface. (Default: Enabled)
Enables/disables the flooding of BPDUs to other ports when global spanning tree is disabled or when spanning tree is disabled on a specific port. When flooding is enabled, BPDUs are flooded to all other ports on the switch or to all other ports within the receiving port's native VLAN as specified by the global spanning tree BPDU Flooding attribute. (Default: Enabled)
Defines the priority used for this port in the Spanning Tree Protocol. If the path cost for all ports on a switch are the same, the port with the highest priority (i.e., lowest value) will be configured as an active link in the Spanning Tree. This makes a port with higher priority less likely to be blocked if the Spanning Tree Protocol is detecting network loops. Where more than one port is assigned the highest priority, the port with lowest numeric identifier will be enabled.
Default: 128
Range: 0-240, in steps of 16
This parameter is used by the STA to determine the best path between devices.
Therefore, lower values should be assigned to ports attached to faster media,
and higher values assigned to ports with slower media. (Path cost takes
precedence over port priority.)
(Range: 0 for auto-configuration, 1-65535 for the short path cost method,
1-200,000,000 for the long path cost method)
By default, the system automatically detects the speed and duplex mode used
on each port, and configures the path cost according to the values shown
below. Path cost "0" is used to indicate auto-configuration mode.
When the short path cost method is selected and the default path cost recommended
by the IEEE 8021w standard exceeds 65,535, the default is set to 65,535.
Administrative path cost cannot be used to directly determine the root port on a switch. Connections to other devices use IEEE 802.1Q-2005 to determine the root port.
The link type attached to this interface.
A connection to exactly one other bridge.
A connection to two or more bridges.
The switch automatically determines if the interface is attached to a point-to-point link or to shared media. (This is the default setting.)
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)
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: Disabled)
Manually configures a port as an Edge Port.
Disables the Edge Port setting.
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.
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.
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).
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).
Automatically re-enables an interface after the specified interval. (Range: 30-86400 seconds; Default: Disabled)
The time to wait before re-enabling an interface. (Range: 30-86400 seconds; Default: 300 seconds)
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).
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)
Stops the propagation of topology change notifications (TCN). (Default: Disabled))
Use the Spanning Tree > STA (Configure Interface - Show Information) page to display the current status of ports or trunks in the Spanning Tree.
Shows if STA has been enabled on this interface.
Shows if BPDUs will be flooded to other ports when spanning tree is disabled globally on the switch or disabled on a specific port.
Displays current state of this port within the Spanning Tree:
Port receives STA configuration messages, but does not forward packets.
Port has transmitted configuration messages for an interval set by the Forward Delay parameter without receiving contradictory information. Port address table is cleared, and the port begins learning addresses.
Port forwards packets, and continues learning addresses.
The rules defining port status are:
A port on a network segment with no other STA compliant bridging device is always forwarding.
If two ports of a switch are connected to the same segment and there is no other STA device attached to this segment, the port with the smaller ID forwards packets and the other is discarding.
All ports are discarding when the switch is booted, then some of them change state to learning, and then to forwarding.
The number of times this port has transitioned from the Learning state to the Forwarding state.
The cost for a packet to travel from this port to the root in the current Spanning Tree configuration. The slower the media, the higher the cost.
The bridge priority and MAC address of the device through which this port must communicate to reach the root of the Spanning Tree.
The port priority and number of the port on the designated bridging device through which this switch must communicate with the root of the Spanning Tree.
The contribution of this port to the path cost of paths towards the spanning tree root which include this port.
The operational point-to-point status of the LAN segment attached to this interface. This parameter is determined by manual configuration or by auto-detection.
This parameter is initialized to the setting for Admin Edge Port (i.e., true or false), but will be set to false if a BPDU is received, indicating that another bridge is attached to this port.
Roles are assigned according to whether the port is part of the active topology, that is the best port connecting a non-root bridge to the root bridge (i.e., root port), connecting a LAN through the bridge to the root bridge (i.e., designated port), is the MSTI regional root (i.e., master port), or is an alternate or backup port that may provide connectivity if other bridges, bridge ports, or LANs fail or are removed. The role is set to disabled (i.e., disabled port) if a port has no role within the spanning tree.
The criteria used for determining the port role is based on root bridge ID, root path cost, designated bridge, designated port, port priority, and port number, in that order and as applicable to the role under question.