Configuring STP Interface Settings on the SG350XG and SG550XG

Objective

Spanning Tree Protocol (STP) is a network protocol that prevents the occurrence of loops in the topology. These loops cause switches to forward traffic an infinite amount of times. This causes the network to flood and use its resources which reduces network efficiency.

STP interface settings are used to increase efficiency of STP on a per port basis. Using the edge port function, fast link increases the speed of STP convergence by setting a port to a forwarding state when a device is connected. The Root Guard and Bridge Protocol Data Unit (BPDU) Guard are used to control the STP topology. This extra control in the topology prevents any occurrence of bridge loops.

The objective of this document is to show you how to configure STP Interface Settings on the SG350XG and SG550XG.

Note: The steps in this document are performed under the Advanced Display Mode. To change to the Advanced Display Mode, go to the top right corner and select Advanced in the Display Mode drop-down list.

Applicable Devices

• SG350XG
• SG550XG

Software Version

• SG350XG – v2.0.0.73
• SG550XG – v2.0.0.73

Configuring STP Interface Settings

Step 1. Log in to the web configuration utility and choose Spanning Tree > STP Interface Settings. The STP Interface Settings page opens:

Step 2. In the Filter: Interface Type equals to drop-down list, select the desired Port of Unit or LAG. Then click Go.

Note: You will have more options (e.g. Port of Unit 2) if there are more units in the stack.

Step 3. The STP Interface Setting Table displays information of all the interfaces currently configured on the switch. Select a radio button and click Edit… to edit its settings in the Edit STP Interface Setting window that appears.

Step 4. In the Interface field, select a radio button. You can choose either Unit and Port or LAG. If you chose LAG, then skip to Step 7.

Step 5. In the Unit drop-down list, select the unit you want to configure.

Step 6. In the Port drop-down list, select the port you want to configure, then skip to Step 8.

Step 7. If you have chosen LAG in step 4, select the desired LAG port that you want to configure.

Step 8. In the STP field, check the Enable box if you want to enable STP on the port. This is checked by default.

Step 9. In the Edge Port field, you can choose either Enable, Auto, or Disable. If Fast Link mode is enabled on a port, the port is automatically set to Forwarding state when the port link is up. Fast Link is also known as port-fast. STP works by “listening” for about 30-45 seconds. With Fast Link enabled, it only listens for about 5 seconds before transitioning into forwarding state.

The options are defined as:

• Enable – Enables Fast Link immediately.
• Auto – Enables Fast Link a few seconds after the interface becomes active. This allows STP to resolve loops before enabling Fast Link.
• Disable – Disables Fast Link.

Step 10. Root Guard option provides a way to enforce the root bridge placement in the network. Check the Enable box if you want to enable Root Guard.

Step 11. Bridge Protocol Data Units (BPDUs) are exchanged across bridges to detect loops in a network topology. The BPDU Guard enables you to enforce the STP domain borders and keep the active topology predictable. The devices behind the ports that have BPDU Guard enabled cannot influence the STP topology. At the reception of BPDUs, the BPDU guard operation disables the port that has BPDU configured. In this case, a BPDU message is received and an appropriate SNMP trap is generated. Check the Enable box if you want to enable BPDU Guard.

Step 12. In theBPDUHandling field, select how BPDU packets are managed when STP is disabled on the port or the device. BPDUs are used to transmit spanning tree information.

The available options are:

• Use Global Settings – Select to use the settings defined in the
• STP Status and Global Settings on the SG350XG and SG550XG page.
• Filtering – Filters BPDU packets when Spanning Tree is disabled on an interface.
• Flooding – Floods BPDU packets when Spanning Tree is disabled on an interface.

Step 13. In the Path Cost field, select either Use Default which uses the default cost generated by the system or User Defined which set the port contribution to the root path cost.

Step 14. In the Priority field, set the priority value of the port. The priority value influences the port choice when a bridge has two ports connected in a loop. The priority is a value from 0 – 240, set in increments of 16. The lowest priority is 0 and the highest priority is 240.

The Port State displays the current STP state of a port.

The states are defined as:

• Disabled – STP is currently disabled on the port. The port forwards traffic while learning MAC addresses.
• Blocking – The port is currently blocked and cannot forward traffic (with the exception of BPDU data) or learn MAC addresses.
• Listening – The port is in listening mode. The port cannot forward traffic and cannot learn MAC addresses.
• Learning – The port is in learning mode. The port cannot forward traffic, but it can learn new MAC addresses.
• Forwarding – The port is in forwarding mode. The port can forward traffic and learn new MAC addresses.

The Designated Bridge ID displays the bridge priority and the MAC address of the designated bridge.

The Designated Port ID displays the priority and interface of the selected port.

The Designated Cost displays the cost of the port participating in the STP topology. Ports with a lower cost are less likely to be blocked if STP detects loops.

The Forward Transitions displays the number of times the port has changed from the Blocking state to Forwarding state.

The Speed displays the speed of the port.

Note: This is not available if you have chosen LAG in Step 4.

The LAG displays the LAG to which the port belongs. If a port is a member of a LAG, the LAG settings override the port settings.

Note: This is not available if you have chosen LAG in Step 4.

Step 15. Click Apply. The interface settings are written to the Running Configuration file.

Step 16. If you want to quickly copy a port’s settings to another port or group of ports, select its radio button in the STP Interface Settings and click the Copy Settings… button.

Step 17. In the Copy Settings window, enter the port(s) that you want to copy to in the text field. You can specify multiple ports, separated by commas, or a range of ports.

Step 18. Click Apply. The settings are copied.

Revision History

Revision	Publish Date	Comments
1.0	13-Dec-2018	Initial Release