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QSW-2900-24T-AC
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Qtech QSW-2900-24T-AC [151/209] Roles of ports

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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.

Qtech QSW-2900-24T-AC [151/209] Roles of ports

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