CIST
A CIST, calculated by using STP or RSTP, connects all the switching devices on a switching
network.
As shown in Figure 7-6, the ISTs and the CST form a complete spanning tree, the CIST.
SST
A Single Spanning Tree (SST) is formed in either of the following situations:
l A switching device running STP or RSTP belongs to only one spanning tree.
l An MST region has only one switching device.
As shown in Figure 7-6, the switching device in B0 forms an SST.
Port Role
Based on RSTP, MSTP has two additional port types. MSTP ports can be root ports, designated
ports, alternate ports, backup ports, edge ports, master ports, and regional edge port.
The functions of root ports, designated ports, alternate ports, and backup ports have been defined
in RSTP. Table 7-2 lists all port roles in MSTP.
NOTE
Except edge ports, all ports participate in MSTP calculation.
A port can play different roles in different spanning tree instances.
Table 7-2 Port roles
Port
Role
Description
Root port A root port is the non-root bridge port closest to the root bridge. Root bridges do
not have root ports.
Root ports are responsible for sending data to root bridges.
As shown in Figure 7-7, S1 is the root; CP1 is the root port on S3; BP1 is the
root port on S2.
Designate
d port
The designated port on a switching device forwards BPDUs to the downstream
switching device.
As shown in Figure 7-7, AP2 and AP3 are designated ports on S1; CP2 is a
designated port on S3.
Alternate
port
l From the perspective of sending BPDUs, an alternate port is blocked after a
BPDU sent by another bridge is received.
l From the perspective of user traffic, an alternate port provides an alternate
path to the root bridge. This path is different than using the root port.
As shown in Figure 7-7, BP2 is an alternate port.
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching 7 MSTP Configuration
Issue 01 (2014-11-30) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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