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CHAPTER
14-1
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
14
Configuring MSTP
This chapter describes how to configure the Cisco implementation of the IEEE 802.1s Multiple STP
(MSTP) on your Catalyst 2950 or Catalyst
2955 switch.
Note The multiple spanning-tree (MST) implementation is a prestandard implementation. It is based on the
draft version of the IEEE standard.
The MSTP enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the
number of spanning-tree instances needed to support a large number of VLANs. The MSTP provides for
multiple forwarding paths for data traffic and enables load balancing. It improves the fault tolerance of
the network because a failure in one instance (forwarding path) does not affect other instances
(forwarding paths). 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 required
in a service-provider environment.
When the switch is in the MST mode, the Rapid Spanning Tree Protocol (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 per-VLAN spanning-tree plus (PVST+) and
rapid per-VLAN spanning-tree plus (rapid PVST+). For information about PVST+ and rapid PVST+,
see
Chapter 13, “Configuring STP.” For information about other spanning-tree features such as Port Fast,
UplinkFast, root guard, and so forth, see Chapter 15, “Configuring Optional Spanning-Tree Features.”
Note For complete syntax and usage information for the commands used in this chapter, see the command
reference for this release.
This chapter consists of these sections:
• Understanding MSTP, page 14-2
• Understanding RSTP, page 14-6
• Configuring MSTP Features, page 14-11
• Displaying the MST Configuration and Status, page 14-22