Information About Mst; Mst Overview; Mst Regions - Cisco Nexus 7000 Series Configuration Manual

Information About MST

Information About MST

MST, which is the IEEE 802.1s standard, allows you to assign two or more VLANs to a spanning tree instance.
MST is not the default spanning tree mode; Rapid per VLAN Spanning Tree (Rapid PVST+) is the default
mode. MST instances with the same name, revision number, and VLAN-to-instance mapping combine to
form an MST region. The MST region appears as a single bridge to spanning tree configurations outside the
region. MST forms a boundary to that interface when it receives an IEEE 802.1D Spanning Tree Protocol
(STP) message from a neighboring device.
Note Spanning tree is used to refer to IEEE 802.1w and IEEE 802.1s. If the IEEE 802.1D Spanning Tree Protocol
is discussed in this publication, 802.1D is stated specifically.

MST Overview

You must enable MST; Rapid PVST+ is the default spanning tree mode.
Note
MST maps multiple VLANs 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 STP instances required to support a large number
of VLANs. MST improves the fault tolerance of the network because a failure in one instance (forwarding
path) does not affect other instances (forwarding paths).
MST provides rapid convergence through explicit handshaking because each MST instance uses the IEEE
802.1w standard, which eliminates the 802.1D forwarding delay and quickly transitions root bridge ports and
designated ports to the forwarding state.
MAC address reduction is always enabled on the device. You cannot disable this feature.
MST improves spanning tree operation and maintains backward compatibility with these STP versions:
• Original 802.1D spanning tree
• Rapid per-VLAN spanning tree (Rapid PVST+)
• IEEE 802.1 was defined in the Rapid Spanning Tree Protocol (RSTP) and was incorporated into
Note
• IEEE 802.1 was defined in MST and was incorporated into IEEE 802.1Q.

MST Regions

To allow devices to participate in MST instances, you must consistently configure the devices with the same
MST configuration information.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
132
IEEE 802.1D.
Configuring MST Using Cisco NX-OS