CHAPTER
15-1
Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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15
Configuring MSTP
This chapter describes how to configure the Cisco implementation of the IEEE 802.1s MultipleSTP
(MSTP) on the Cisco ME 3400 Ethernet Access switch. On the Cisco ME switch, STP is supported only
on network node interfaces (NNIs). User network interfaces (UNIs) on the switch do not participate in
STP and forward traffic immediately when they are brought up.
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 n etwork requi r ed
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 rapi d convergence of the spa nning tre e t hr oug h
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+ ) an d 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 Opt ional Spann ing-Tree Feature s.”
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 15-2
•Understanding RSTP, page 15-6
•Configuring MSTP Features, page 15-11
•Displaying the MST Configuration and Status, page 15-23