How to configure MSTP on L2 Managed switches using the new GUI

MSTP (Multiple Spanning Tree Protocol) is the extension of STP and RSTP. In STP and RSTP, once a link is blocked, all traffic are not allowed to transmit through this link. This will cause load imbalance, because other links have to take on more traffic. To solve this problem, MSTP appears.

MSTP blocks ports based on VLAN. For example, port 1 belongs to VLAN 1 and VLAN 2. MSTP may block port 1 in VLAN 1, but still keep port 1 up in VLAN 2. Thus, port 1 is not completely blocked and still can forward the packets from VLAN 2.

Here is an example for MSTP. As the figure shows below, the network consists of three switches. Traffic in VLAN 101-VLAN 106 is transmitted in this network. The link speed between the switches is 100Mb/s (this means the default path cost of the port is 200000). It is required that traffic in VLAN 101-VLAN 103 and traffic in VLAN 104-VLAN 106 should be transmitted along different paths.

Configuration Scheme:

We can configure MSTP to meet the requirement. By configuring MSTP, you can manually design two different links for VLAN 101-VLAN 103 and VLAN 104-VLAN 106. To achieve this goal, we need to create two instances and map them to VLAN 101-VLAN 103 and VLAN 104-VLAN 106 respectively. Then in each instance, manually designate the root switch and specify the path cost for each port so that we can decide which port would be blocked.

The figure below shows the paths we design for the VLANs. We create instance 1 and instance 2, and map them to VLAN101-VLAN103 and VLAN 104-VLAN 106 respectively. In instance 1, we will block port 1/0/2 on switch A; in instance 2, we will block port 1/0/1 on switch B. Thus the traffic in VLAN 101-VLAN 103 and VLAN 104-VLAN 106 will be transmitted through different paths.

The overview of configuration is as follows:

Enable MSTP globally on each switch. Enable spanning tree on the corresponding ports.
Configure Switch A, Switch B and Switch C in the same region. Configure the region name as 1, and the revision level as 100. Map VLAN 101-VLAN 103 to instance 1 and VLAN 104-VLAN 106 to instance 2.
Configure the priority of Switch B as 4096 to set it as the root bridge in instance 1; configure the priority of Switch C as 4096 to set it as the root bridge in instance 2.
Configure the path cost to block the specified ports. For instance 1, set the path cost of port 1/0/1 of Switch A to be greater than the default path cost (200000); for instance 2, set the path cost of port 1/0/2 of Switch B to be greater than the default path cost (200000). After this configuration, port 1/0/2 of Switch A in instance 1 and port 1/0/1 of Switch B in instance 2 will be blocked for they cannot be neither root port nor designated port.

Note:

Please configure MSTP for each switch first and then connect them together to avoid broadcast storm.

Configuring Switch A:

Go to L2 FEATURES > Spanning Tree > STP Config > STP Config page. Enable MSTP function globally. Here we leave the other global parameters as default settings.

Go to L2 FEATURES > Spanning Tree > STP Config > Port Config page. Enable spanning tree function on port 1/0/1 and port 1/0/2. Leave the other parameters as default settings.

Go to L2 FEATURES > Spanning Tree > MSTP Instance > Region Config page. Set the region name as 1 and the revision level as 100.

Go to L2 FEATURES > Spanning Tree > MSTP Instance > Instance Config page. Map VLAN101-VLAN103 to instance 1 and set the priority as 32768; map VLAN104-VLAN106 to instance 2 and set the priority as 32768.

5. Go to L2 FEATURES > Spanning Tree > MSTP Instance > Instance Port Config page. Set the path cost of port 1/0/1 in instance 1 as 300000 so that port 1/0/1 of switch C can be selected as the designated port.

Configuring Switch B:

Go to L2 FEATURES > Spanning Tree > STP Config > STP Config page. Enable MSTP function globally. Leave the other global parameters as default settings.

Go to L2 FEATURES > Spanning Tree > STP Config > Port Config page. Enable spanning tree function on port 1/0/1 and port 1/0/2. Leave the other parameters as default settings.

Go to L2 FEATURES > Spanning Tree > MSTP Instance > Region Config page. Set the region name as 1 and the revision level as 100.

Go to L2 FEATURES > Spanning Tree > MSTP Instance > Instance Config page. Map VLAN101-VLAN103 to instance 1 and set the priority as 4096; map VLAN104-VLAN106 to instance 2 and set the priority as 32768.

Go to L2 FEATURES > Spanning Tree > MSTP Instance > Instance Port Config page. Set the path cost of port 1/0/2 in instance 2 as 300000 so that port 1/0/1 of switch A can be selected as the designated port.

Configuring Switch C:

Go to L2 FEATURES > Spanning Tree > STP Config > STP Config page. Enable MSTP function globally. Leave the other global parameters as default settings.

Go to L2 FEATURES > Spanning Tree > STP Config > Port Config page. Enable spanning tree function on port 1/0/1 and port 1/0/2. Leave the other parameters as default settings.

Go to L2 FEATURES > Spanning Tree > MSTP Instance > Region Config page. Set the region name as 1 and the revision level as 100.

Go to L2 FEATURES > Spanning Tree > MSTP Instance > Instance Config page. Map VLAN101-VLAN103 to instance 1 and set the priority as 32768; map VLAN104-VLAN106 to instance 2 and set the priority as 4096.