The main purpose of MST is to reduce the total number of spanning tree
instances to match the physical topology of the network and thus reduce the CPU
loading of a switch. The instances of spanning tree are reduced to the number
of links (i.e. active paths) that are available. If the example in the diagram
were implemented via PVST+, there could potentially be 4094 instances of
spanning tree, each with their respective BPDU conversations, root bridge
election and path selections.
In this example, the goal is to achieve
load distribution with VLANS 1-500 using one path and VLANS 501-1000 using the
other path, with only two instances of spanning tree. The two ranges of VLANs
are mapped to two MST instances (MSTI) respectively. Rather than maintaining
1000 spanning trees, each switch needs to maintain only two. Implemented in
this fashion, MST converges faster than Per VLAN Spanning Tree+ (PVST+) and is
backward compatible with 802.1D STP, 802.1w (RSTP), and the Cisco PVST+
architecture. Implementation of MST is not required if the Enterprise Composite
Model is being employed, as the number of active VLAN instances, and hence, the
STP instances would be small in number and very stable due to the design.
MST allows you to build multiple spanning trees over trunks by grouping and
associating VLANs to spanning tree instances. Each instance can have a topology
independent of other spanning tree instances. This architecture provides
multiple active forwarding paths for data traffic and enables load balancing.
Network fault tolerance is improved over CST because a failure in one instance
(forwarding path) does not necessarily affect other instances. This VLAN to MST
grouping must be consistent across all bridges within an MST region.
In large networks, you can more easily administer the network and use
redundant paths by locating different VLAN and spanning tree assignments in
different parts of the network. A spanning tree instance can exist only on
bridges that have compatible VLAN instance assignments.
You must
configure a set of bridges with the same MST configuration information, which
allows them to participate in a specific set of spanning tree instances.
Interconnected bridges that have the same MST configuration are referred to as
an "MST region." Bridges with different MST configurations or legacy
bridges running 802.1D are considered separate MST regions.
In a Cisco
PVST+ environment, the spanning tree parameters are tuned so that half of the
VLANs are forwarding on each uplink trunk. This is easily achieved by electing
bridge D1 to be the root for VLAN501–1000, and bridge D2 to be the root for
VLAN1–500. In this configuration, the following is true:
- Optimum load balancing is achieved.
- One spanning tree instance for each VLAN is maintained, which means 1000
instances for only two different logical topologies. This consumes resources
for all the switches in the network (in addition to the bandwidth used by each
instance sending its own BPDUs).
MST (IEEE 802.1s) combines the best aspects from both PVST+ and 802.1Q.
The idea is that several VLANs can be mapped to a reduced number of
spanning-tree instances because most networks do not need more than a few
logical topologies.