Huawei Quidway S3000 Series Operation Manual page 29

Operation Manual - STP
Quidway S3000 Series Ethernet Switches
II. MSTI calculation
Inside an MST region, MSTP generates different MSTIs for different VLANs according
to the association between VLAN and the spanning tree. The calculation process of
MSTI is same like RSTP.
In this way, the packets of a VLAN travel along the corresponding MSTI inside the MST
region and the CST between different regions.
Followed introduce the calculation process of one MSTI.
The fundamental of STP is that the switches exchange a special kind of protocol packet
(which is called configuration Bridge Protocol Data Units, or BPDU, in IEEE 802.1D) to
decide the topology of the network. The configuration BPDU contains the information
enough to ensure the switches to compute the spanning tree.
The configuration BPDU mainly contains the following information:
1) The root ID consisting of root priority and MAC address
2) The cost of the shortest path to the root
3) Designated switch ID consisting of designated switch priority and MAC address
4) Designated port ID consisting of port priority and port number
5) The age of the configuration BPDU: MessageAge
6) The maximum age of the configuration BPDU: MaxAge
7) Configuration BPDU interval: HelloTime
8) Forward delay of the port: ForwardDelay.
What are the designated switch and designated port?
Switch B
Figure2-3 Designated switch and designated port
For a switch, the designated switch is a switch in charge of forwarding packets to the
local switch via a port called the designated port accordingly. For a LAN, the designated
switch is a switch that in charge of forwarding packets to the network segment via a port
called the designated port accordingly.
forwards data to Switch B via the port AP1. So to Switch B, the designated switch is
Switch A
AP1 AP2
BP1
Switch C
BP2
LAN
CP1
CP2
As illustrated in the Figure2-3, Switch A
2-5
Chapter 1 RSTP Configuration