QTECH
Software Configuration Manual
15-194
As an attribute of an MST region, the VLAN-to-instance mapping table describes the mapping relationships
between VLANs and MST instances. In Figure 4, for example, the VLAN-to-instance mapping table of region A0
describes that the same region name, the same VLAN-to-instance mapping (VLAN1 is mapped to MST instance 1,
VLAN2 to MST instance 2, and the rest to CIST.
3) IST
Internal spanning tree (IST) is a spanning tree that runs in an MST region, with the instance number of 0.
ISTs in all MST regions and the common spanning tree (CST) jointly constitute the common and internal spanning
tree (CIST) of the entire network. An IST is a section of the CIST in an MST region. In Figure 4, for example, the
CIST has a section is each MST region, and this section is the IST in each MST region.
4) CST
The CST is a single spanning tree that connects all MST regions in a switched network. If you regard each
MST region as a “device”, the CST is a spanning tree computed by these devices through MSTP. For example, the
red lines in Figure 4 describe the CST.
5) CIST
Jointly constituted by ISTs and the CST, the CIST is a single spanning tree that connects all devices in a
switched network. In Figure 4, for example, the ISTs in all MST regions plus the inter-region CST constitute the
CIST of the entire network.
6) MSTI
Multiple spanning trees can be generated in an MST region through MSTP, one spanning tree being
independent of another. Each spanning tree is referred to as a multiple spanning tree instance (MSTI). In Figure 4, for
example, multiple spanning tree can exist in each MST region, each spanning tree corresponding to a VLAN. These
spanning trees are called MSTIs.
7) Regional root bridge
The root bridge of the IST or an MSTI within an MST region is the regional root bridge of the MST or that
MSTI. Based on the topology, different spanning trees in an MST region may have different regional roots. For
example, in region D0 in Figure 4, the regional root of instance 1 is device B, while that of instance 2 is device C.
8) Common root bridge
The root bridge of the CIST is the common root bridge. In Figure 4, for example, the common root bridge is
a device in region A0.
9) Boundary port
A boundary port is a port that connects an MST region to another MST configuration, or to a single
spanning-tree region running STP, or to a single spanning-tree region running RSTP.
During MSTP computing, a boundary port assumes the same role on the CIST and on MST instances.
Namely, if a boundary port is master port on the CIST, it is also the master port on all MST instances within this
region. In Figure 4, for example, if a device in region A0 is interconnected with the first port of a device in region D0
and the common root bridge of the entire switched network is located in region A0, the first port of that device in
region D0 is the boundary port of region D0.
&
Note :
Currently, the device is not capable of recognizing boundary ports. When the device interworks with a
third party’s device that supports boundary port recognition, the third party’s device may malfunction
in recognizing a boundary port.
10) Roles of ports
In the MSTP computing process, port roles include designated port, root port, master port, alternate port,
backup port, and so on.
·
Root port : a port responsible for forwarding data to the root bridge.
·
Designated port : a port responsible for forwarding data to the downstream network segment or device.
·
Master port : A port on the shortest path from the entire region to the common root bridge, connecting the
MST region to the common root bridge.
·
Alternate port : The standby port for the root port or master port. If a root port or master port is blocked,
the alternate port becomes the new root port or master port.
·
Backup port : The backup port of designated ports. When a designated port is blocked, the backup port
becomes a new designated port and starts forward data without delay. When a loop occurs while two ports of
the same MSTP device are interconnected, the device will block either of the two ports, and the backup port
is that port to be blocked.
A port can assume different roles in different MST instances.
