Spanning Tree Mode

Spanning Tree Mode

The Cisco 2960 switch supports these spanning-tree modes:

Mode	Description
Per-VLAN Spanning Tree Protocol (PVST+ or PVST)	Per-VLAN Spanning Tree Protocol (PVST+ or PVST) is a spanning-tree mode based on the 802.1d standard and Cisco proprietary extensions. It is the default spanning-tree mode used on all Ethernet port-based VLANs. PVST+ characteristics include the following: Layer 2 load balancing for the VLAN on which it runs Each instance of PVST+ on a VLAN has a single root switch A short aging time for learned MAC address entries PVST+ includes features such as PortFast and UplinkFast to speed convergence.
Rapid PVST+	Rapid PVST+ is the same as PVST+ except that it uses a rapid convergence based on the 802.1w standard. To provide rapid convergence, the rapid PVST+ deletes learned MAC address entries on a per-port basis upon receiving a topology change. Rapid PVST+ characteristics include the following: A similar configuration as PVST+ Needs a minimal amount of extra configuration from PVST+ Each VLAN runs its own spanning-tree instance up to the maximum number supported
Multiple STP (MSTP)	Multiple STP (MSTP) is the spanning tree mode based on the 802.1s standard. With MSTP you can map multiple VLANs to the same spanning-tree instance. MSTP characteristics include the following: Supports a large number of VLANs with spanning-tree instances Runs on top of the Rapid Spanning Tree Protocol (RSTP) MSTP cannot run without RSTP  Note: You cannot run both MSTP and PVST+ or both MSTP and rapid PVST+ at the same time.

Be aware of the following regarding spanning tree on a Cisco switch:

• To optimize how spanning tree works when multiple VLANs exist, a switch runs multiple instances of the spanning tree protocol.
  • Each instance includes a single VLAN (each VLAN can be part of only one spanning tree instance).
  • Ports associated with a VLAN participate in the spanning tree instance assigned to the VLAN. Because a port can only be a member of one VLAN, each port is associated with only one instance of spanning tree.
  • Each instance of spanning tree elects its own root bridge. A single switch might be the root bridge for all spanning tree instances, or it might be the root bridge for only one of the instances running on the switch.
• By default, spanning tree is enabled with a single instance of the spanning tree protocol for VLAN1. By default, all switch ports are members of VLAN1, therefore all ports participate in spanning tree by default.
• When you create a new VLAN, a new instance of spanning tree runs automatically.
• You cannot disable spanning tree for a switch port. You can, however, disable it for an entire VLAN or the entire switch. In practice, there are few reasons to do this as disabling spanning tree makes bridging loops possible.