Ethernet Port Spanning Tree Configuration

About this task

Spanning Tree Protocol (STP) (IEEE 802.1D standard) configures a meshed network for robustness by eliminating loops within the network and calculating and storing alternate paths to provide fault tolerance.

As the port comes up and STP calculation takes place, the port is set to Blocked state. In this state, no traffic can pass through the port. Since STP calculations take up to a minute to complete, the port is not operational thereby effecting the network behind the port. When the STP calculation is complete, the port‘s state is changed to Forwarding and traffic is allowed.

Multiple Spanning Tree Protocol (MSTP) provides an extension to RSTP to optimize the usefulness of VLANs. MSTOP allows for a separate spanning tree for each VLAN group, and blocks all but one of the possible alternate paths within each spanning tree topology.

If there is only one VLAN in the access point managed network, a single spanning tree works fine. However, if the network contains more than one VLAN, the network topology defined by single STP would work, but it is possible to make better use of the alternate paths available by using an alternate spanning tree for different VLANs or groups of VLANs.

An MSTP supported deployment uses multiple MST regions with multiple MST instances (MSTIs). Multiple regions and other STP bridges are interconnected using one single common spanning tree (CST). MSTP includes all of its spanning tree information in a single Bridge Protocol Data Unit (BPDU) format. BPDUs are used to exchange information bridge IDs and root path costs. Not only does this reduce the number of BPDUs required to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with RSTP.

MSTP encodes additional region information after the standard RSTP BPDU as well as a number of MSTI messages. Each MSTI message conveys spanning tree information for each instance. Each instance can be assigned a number of configured VLANs. The frames assigned to these VLANs operate in this spanning tree instance whenever they are inside the MST region. To avoid conveying their entire VLAN to spanning tree mapping in each BPDU, the access point encodes an MD5 digest of their VLAN to an instance table in the MSTP BPDU. This digest is used by other MSTP supported devices to determine if the neighboring device is in the same MST region as itself.

To configure spanning tree settings for the selected Ethernet port:


  1. Select Profiles.
    The profile name list opens.
  2. Select a profile from the existing list.
  3. Select Interface > Ethernet > Ethernet name > Spanning Tree.
  4. Select the Portfast slider to configure portfast settings:
    BPDU Filter BPDUs are messages that are exchanged when controllers gather information about the network topology.

    Use the drop-down list box to invoke BPDU filter settings

    When activated, Portfast enabled ports do not transmit BPDU messages. When this value is set to Default, the BPDU Filter value is set to the bridge's BPDU filter value

    BPDU Guard Use the drop-down list box to invoke BPDU guard for this portfast enabled port.

    When selected, Portfast enabled ports are forced to shut down when they receive BPDU messages. When this value is set to Default, the Portfast BPDU Guard value is set to the bridge's BPDU guard value

  5. Configure the following MSTP settings:
    Link Type Select Point-to-Point or Shared
    • Point-to-Point - Port is treated as connected to a point-to-point link

      An example of a Point-to-Point connection is a port that is connected to a controller or service platform

    • Shared - Port is shared between multiple devices

      An example of a Shared connection is a port that is connected to a hub

    Cisco Interoperability Enable or Disable interoperability with Cisco‘s version of MSTP over the port. Cisco's version of MSTP is incompatible with standard MSTP
    Force Protocol Version Select STP to use the standard Spanning Tree Protocol

    Select RSTP to use Rapid Spanning Tree Protocol

    Select MSTP to use Multiple Spanning Tree Protocol

    Select Not Supported to deactivate spanning tree protocol for this interface

    MSTP is the default setting

    Guard Select Root radio to ensure that the port is a designated port.

    Typically, each guard root port is a designated port, unless two or more ports (within the root bridge) are connected together. If the bridge receives superior BPDUs on a guard root-enabled port, the guard root moves the port to a root-inconsistent STP state. This state is equivalent to a listening state. No data is forwarded across the port. Thus, the guard root enforces the root bridge position

    Select None to deactivate this feature

  6. Select Add to create a new Port Cost and configure the following settings:
    Instance Index Set a value between 0 to 15
    Cost This is the cost for a packet to traverse the current network segment. The cost of a path is the sum of all costs of traversal from the source to the destination. The default rule for the cost of a network segment is, the faster the media, the lower the cost
    Set a cost between 1 to 200000000
    Note: The default path cost depends on the user-defined speed of the port. The cost helps determine the role of the port in the MSTP network. The designated cost is the cost for a packet to travel from this port to the root in the MSTP configuration. The slower the media, the higher the cost

    Select to delete a port cost.

  7. Select Add to create a new Port Priority and configure the following settings:
    Instance Index Set a value between 0 to 15
    Priority Set a value between 0 to 240

    This is the priority for this port becoming a designated root. The default rule is, the lower this value, the higher the chance that the port is assigned as a designated root

    Select to delete a port priority.

  8. Click Save to update the changes and overrides made to the Ethernet port's Spanning Tree configuration.