Operation Manual – MSTP
H3C S3100 Series Ethernet Switches
Chapter 1 MSTP Configuration
1-17
z
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:
(Instance bridge ID, Internal path costs, Designated bridge ID, ID of sending port, ID of
receiving port), so the compared as follows
z
The smaller the Instance bridge ID of the configuration BPDU is, the higher the
priority of the configuration BPDU is.
z
For configuration BPDUs with the same Instance bridge IDs, Internal path costs
are compared.
z
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:
z
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.
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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.
z
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.
1.2.4 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, H3C series switches also provide
the following functions for users to manage their switches.