Nokia ESB26 User Manual page 152

14.
T e r m
MST Region
Single Spanning
Tree Bridge
Multiple Spanning-Tree Regions
To enable switches to participate in multiple spanning-tree instances (MSTIs), 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 except for the case where the switches are connected through a shared media
(i.e., LAN).
The MST configuration determines to which MST region each switch belongs. The
configuration includes the name of the region, the revision number, and the MST instance-to-
VLAN assignment map.
A region can have one member or multiple members with the same MST configuration. Each
member must be capable of processing RSTP BPDUs. There is no limit to the number of
MST regions in a network, but each region can support up to 16 spanning-tree instances. You
can assign a VLAN to only one spanning-tree instance at a time.
IST, CIST, and CST
Table 14-1 describes the conventional terms and acronyms used in MSTP.
The MSTP establishes and maintains two types of spanning-trees:
• IST - An internal spanning tree, which is the spanning tree that runs in an MST region.
Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance
0 is a special instance for a region, known as the internal spanning tree (IST). All other
MST instances are numbered from 1 to 15.
The IST is the only spanning-tree instance that sends and receives BPDUs. All other
spanning-tree instance information is contained in M-records, which are encapsulated
within MSTP BPDUs. Because the MSTP BPDU carries information for all instances, the
number of BPDUs that need to be processed by a switch to support multiple spanning-tree
instances is significantly reduced. All MST instances within the same region share the
same protocol timers, but each MST instance has its own topology parameters, such as
root switch ID, root path cost, and so forth. By default, all VLANs are assigned to the IST.
An MST instance is local to the region; for example, MST instance 1 in region A is
MN700004 Rev 01
Multiple Spanning Tree Protocol (MSTP)
A c r o n y m D e f i n i t i o n
-- A set of LANs and MST Bridges physically connected via
Ports on those MST Bridges, where each LAN's CIST
Designated Bridge is an MST Bridge, and each Port is
either the Designated Port on one of the LANs, or else a
non-Designated Port of an MST Bridge that is connected
to one of the LANs, whose MCID matches exactly the
MCID of the Designated Bridge of that LAN.
NOTE—It follows from this definition that the MCID is
the same for all LANs and Ports in the Region, and that
the set of MST Bridges in the region are interconnected
by the LANs.
SST Bridge A Bridge capable of supporting only a single spanning
tree, the CST. The single spanning tree may be
supported by the Spanning Tree Algorithm and Protocol
(STP) or by the Rapid Spanning Tree Algorithm and
Protocol (RSTP).
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