3Com Switch 7754, Configuration Manual

Page: 245 / 940

MSTP Overview 245
Implementation of MSTP MSTP divides a network into multiple MST regions at Layer 2. The CST is
generated between these MST regions, and multiple spanning trees (or, MSTIs)
can be generated in each MST region. As well as RSTP, MSTP uses configuration
BPDUs to generate spanning trees. The only difference is that the configuration
BPDUs for MSTP carry the MSTP configuration information on the switches.
Generating the CIST
Through configuration BPDU comparing, the switch that is of the highest priority
in the network is chosen as the root of the CIST. In each MST region, an IST is
figured out by MSTP. At the same time, MSTP regards each MST region as a switch
to figure out the CST of the network. The CST, together with the ISTs, forms the
CIST of the network.
Generating an MSTI
In an MST region, different MSTIs are generated for different VLANs depending on
the VLAN-to-spanning-tree mappings. Each spanning tree is figured out
independently, in the same way as STP/RSTP.
Implementation of STP algorithm
In the beginning, each switch regards itself as the root, and generates a
configuration BPDU for each port on it as a root, with the root path cost being 0,
the ID of the designated bridge being that of the switch, and the designated port
being itself.
1 Each switch sends out its configuration BPDUs and operates in the following way
when receiving a configuration BPDU on one of its ports from another switch:
■
If the priority of the configuration BPDU is lower than that of the configuration
BPDU of the port itself, the switch discards the BPDU and does not change the
configuration BPDU of the port.
■
If the priority of the configuration BPDU is higher than that of the
configuration BPDU of the port itself, the switch replaces the configuration
BPDU of the port with the received one and compares it with those of other
ports on the switch to obtain the one with the highest priority.
2 Configuration BPDUs are compared as follows:
■
The smaller the root ID of the configuration BPDU is, the higher the priority of
the configuration BPDU is.
■
For configuration BPDUs with the same root IDs, the comparison is based on
the path costs. Suppose S is the sum of the root path cost and the
corresponding path cost of the port. The less the S value is, the higher the
priority of the configuration BPDU is.
■
For configuration BPDUs with both the same root ID and the same root path
cost, the designated bridge ID, designated port ID, the ID of the receiving port
are compared in turn.
3 A spanning tree is figured out as follows:
■
Determining the root bridge
The root bridge is selected by configuration BPDU comparing. The switch with the
smallest root ID is chosen as the root bridge.
■
Determining the root port