Cisco IE 4000, Software Configuration Manual

Page: 337 / 1066

333
Cisco Systems, Inc. www.cisco.com
Configuring MSTP
Information About Configuring M
STP
This chapter describes how to configure the Cisco implementation of the IEEE 802.1s Multiple STP (MSTP) on the switch.
Note: The multiple spanning-tree (MST) implementation is based on the IEEE 802.1s 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+).
MSTP
MSTP, which uses RSTP for rapid convergence, enables VLANs to be grouped into a spanning-tree instance, with each
instance having a spanning-tree topology independent of other spanning-tree instances. This architecture provides
multiple forwarding paths for data traffic, enables load balancing, and reduces the number of spanning-tree instances
required to support a large number of VLANs.
Multiple Spanning-Tree Regions
For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure the switches with
the same MST configuration information. A collection of interconnected switches that have the same MST configuration
comprises an MST region as shown in
Figure 1 on page 3 .
The MST configuration controls to which MST region each switch belongs. The configuration includes the name of the
region, the revision number, and the MST VLAN-to-instance assignment map. You configure the switch for a region by
using the
spanning-tree mst configuration global configuration command, after which the switch enters the MST
configuration mode. From this mode, you can map VLANs to an MST instance by using the instance MST configuration
command, specify the region name by using the name MST configuration command, and set the revision number by
using the revision MST configuration command.
A region can have one or multiple members with the same MST configuration. Each member must be capable of
processing RSTP bridge protocol data units (BPDUs). There is no limit to the number of MST regions in a network, but
each region can support up to 65 spanning-tree instances. Instances can be identified by any number in the range from
0 to 4096. You can assign a VLAN to only one spanning-tree instance at a time.