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.
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:
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.
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:
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:
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:
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.
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.
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. |
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. |
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. |
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 |