• Partitioning the network into a large number of regions is not recommended. However, if this situation is unavoidable, we recommend that you partition the switched LAN into smaller LANs interconnected by routers or non-Layer 2 devices. • A region can have one member or multiple members with the same MST configuration; each member must be capable of processing rapid spanning tree protocol (RSTP) Bridge Protocol Data Units (BPDUs). There is no limit to the number of MST regions in a network, but each region can only support up to 65spanning-tree instances. You can assign a VLAN to only one spanning-tree instance at a time.
Information About Multiple Spanning Tree Protocol The following sections provide information about Multiple Spanning-Tree Protocol (MSTP): Multiple Spanning Tree Protocol Configuration Multiple Spanning-Tree Protocol (MSTP), which uses Rapid Spanning-Tree Protocol (RSTP) for rapid convergence, enables multiple VLANs to be grouped into and mapped to the same spanning-tree instance, reducing the number of spanning-tree instances that are needed to support many VLANs. The MSTP provides for multiple forwarding paths for data traffic, enables load balancing, and reduces the number of spanning-tree instances that are required to support many VLANs.
It improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths). The multiple spanning-tree (MST) implementation is based on the IEEE 802.1s standard. Note The most common initial deployment of MSTP is in the backbone and distribution layers of a Layer 2 switched network. This deployment provides the highly available network that is required in a service-provider environment. When the device is in the MST mode, the RSTP, which is based on IEEE 802.1w, is automatically enabled.
The RSTP provides rapid convergence of the spanning tree through explicit handshaking that eliminates the IEEE 802.1D forwarding delay and quickly transitions root ports and designated ports to the forwarding state. Both MSTP and RSTP improve the spanning-tree operation and maintain backward compatibility with equipment that is based on the (original) IEEE 802.1D spanning tree, with existing Cisco-proprietary Multiple Instance STP (MISTP), and with existing Cisco PVST+ and rapid per-VLAN spanning-tree plus (Rapid PVST+). A device stack appears as a single spanning-tree node to the rest of the network, and all stack members use the same device ID.
Multiple Spanning Tree Protocol Configuration Guidelines • When you enable MST by using the spanning-tree mode mst global configuration command, RSTP is automatically enabled. • For configuration guidelines about UplinkFast, BackboneFast, and cross-stack UplinkFast, see the relevant sections in the Related Topics section. Configuring Multiple Spanning-Tree Protocol Information About Multiple Spanning Tree Protocol Configuring Multiple Spanning-Tree Protocol 2