After a switching device becomes the root, it sends BPDUs at Hello intervals. Non-root switching
devices adopt the Hello Time value set for the root.
Huawei datacom devices allow the maximum number of BPDUs sent by a port at a Hello interval
to be configured as needed.
The greater the Hello Time value, the more BPDUs sent at a Hello interval. Setting the Hello
Time to a proper value limits the number of BPDUs sent by a port at a Hello interval. This helps
prevent network topology flapping and avoid excessive use of bandwidth resources by BPDUs.
7.2.4 MSTP Topology Calculation
MSTP Principle
In MSTP, the entire Layer 2 network is divided into multiple MST regions, which are
interconnected by a single CST. In an MST region, multiple spanning trees are calculated, each
of which is called an MSTI. Among these MSTIs, MSTI 0 is also known as the internal spanning
tree (IST). Like STP, MSTP uses configuration messages to calculate spanning trees, but the
configuration messages are MSTP-specific.
Vectors
Both MSTIs and the CIST are calculated based on vectors, which are carried in MST BPDUs.
Therefore, switching devices exchange MST BPDUs to calculate MSTIs and the CIST.
l
Vectors are described as follows:
–
The following vectors participate in the CIST calculation:
{ root ID, external root path cost, region root ID, internal root path cost, designated
switching device ID, designated port ID, receiving port ID }
–
The following vectors participate in the MSTI calculation:
{ regional root ID, internal root path cost, designated switching device ID, designated
port ID, receiving port ID }
The priorities of vectors in braces are in descending order from left to right.
Table 7-7
Vector description
Vector Name
Description
Root ID
Identifies the root switching device for the CIST. The root identifier
consists of the priority value (16 bits) and MAC address (48 bits).
The priority value is the priority of MSTI 0.
External root path
cost (ERPC)
Indicates the path cost from a CIST regional root to the root. ERPCs
saved on all switching devices in an MST region are the same. If
the CIST root is in an MST region, ERPCs saved on all switching
devices in the MST region are 0s.
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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