Mstp Implementation On Switches - 3Com 4200G 12-Port Configuration Manual

For configuration BPDUs with both the same Root bridge ID and the same External path costs,
Master bridge ID, Internal path cost, Designated bridge ID,ID of sending port,ID of receiving port
are compared in turn.
For MSTP, MSTI configuration information is generally expressed as follows:
(Instace bridge ID, Internal path costs, Designated bridge ID,ID of sending port,ID of receiving port),so
the compared as follows
The smaller the Instance bridge ID of the configuration BPDU is, the higher the priority of the
configuration BPDU is.
For configuration BPDUs with the same Instance bridge IDs, Internal path costs are compared.
For configuration BPDUs with both the same Instance bridge ID and the same Internal path costs,
Designated bridge ID,ID of sending port,ID of receiving port are compared in turn.
3) A spanning tree is calculated as follows:
Determining the root bridge
Root bridges are selected by configuration BPDU comparing. The switch with the smallest root ID is
chosen as the root bridge.
Determining the root port
For each switch in a network, the port on which the configuration BPDU with the highest priority is
received is chosen as the root port of the switch.
Determining the designated port
First, the switch calculates a designated port configuration BPDU for each of its ports using the root port
configuration BPDU and the root port path cost, with the root ID being replaced with that of the root port
configuration BPDU, root path cost being replaced with the sum of the root path cost of the root port
configuration BPDU and the path cost of the root port, the ID of the designated bridge being replaced
with that of the switch, and the ID of the designated port being replaced with that of the port.
The switch then compares the calculated configuration BPDU with the original configuration BPDU
received from the corresponding port on another switch. If the latter takes precedence over the former,
the switch blocks the local port and keeps the port's configuration BPDU unchanged, so that the port
can only receive configuration messages and cannot forward packets. Otherwise, the switch sets the
local port to the designated port, replaces the original configuration BPDU of the port with the calculated
one and advertises it regularly.

MSTP Implementation on Switches

MSTP is compatible with both STP and RSTP. That is, MSTP-enabled switches can recognize the
protocol packets of STP and RSTP and use them for spanning tree calculation. In addition to the basic
MSTP functions, 3com switches also provide the following functions for users to manage their switches.
Root bridge hold
Root bridge backup
Root guard
BPDU guard
Loop guard
TC-BPDU attack guard
BPDU packet drop
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