H3C Technologies H3C WX3000 Series Unified Switches - Manual (Page 180)

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mapped to MSTI 1; VLAN 2 is mapped to MSTI 2; and other VLANs are mapped to CIST. In an MST
region, load balancing is implemented according to the VLAN mapping table.
IST
An internal spanning tree (IST) is a spanning tree in an MST region.
ISTs together with the common spanning tree (CST) form the common and internal spanning tree (CIST)
of the entire switched network. An IST is a special MSTI; it is a branch of CIST in the MST region.
In Figure 22-4 , each MST region has an IST, which is a branch of the CIST.
CST
A CST is a single spanning tree in a switched network that connects all MST regions in the network. If
you regard each MST region in the network as a device, then the CST is the spanning tree generated by
STP or RSTP running on the "devices".
CIST
A CIST is the spanning tree in a switched network that connects all devices in the network. It comprises
the ISTs and the CST.
In Figure 22-4 , the ISTs in the MST regions and the CST connecting the MST regions form the CIST.
Region root
A region root is the root of the IST or an MSTI in an MST region. Different spanning trees in an MST
region may have different topologies and thus have different region roots.
In region D0 shown in Figure 22-4 , the region root of MSTI 1 is Device B, and the region root of MSTI 2
is Device C.
Common root bridge
The common root bridge is the root of the CIST. The common root bridge of the network shown in Figure
22-4 is a device in region A0.
Port role
During MSTP calculation, the following port roles exist: root port, designated port, master port, region
edge port, alternate port, and backup port.
z
A root port is used to forward packets to the root.
z
A designated port is used to forward packets to a downstream network segment or device.
z
A master port connects an MST region to the common root. The path from the master port to the
common root is the shortest path between the MST region and the common root.
z
A region edge port is located on the edge of an MST region and is used to connect one MST region
to another MST region, an STP-enabled region or an RSTP-enabled region
z
An alternate port is a secondary port of a root port or master port and is used for rapid transition.
With the root port or master port being blocked, the alternate port becomes the new root port or
master port.
z
A backup port is the secondary port of a designated port and is used for rapid transition. With the
designated port being blocked, the backup port becomes the new designated port fast and begins
to forward data seamlessly. When two ports of an MSTP-enabled device are interconnected, the
device blocks one of the two ports to eliminate the loop that occurs. The blocked port is the backup
port.
In Figure 22-5 , device A, device B, device C, and device D form an MST region. Port 1 and port 2 on
device A connect upstream to the common root. Port 5 and port 6 on device C form a loop. Port 3 and