STP Window

This window appears when you choose Device > STP on the menu bar. You can also click here to launch it.

This window has these tabs:

Begin by selecting a switch from the Host Name list. The information on the tabs pertains to the selected switch.

When you finish configuring STP, click OK.

STP Status

This 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. In the table, select one or more VLANs on which you want to disable (or enable) STP.
  2. Click Modify to display the Modify STP Status window.
  3. Select Disable (or Enable) from the STP Protocol list in the window.

Current Roots

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. The Current Roots Table describes these settings.

Root Parameters

This tab displays root switch parameters 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 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.
  2. Click Modify to display the Modify STP Root Parameters window.
  3. Enter the new values.
    Review the Root Parameters Table table if you need help.
  4. Click OK to put your changes in effect and to close the window.

Port Parameters

This tab has two purposes:

To change port parameters for a VLAN:

  1. Select the ID of the VLAN from the list in the VLAN ID area.
  2. In the table of ports, find the port whose parameters you want to change and select its row.
  3. Click Modify to display the Modify STP Port Parameters window.
  4. Enter the parameter changes you want.
  5. Click OK to put your changes into effect and to close the window.

UplinkFast

UplinkFast speeds the selection of a new root port (fast convergence) when a link or switch fails or when STP reconfigures itself. It 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 Cisco IOS Release 12.0(5)XU or later for UplinkFast to operate properly.

To enable UplinkFast:

  1. From the UplinkFast 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.
  2. In the 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.

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.

Cross-stack UplinkFast extends UplinkFast support across a stack of Catalyst switches that use GigaStack GBIC modules connected in a cascade configuration. If this feature is not enabled and a forwarding Gigabit Ethernet uplink fails, STP can take up to 50 seconds to place a redundant Gigabit Ethernet uplink into a forwarding state and restore the stack connectivity to the backbone. If this feature is enabled, the failover time is reduced to less than 1 second, irrespective of factors external to the stack.

If the selected device has one or more GigaStack GBICs installed, the UplinkFast tab displays a table in which you can enable cross-stack UplinkFast. If no GigaStack GBIC is installed, the cross-stack UplinkFast table does not appear. Instead, a message is displayed, stating that no GigaStack GBIC was detected and that this feature is only for GigaStack GBICs.

To enable cross-stack UplinkFast:

  1. Click in the Stack Port cell for the GigaStack GBIC port on which you want to enable cross-stack UplinkFast. A list drops down.
  2. Select Enable from the list.
    Note: You can enable cross-stack UplinkFast on only one GigaStack GBIC port.

If you disable UplinkFast, you also disable cross-stack UplinkFast.

Root Guard

Use Root Guard to put an interface into a blocked state. It then cannot become a path to the root switch, nor can it reside on the root switch. Root Guard is useful when a Layer 2 network includes connections to switches that it does not controlfor example, a customer network and its connections to a service provider network. The customer network would want to enable Root Guard on interfaces that connect to the service provider network.

To enable or disable Root Guard on an interface:

  1. Select an interface.
    Note: If the status of the interface is NA-line protocol is down, CMS cannot determine whether Root Guard is enabled or disabled on the interface.
  2. Click Modify and use the Modify Root Guard window.

Backbone Fast

Check the Enable box to enable Backbone Fast on the selected switch.

The status window shows the status of Backbone Fast (enabled or not) and displays these statistics:

Current Roots Table

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

FieldDescription
VLAN IDThe VLAN to which these settings apply when the switch acts as the root.
MAC AddressThe MAC address of the root switch.
PriorityIdentifies 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 AgeSets 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 TimeSets 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 DelaySets 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 CostA 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 instance 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 these meanings:

FieldDescription
VLAN IDThe VLAN to which these root settings apply (read-only).
ProtocolThe protocol used on this VLAN, IEEE or IBM.
PriorityIdentifies 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 AgeSets 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 TimeSets 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 DelaySets 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 these meanings:

ParameterDescription
PortPort to which these settings apply (read-only).
StateOne of several states in which ports exist (read-only). See the Port State Table for state descriptions.
Root CostThe 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). You can choose to enable it for static-access ports only or for static-access and trunk ports.
Path CostA weight assigned to a port based on its speed. A lower path cost represents higher-speed transmission. See the Path Cost Table for details.
PriorityA weight assigned to a port to affect its selection to carry traffic.

Port State Table

A port can be in one of these states:

StateDescription
BlockingThe port is not participating in the frame-forwarding process and is not learning new addresses.
ListeningThe port is not participating in the frame-forwarding process but is progressing toward a forwarding state. The port is not learning addresses.
LearningThe port is not forwarding frames but is learning addresses.
Forwarding The port is forwarding frames and learning addresses.
DisabledThe port has been removed from STP operation.
DownThe port has no physical link.
BrokenOne 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 CostSpeed
10010 Mbps
19100 Mbps
14155 Mbps
41 Gbps
210 Gbps
1speeds greater than 10 Gbps