Spanning-Tree Protocol

With the Spanning-Tree Protocol (STP) window, you can:

The STP is a standardized technique for maintaining a network of multiple bridges or switches. When the topology changes, STP transparently reconfigures bridges and switches to avoid the creation of loops by placing ports in a forwarding or blocking state. Each VLAN is treated as a separate bridge and a separate instance of STP is applied to each.

STP parameters are set for each VLAN. For each spanning-tree instance, you can configure a set of global options and a set of port parameters. The Port Parameter list contains only ports that are members of a given VLAN. A maximum of 64 spanning-tree instances are supported.

Enabling and Disabling STP

The STP Status tab shows whether STP is enabled for each VLAN on the switch. STP is enabled by default. However, by disabling STP, you can avoid the 30-second delay in packet forwarding from a port when a switch reconfigures.

Note: Disable STP only if you are sure there are no loops in your network topology. If STP is disabled and loops are present in the topology, network performance is degraded by excessive traffic and indefinite packet duplication.

To disable (or enable) STP:

  1. Select the STP Status tab on the Spanning-Tree Protocol window.
  2. In the STP Status table, select one or more VLANs on which you want to disable (or enable) STP.
  3. Click Modify to display the STP Protocol Configuration dialog box.
  4. Select Disable (or Enable) from the drop-down list in the dialog box.
  5. Click OK to disable (or enable) STP on the selected VLANs and close the dialog box.
    Your change is recorded on the STP Status table.
  6. Click OK to record your changes and close the Spanning-Tree Protocol window.

Viewing the Current Root Settings

For each VLAN, the Current Root tab (a read-only tab) displays the STP settings on the current root switch. These settings, which could be defined on another switch, define the parameters that take effect when the switch is acting as the VLAN root. See the Current Roots Table for a description of these settings.

Changing Parameters on the STP Root Switch

The Root Parameters tab displays a list of configurable settings that specify root switch behavior for the selected switch. See the Root Parameters Table for an explanation of these parameters.

To change the configuration of STP for a root switch:

  1. Select the Root Parameters tab on the Spanning Tree Protocol window.
  2. In the table on this tab, select one or more VLANs that you want to change.
    To select multiple VLANs, hold down the Ctrl key and select VLANs individually; or, hold down the Shift key and select the first and last VLAN in a range.
  3. Click Modify to display the STP Root Configuration dialog box.
  4. Enter the new values.
    Review the Root Parameters Table table if you need help.
  5. Click OK on the STP Root Configuration dialog box to put your changes in effect and close the dialog box.
  6. Click OK to close the Spanning-Tree Protocol window.

Changing Spanning-Tree Parameters on a Port

The Port Parameters tab displays a list of parameters for VLAN ports on a particular switch. These parameters affect how the port responds if a loop is formed, and some settings cannot be changed. Refer to the Port Parameters Table for a description of each field.

To change port-specific parameters for VLANs:

  1. Select the Port Parameters tab on the Spanning Tree Protocol window.
  2. In the table on this tab, select one or more VLANs that you want to change.
    To select multiple VLANs, hold down the Ctrl key and select VLANs individually; or, hold down the Shift key and select the first and last VLAN in a range.
  3. Click Modify to display the STP Port Configuration dialog box.
  4. Enter your new parameter settings using information in the Port Parameters Table and these guidelines:
    Port Fast: Enable the Port Fast feature only if the port is connected to an end station. The Port Fast feature is automatically enabled on VMPS dynamic-access ports, and cannot be enabled on ATM ports.
    Path Cost: Enter a number from 1 to 65535. A lower path cost represents higher-speed transmission; this setting can affect which port remains enabled in the event of a loop. See the Path Cost Table.
    Priority: Enter a number from 0 to 255 for IEEE or IBM. The default for all three protocols is 128. The lower the number, the higher the priority.
  5. Click OK on the STP Port Configuration dialog box to put your changes in effect and close the dialog box.
  6. Click OK to close the Spanning-Tree Protocol window.

Configuring UplinkFast for Fast Convergence

UplinkFast is an enhancement to STP that speeds the selection of a new root port (fast convergence) when a link or switch fails or when STP reconfigures itself. UplinkFast enables the switch to begin using the alternate paths as soon as STP selects a new root port. The root port immediately transitions to the forwarding state without going through the listening and learning states, as it would with normal STP procedures. UplinkFast is most useful in hierarchical switch networks where edge or access switches have at least one redundant link that is blocked by STP to prevent loops.

Note: UplinkFast might not be appropriate for backbone devices.

UplinkFast settings apply to all STP instances. The switch must be running IOS release 12.0(5)XU for UplinkFast to operate properly.

