Objective

A bridging loop or spanning tree loop can cause a network outage because packet sent on the network can loop forever, slowing down the network. Spanning-Tree Protocol (STP) prevents loops from being formed when switches or bridges are interconnected via multiple paths. Spanning-Tree Protocol implements the 802.1D IEEE algorithm by exchanging Bridge Protocol Data Unit (BPDU) messages with other switches to detect loops and then removes the loop by shutting down selected bridge interfaces. This algorithm guarantees that there is one and only one active path between two network devices. The SG350XG and the SG550XG offers Classic STP, Rapid STP (RSTP), and Multiple STP (MSTP).

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

Note: The steps in this document are performed under the Advanced Display Mode. To change to the Advance 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 Global Settings

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

Step 2. In the Spanning Tree State field, check the Enable box to enable STP. It is checked by default.

Step 3. STP Loopback Guard provides additional protection against Layer 2 forwarding loops. A loop is created when an STP blocking port in a redundant topology incorrectly transitions to the forwarding state. This usually happens because one of the ports of a physically redundant topology (not necessarily the STP blocking port) no longer receives STP BPDUs. If you want to enable STP Loopback Guard, check the Enable box to enable STP Loopback Guard.

Step 4. Select the STP Operation Mode that you desire to use.

The available options are:

- Classic STP – STP is a link layer network protocol that ensures a loop-free topology for any bridged LAN. The basic function of STP is to prevent bridge loops and to ensure broadcast radiation.

- Rapid STP – Rapid Spanning Tree Protocol (RSTP) is a layer 2 network protocol used to obtain a loop free topology. RSTP is an enhanced version of Spanning Tree Protocol (STP) that provides faster convergence to obtain a loop free topology.

- Multiple STP –Multiple STP is based on Rapid STP. It detects Layer 2 loops and attempts to mitigate them by preventing the involved port from transmitting traffic. Since loops exist on a per-Layer 2-domain basis, a situation can occur when a port is blocked to eliminate a STP loop. Traffic will be forwarded to the port that is not blocked and no traffic will be forwarded to the port that is blocked. This is not an efficient usage of bandwidth as the blocked port will always be unused.

Step 5. In the BPDU Handling field, select the desired radio button. BPDU Handling is how Bridge Protocol Data Unit (BPDU) packets are managed when STP is disabled on the port or the device. BPDUs are used to transmit spanning tree information. This field is only available if you did not enable Spanning Tree State in Step 2.

The available options are:

- 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 6. In the Path Cost Default Values field, select the desired method that you want to use to assign default path costs to the STP ports. The default path cost assigned to an interface varies according to the selected method.

The available options are:

- Short – Specifies the range 1 through 65,535 for port path costs.

- Long – Specifies the range 1 through 200,000,000 for port path costs.

Configuring Bridge Settings

Step 1. The priority sets the bridge priority value. After exchanging BPDUs, the device with the lowest priority becomes the Root Bridge. In the case that all bridges use the same priority, then their MAC addresses are used to determine the Root Bridge. The bridge priority value is provided in increments of 4096. For example, 4096, 8192, 12288, and so on. In the Priority field, enter in the value from 0 – 61440. The default value is 32768.

Step 2. In the Hello Time field, set the interval (in seconds) that a Root Bridge waits between configuration messages. This ranges from 1-10 and the default value is 2.

Step 3. Set the interval (in seconds) in the Max Age field. This tells how long the device can wait without receiving a configuration message before attempting to redefine its own configuration. The range is from 6 – 40 and the default value is 20.

Step 4. In the Forward Delay field, set the interval (in seconds) that a bridge remains in a learning state before forwarding packets. This ranges from 4 – 30 and the default value is 15.

Note: For more information, refer to Configuring STP Interface Settings on the SG350XG and SG550XG.

Step 5. Click Apply. The STP Global settings are written to the Running Configuration file.

Designated Root

A designated root is when you force a specific device to be the root device in an STP (Spanning Tree Protocol) domain instead of having the devices figure it out on their own. This section of the document shows the details on the designated root.

The Bridge ID field shows the bridge priority concatenated with the MAC address of the device.

The Root Bridge ID field shows the Root Bridge priority concatenated with the MAC address of the Root Bridge.

The Root Port field is the port that offers the lowest cost path from this bridge to the Root Bridge.

Note: This is significant when the bridge is not the root.

The Root Path Cost field is the cost of the path from this bridge to the root.

The Topology Changes Counts field is the total number of STP topology changes that have occurred.

The Last Topology Change field is the time interval that elapsed since the last topology change occurred. The time appears in a days/hours/minutes/seconds format.