Cisco Systems 2950 Configuring MSTP

CHAPTER
15-1
Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
78-11380-10
15
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 pre-standard 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, thereby reducing
the number of spanning-tree instances needed to suppo rt a large number of VLANs. The MSTP provides
for multiple forwarding paths for data traffic and enables load balancing. It impr o ves the fault tolerance
of the network because a failure in one instance (forwarding pat h) do es not affect ot her inst an ces
(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 re quir ed
in a service-provider environmentt.
When the switch is in the MST mode, the Rapid Spanning Tree Protocol (RSTP), whi ch is bas ed 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) 802.1D spanning tree, with existing Cisco-propr ietary Multiple
Instance STP (MISTP) and with existing Cisco per-VLAN spanning-tree plus (PVST +) a nd ra pid
per-VLAN spanning-tree plus (rapid PVST+). For information about PVST+ and rapid PVST+, see
Chapter 14, “Configuring STP.” For information about other spanning-tree features such as Port Fast,
UplinkFast, root guard, and so forth, see Chapter 16, “Configuring Optional Spanning-Tree Feature s.”
Note For complete syntax and usage information for the commands used in this chapter, refer to the
command reference for this release.
This chapter consists of these sections:
•Understanding MSTP, page 15-2
•Understanding RSTP, page 15-6
•Configuring MSTP Features, page 15-11
•Displaying the MST Configuration and Status, page 15-23