Readers will learn how to verify which ports are being blocked by the Spanning Tree Protocol (STP) by looking at the traffic statistics.
EdgeSwitch Network Using Spanning Tree (STP)
The example network below consists of two EdgeSwitch 8 150W (ES-8-150W) switches and a single EdgeSwitch 10X (ES-10X) switch. The switches are interconnected using the SFP ports using Multi-Mode fiber (UF-MM-1G) SFP modules and fiber cables. The Rapid Spanning Tree Protocol (RSTP) is running on each switch in order to prevent a bridging loop in the network.
Example EdgeSwitch (ES) and EdgeSwitch X (ES-X) network that utilizes the Spanning Tree Protocol (STP).
It is possible to alter the STP settings in order to optimize the traffic flow in a network. The way that Spanning Tree is preventing a bridging loop from occurring is by blocking one of the interconnecting ports between the switches. Depending on the STP settings on each switch, one of the following connections will be blocked by STP:
ES-1 / ES-10XThe SFP fiber connection between Port 9 on ES-1 and Port 9 on the ES-10X is blocked.
ES-2 / ES-10XThe SFP fiber connection between Port 9 on ES-2 and Port 10 on the ES-10X is blocked.
ES-1 / ES-2The SFP fiber connection between Port 10 on ES-1 and Port 10 on ES-2 is blocked.
We can look at the Tx and Rx traffic statistics on each switch to determine which of the above connections is blocked by STP.
Follow the steps below to verify the port traffic statistics on each switch:
Navigate to the Dashboard tab and verify the Rx and Tx traffic columns for the SFP ports (Port 9 and 10). On ES-1:
On the ES-10X:
By looking at the traffic statistics, we can determine that there is minimal traffic going over the SFP fiber connection between Port 9 on ES-1 and Port 9 on the ES-10X. This means that whenever hosts that connect through ES-10X are accessing the Internet, the traffic will take the ES-10X > ES-2 > ES-1 > ER-4 path.
Based on the port statistics, we can make the determination that ES-2 is the STP root bridge as both interconnected links (ES-1 / ES-2 and ES-2 / ES-10X) are showing traffic. We can change the path that the traffic is taking by lowering the STP priority on the EdgeSwitch that should be the STP root bridge. In this example, the priority is lowered on ES-1, causing it to become the new root bridge. This has the added effect that the ES-1 / ES-10X path will start forwarding traffic and the ES-2 / ES-10X path will be blocked.
Follow the steps below to change the STP priority on ES-1:
1. Navigate to the Spanning Tree Protocol section in the Settings tab.
2. Change the STP priority from 32768 (default) to a lower number using increments of 4096.
3. In this example, the STP priority is lowered to 4096:
Settings > Spanning Tree Protocol
4. Apply the changes.
To verify the changes, log back into the ES-10X switch:
Navigate to the Dashboard tab and verify the Rx and Tx traffic columns for the SFP ports (Port 9 and 10).
After the STP priority change on ES-2, we can see that the traffic is now going over the ES-1 / ES-10X path.