Preparing for spanning tree configuration

Before configuring a spanning tree:

  • Determine the spanning tree protocol to be used: RPVST+ or MSTP. RPVST+ is ideal in networks having fewer than 100 VLANs. In networks having 100 or more VLANs, MSTP is the recommended spanning tree choice due to the increased load on the switch CPU.
  • Plan the device roles (the root bridge or leaf node) by adjusting instance priority.

When you configure spanning tree protocols, follow these guidelines:

  • If MSTP is enabled on the switch, MSTP takes all MSTI information along with the packet. To advertise a specific VLAN within the network through MSTP, make sure that the VLAN is mapped to an MSTI when you configure the VLAN-to-instance table.
  • STP is mutually exclusive with loop protection. If STP and loop protection are both enabled on the same VLAN, STP takes precedence. This means that loop protection does not take any action on a port blocked by STP.

  • A switch running RPVST+ transmits IEEE spanning tree BPDUs. It interoperates with IEEE RSTP and MSTP spanning tree regions, and opens or blocks links from these regions as needed to maintain a loop-free topology with one physical path between regions. RPVST+ interoperates with RSTP and MSTP only on VLAN 1.

  • One spanning tree variant can be run on the switch at any given time. On a switch running RPVST+, MSTP cannot be enabled. However, any MSTP-specific configuration settings in the startup configuration file will be maintained.

  • The following features cannot run concurrently with RPVST+:

    • Features that dynamically assign ports to VLANs: MVRP, RADIUS-based VLAN assignments (802.1X, WebAuth, MKA:C auth), and MAC-based VLANs.

    • Filter multicast in RPVST+ mode (The multicast MAC address value cannot be set to the PVST MAC address 01:00:0c:cc:cc:cd.)

  • Spanning tree mode cannot be set to RPVST+ when GVRP or MVRP is enabled. GVRP or MVRP cannot be enabled when RPVST+ is enabled.

  • Configurations made on an aggregation member port can take effect only after the port is removed from the aggregation group.
  • After you enable a spanning tree protocol on a layer 2 aggregate interface, the system performs spanning tree calculation on the layer 2 aggregate interface. It does not perform the spanning tree calculation on the aggregation member ports. The spanning tree protocol enable state and forwarding state of each selected member port is consistent with the state of the corresponding layer 2 aggregate interface.
  • The member ports of an aggregation group do not participate in spanning tree calculations. However, the ports reserve their spanning tree configurations for participation in spanning tree calculations after leaving the aggregation group.