To configure UplinkFast:

  1. Select the UplinkFast tab from the Spanning Tree Protocol window.
  2. From the UplinkFast drop-down list, select Enable.
    When you enable UplinkFast, the priority of all VLANs on the switch is increased to 49152, and the path cost of all ports and VLAN trunks is increased by 3000 (not exceeding 65535). These changes reduce the likelihood that the switch will become the root or that its ports will become the designated port on a LAN segment.
    Note: If you disable STP on a VLAN, UplinkFast has no effect (see Enabling and Disabling STP).
  3. In the Max. Update Rate (0 -1000 Packets/Seconds) field, specify the number of multicast packets per second the switch generates to permit upstream switches to relearn addresses.
    Through this field, you limit the bursts of multicast traffic (one packet for each address that was learned on the port) that flood the network when STP reconfigures.
    The default is 500 packets per second; the range is from 0 to 1000.
    If you set the value to 0, multicast packet generation is disabled, and the STP topology converges more slowly after a loss of connectivity.
  4. Click Apply to put your changes on this tab in effect.
  5. Click OK to close the Spanning Tree Protocol window.

Note: When you disable UplinkFast, the bridge priorities of all VLANs on the switch and the path costs of all ports are set to their default values or the values you set.

Current Roots Table

These fields define the parameters that take effect when a switch is acting as the root:

Field Description
VLAN ID The VLAN to which these settings apply when the switch acts as the root.
MAC Address The MAC address of the root switch.
Priority Identifies the root bridge. The switch with the lowest value has the highest priority and is selected as the root. For IEEE and IBM, enter a number from 0 to 65535. The default for IEEE and IBM is 32768.
Max Age Sets the number of seconds a switch waits without receiving STP configuration messages before it attempts a reconfiguration. For IEEE and IBM, enter a number from 6 to 200. The default for IEEE is 20 seconds; the default for IBM is 10 seconds.
Hello Time Sets the number of seconds between STP configuration messages. For IEEE and IBM, enter a number from 1 to 10. The default for IEEE and IBM is 2 seconds.
Forward Delay Sets the number of seconds a port waits before changing from its STP learning and listening states to the forwarding state. This delay time is necessary to ensure that no loop is formed before the switch forwards a packet. For IEEE and IBM, enter a number from 4 to 200. The default for IEEE is 15 seconds; the default for IBM is 4 seconds.
Path Cost A relative measure used to determine the most favorable path to a destination. See the path cost table for details.
Port The port to which these settings apply.

Note: Each switch in a spanning tree adopts the hello, delay, and max age parameters of the root bridge regardless of how it is configured.

Root Parameters Table

Fields in the Root Parameters tab are configurable and have the following meaning:

Field Description
VLAN ID The VLAN to which these root settings apply (read-only).
Protocol The protocol used on this VLAN, IEEE or IBM.
Priority Identifies the root bridge. The switch with the lowest value has the highest priority and is selected as the root. For IEEE and IBM, enter a number from 0 to 65535. The default for IEEE and IBM is 32768.
Max Age Sets the number of seconds a switch waits without receiving STP configuration messages before it attempts a reconfiguration. Enter a number from 6 to 200. The default for IEEE is 20 seconds; the default for IBM is 10 seconds.
Hello Time Sets the number of seconds between STP configuration messages. Enter a number from 1 to 10. The default is 2 seconds for IEEE and IBM.
Forward Delay Sets the number of seconds a port waits before changing from its STP learning and listening states to the forwarding state. This delay time is necessary to ensure that no loop is formed before the switch forwards a packet. Enter a number from 4 to 200. The default for IEEE is 15 seconds; the default for IBM is 4 seconds.

Port Parameters Table

Fields on the Port Parameters tab have the following meaning:

Parameter Description
Port Port to which these settings apply (read-only).
State One of several states in which ports exist (read-only). See the Port State Table for state descriptions.
Root Cost The path cost to the VLAN root (read-only). For example, a 100-Mbps port with a path cost of 19 that is 3 hops away from the VLAN root has a root cost of 57.
Port Fast Port Fast immediately brings a port from the blocking state into the forwarding state by eliminating the forward delay (the amount of time a port waits before changing from its STP learning and listening states to the forwarding state).
Path Cost A weight assigned to a port based on its speed. A lower path cost represents higher-speed transmission. See the Path Cost Table for details.
Priority A weight assigned to a port to affect its selection to carry traffic.

Port State Table

A port can be in one of the following states:

State Description
Blocking The port is not participating in the frame-forwarding process and is not learning new addresses.
Listening The port is not participating in the frame-forwarding process but is progressing toward a forwarding state. The port is not learning addresses.
Learning The port is not forwarding frames but is learning addresses.
Forwarding The port is forwarding frames and learning addresses.
Disabled The port has been removed from STP operation.
Down The port has no physical link.
Broken One end of the link is configured as an access port and the other end is configured as an 802.1Q trunk port; or, both ends of the link are configured as 802.1Q trunk ports but have different native VLAN IDs.

Path Cost Table

This table explains default path-cost settings for different speeds.

Path Cost Speed
100 10 Mbps
19 100 Mbps
14 155 Mbps
4 1 Gbps
2 10 Gbps
1 speeds greater than 10 Gbps