Spanning Tree Protocol (STP) Interface Settings on ESW2-550X Switch

Objective

Spanning Tree Protocol (STP) is a network protocol that prevents loops from occurring in the topology. These loops if caused in a network use all the resources available on the network and reduce network efficiency.

STP interface settings are used to increase efficiency of STP on a per port basis. The edge port function fast link increases the speed of STP convergence by use of a port set 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 the bridge loops from occurring.

This article explains how to configure STP interface settings on the ESW2-550X stackable managed switches.

Applicable Devices

• ESW2-550X
• ESW2-550X-DC

Software Version

• v1.2.9.44

STP Interface Settings

Before the configuration of the STP interface settings, the user should choose the Classic STP mode of operation from Spanning Tree > STP Status & Global Settings. Refer to the article Configure Spanning Tree Protocol (STP) Status and Global Settings on ESW2-550X Switch.

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

Step 2. Choose the desired interface from the Filter: Interface Type equals to drop-down list. Click Go.

Step 3. Click the radio button of the port or LAG that you want to modify and click Edit. The Edit STP Interface Settings window appears:

 

Step 4. (Optional) Click the radio button of the interface to be edited in the Interface field.

• Unit/Slot and Port — The unit identifies the whether the switch is active or a member in the stack (unit 1 is active and unit 2 is member). If you are unfamiliar with the terms used, check out Cisco Business: Glossary of New Terms. The slot identifies whether the switch is ESW2-550 or ESW2-550X (slot 1 is ESW2-550 and slot 2 is ESW2-550X). Choose the desired option from the Unit/Slot drop-down list and choose the desired port from the Port drop-down list.

• LAG — Choose on which LAG the STP is advertised from the LAG drop-down list. A Link Aggregate Group (LAG) is used to link multiple ports together. LAGs multiply bandwidth, increase port flexibility, and provide link redundancy between two devices to optimize port usage.

Step 5. Check the Enable check box in the STP field to enable STP on the interface.

Step 6. Click the radio button that corresponds to the desired use of fast link in the Edge Port field. Fast link is used to automatically set a port or lag to forwarding state when the port or lag links up. Fast link optimizes the STP convergence.

• Enable — Enables fast link immediately.

• Auto — Enables fast link a few seconds after the interface becomes active. This allows STP to resolve loops before fast link is enabled.

• Disable — Disables fast link.

Step 7. Check the Enable check box in the Root Guard field to enable Root Guard on the interface. This option provides a way to enforce the root bridge placement in the network. Root Guard is used to prevent newly connected devices from taking root and affecting the STP topology.

Step 8. Check the Enable check box in the BPDU Guard field to enable Bridge Protocol Data Unit (BPDU) Guard on the interface. BPDU Guard allows you to enforce the STP domain borders on port and keep the active topology predictable. Devices that are connected to a port that have BPDU enabled cannot affect the STP topology. When the switch receives a BPDU on the port with BPDU Guard enabled, the port is disabled and a SNMP trap is generated.

Step 9. Click the radio button that corresponds to the desired option in the BPDU Handling field. This determines how BPDU packets are managed when STP is disabled on the port. BPDUs are used to transmit STP information.

• Use Global Settings — Uses the settings defined on the STP Status and Global Settings page.

• Filtering — Filters BPDU packets when STP is disabled on the interface.

• Flooding— Floods BPDU packets when STP is disabled on the interface.

Step 10. Path cost is calculated by the bandwidth between the switches. Root port is selected based on the path cost. The port with the lowest path cost to the root bridge becomes the root port. Click the radio button that corresponds to the desired path cost in the Path Cost field. 

• Use Default — Use the default cost generated by the system.

• User Defined — Enter a value for the path cost in the User Defined field.

Step 11. From the Priority drop-down list choose the priority value of the interface. The priority value determines port choice when a bridge has two ports which can form a loop. The lower the value, the higher priority the port will have on the bridge will be in forwarding state.

The following fields display statistics of the Interface.

• Port State — Current state of the specified port.

– Disabled — STP is disabled on the port. The port forwards traffic and learns MAC addresses.

– Blocking — The port is blocked. The port cannot forward traffic or learn MAC addresses. The port can forward BPDU data.

– Listening — The port cannot forward traffic and cannot learn MAC addresses.

– Learning — The port cannot forward traffic but it can learn new MAC addresses.

– Forwarding — The port can forward traffic and can learn new MAC addresses.

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

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

• Designated Cost — The cost of the port that is part of the STP topology. Ports with lower costs are less likely to be blocked if STP detects loops.

• Forward Transitions — The number of times the port has changed from the blocking state to the forwarding state.

• Speed — The speed of the port.

• LAG — The LAG to which the port belongs. The LAG settings will override the port settings.

Step 12. Click Apply.

Step 13. (Optional) Click Copy Settings to copy the settings of one interface to others. The Copy Settings page opens:

Step 14. Enter the interface it needs to be copied to in the provided field.

Step 15. Click Apply.