Preparing for spanning tree configuration

Before configuring a spanning tree:

  • Determine the spanning tree protocol to be used (STP, RSTP, PVST, or MSTP). RPVST+ is ideal in networks having less 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).

When you configure spanning tree protocols, follow these guidelines:

  • If both MSTP and a spanning tree protocol are enabled on a device, MSTP packets are forwarded along MSTIs. To advertise a specific VLAN within the network through MSTP, make sure this VLAN is mapped to an MSTI when you configure the VLAN-to-instance table.
  • Spanning tree configurations are mutually exclusive with service loopback and Smart Link.
  • A switch running RPVST+ transmits IEEE spanning tree BPDUs, therefore it can interoperate 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: GVRP, MVRP, RADIUS-based VLAN assignments (802.1X, WebAuth, MKA:C auth), and MAC-based VLANs.

    • Multi-Chassis LAG.

    • 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, and 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 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 those of the corresponding layer 2 aggregate interface.
  • The member ports of an aggregation group do not participate in spanning tree calculation. However, the ports still reserve their spanning tree configurations for participating in spanning tree calculation after leaving the aggregation group.