Руководство По Проектированию для Cisco Cisco Nexus 5010 Switch
© 2012 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information.
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Fundamental Spanning-Tree Concepts
The first important choice to make in the design of a data center network is the spanning-tree choice. It is beyond
the scope of this document to describe the spanning-tree algorithms in detail. At the time of this writing, the choice
is between these two algorithms:
●
Rapid Per-VLAN Spanning Tree Plus (PVST+): Information about Rapid PVST+ is available at
●
Multiple Spanning Tree (MST): Information about MST is available at
This document does not discuss Rapid PVST+ in detail because this protocol is already well known in the industry.
MST is discussed in detail here for the following reasons:
●
With virtual PortChannel (vPC) deployments, VLAN load balancing is achieved automatically with no need
to change spanning -tree priorities. Spanning-tree VLAN load balancing is much easier in Rapid PVST+
than it is in MST. With vPC topologies, this advantage is not applicable.
●
MST scales better than Rapid PVST+ because the switch generates only one Bridge Protocol Data Unit
(BPDU), which summarizes all the necessary information for the specific instance. With the increase in size
of the Layer 2 domain, MST may become of greater interest because of its greater scalability and its
capability to maintain a regional topology.
MST Compared to Rapid PVST+
MST allows you to assign two or more VLANs to a spanning-tree instance. MST is not the default spanning-tree
mode; Rapid PVST+ is the default mode on Cisco
®
switches.
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.
The advantages of MST over Rapid PVST+ are as follows:
●
MST is an IEEE standard.
●
MST is more resource efficient. In particular, the number of BPDUs transmitted by MST does not depend on
the number of VLANs, as Rapid PVST+ does.
●
MST decouples the creation of VLANs from the definition for forwarding the topology.
●
MST simplifies the deployment of stretched Layer 2 networks, because of its ability to define regions.
For all these reasons, it is advisable for many deployments to migrate to an MST-based topology.
Table 1 summarizes the differences between MST and Rapid PVST+.
Table 1.
MST Compared to Rapid PVST+
MST
Rapid PVST+
Number of instances
Typically 2 are enough
1 per VLAN
Scalability
Much greater than Rapid PVST+ (see limits)
Interoperability with multichassis
EtherChannels with vPC ports
EtherChannels with vPC ports
Yes, but make sure that the MST region
definition matches for primary and secondary
vPC peers
definition matches for primary and secondary
vPC peers
Yes