Switch User Manual

1-14
For MSTP, CIST configuration information is generally expressed as follows:
(Root bridge ID, External path cost, Master bridge ID, Internal path cost, Designated bridge ID, ID of
sending port, ID of receiving port), so the compared as follows
z The smaller the Root bridge ID of the configuration BPDU is, the higher the priority of the
configuration BPDU is.
z For configuration BPDUs with the same Root bridge IDs, the External path costs are compared.
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 through the comparison of configuration BPDUs. The device with the
smallest root ID is chosen as the root bridge.
z Determining the root port
For each device in a network, the port on which the configuration BPDU with the highest priority is
received is chosen as the root port of the device.
z Determining the designated port
First, the device 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 device, and the ID of the designated port being replaced with that of the port.
The device then compares the calculated configuration BPDU with the original configuration BPDU
received from the corresponding port on another device. If the latter takes precedence over the former,
the device 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 device 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 the Device
MSTP is compatible with both STP and RSTP. That is, an MSTP-enabled device can recognize the
protocol packets of STP and RSTP and use them for spanning tree calculation. In addition to the basic
MSTP functions, the device also provides the following functions for users to manage their devices.
z Root bridge hold
z Root bridge backup
z Root guard