Spanning-Tree Protocol

With the Spanning-Tree Protocol (STP) page, 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 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.

Configuring UplinkFast for Fast Convergence

UplinkFast, which is offered with the Enterprise Edition Software, 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 is most commonly used in hierarchical switch networks composed of backbone, distribution, and access switches in which distribution and access switches each have at least one redundant link that is blocked by STP to prevent loops.

Note: UplinkFast is most useful in edge or access switches and might not be appropriate for backbone devices.

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.

Note: If you disable STP on a VLAN, UplinkFast has no effect. All 2900 and 3500 XL switches must be running IOS release 12.0(5)XP for UplinkFast to operate properly.

To configure UplinkFast:

  1. Select the Enable UplinkFast checkbox.
    You enable UplinkFast on the entire switch; it applies to all STP instances.
  2. In the UplinkFast Max Update Rate 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.
  3. Click Apply.
    Note: When you enable UplinkFast, the priority of all VLANs 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 chance that the switch will become the root or that its ports will become the designated port on a LAN segment.

Note: To disable UplinkFast, deselect the Enable UplinkFast checkbox. The bridge priorities of all VLANs and the path costs of all ports are set to their default values or the values you set.

Modifying Spanning-Tree Parameters for Existing VLANs

To modify spanning-tree parameters for existing VLANs:

  1. In the VLAN ID list box, select a VLAN ID.
  2. Click Modify STP Parameters.
    The Spanning-Tree Protocol page displays the current spanning-tree parameter settings.
  3. Modify STP parameters as necessary:
    Disable STP.
    View the current spanning-tree root settings.
    Configure spanning-tree options.
    Change spanning-tree parameters on a port.
    Check the port forwarding status.

    For more information, review the field descriptions.

Disabling STP

STP is enabled by default.

Note: You can avoid the 30-second delay before a port can forward packets by disabling STP. However, disable STP only if you are sure there are no loops in your network topology. With STP disabled and loops present in the topology, network performance is degraded by excessive traffic and indefinite packet duplication.

To disable STP:

  1. Deselect the Enable Spanning Tree checkbox.
  2. Click Apply.

Viewing the Current Spanning-Tree Root Settings

The Current Spanning-Tree Root section displays the read-only STP settings for the current root switch, which could be defined on another switch.

The MAC Address field contains the MAC address of the switch currently acting as the root. For more information, review the field descriptions.

Configuring Spanning-Tree Options

The Spanning-Tree Options section displays a list of options that this switch will use as the root switch.

To change the configuration of STP on this switch:

  1. Enter values for the Protocol, Priority, Max Age, Hello Time, Forward Delay fields.
    For more information, review the field descriptions.
  2. Click Apply.

Changing Spanning-Tree Parameters on a Port

To change port-specific parameters that affect how the port responds if a loop is formed:

  1. In the Path Cost field, enter a number from 1 to 65535. This table contains default path-cost settings:

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

    A lower path cost represents higher-speed transmission; this setting can affect which port remains enabled in the event of a loop.

  2. In the Priority field, enter a number from 0 to 255 for IEEE, DEC, or IBM.
    The default for all three protocols is 128.
    The lower the number, the higher the priority.
  3. In the Port Fast column, select the Port Fast checkbox to enable the Port Fast feature (if the port is connected to an end station).
    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).
    Note: The Port Fast feature does not apply to ATM ports. In Enterprise Edition Software, the Port Fast feature is automatically enabled on dynamic-access ports.
  4. Click Apply.

Checking the Port Forwarding Status

To check the port forwarding status, view the State column in the Ports Parameters 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.

Field Descriptions

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

Field Description
Protocol Determines which implementation of STP to use (IEEE, DEC, and IBM). IEEE is the default.
Priority Identifies the root bridge. The switch with the lowest value has the highest priority and is selected as the root. For IEEE, DEC, and IBM, enter a number from 0 to 65535. The default for IEEE and IBM is 32768; the default for DEC is 128.
Max Age Sets the number of seconds a switch waits without receiving STP configuration messages before it attempts a reconfiguration. For IEEE, DEC, and IBM, enter a number from 6 to 200. The default for IEEE is 20 seconds; the default for DEC is 15 seconds; the default for IBM is 10 seconds.
Hello Time Sets the number of seconds between STP configuration messages. For IEEE, DEC, and IBM, enter a number from 1 to 10. The default for IEEE and IBM is 2 seconds; the default for DEC is 1 second.
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, DEC, and IBM, enter a number from 4 to 200. The default for IEEE is 15 seconds; the default for DEC is 30 seconds; the default for IBM is 4 seconds.

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.