SMC Networks 7724M/VSW - annexe 1, Руководство пользователя

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CLI Management Guide
TigerAccess™ EE
260 SMC7824M/VSW
Switch B
Switch A
Priority : 9
Switch C
Switch D
Priority : 10
Priority : 8
Root Switch
Designated
Switch
Path-cost
50
Path-cost
100
Path-cost
100
Path 2
Path 1
(PATH 1 = 50 + 100 = 150, PATH 2 = 100 + 100 = 200, PATH 1 < PATH 2, ∴ PATH 1 selected
Path-cost
100
Fig. 8.12 Designated Switch
In case of the above picture showing SWITCH C sends packet, path-cost of PATH 1 is
150 and path- cost of PATH 2 is total 200(100 + 100 ; path-cost of SWITCH C to B + path-
cost of SWITCH B to C). Therefore lower path-cost, PATH 1 is chosen. In this case, port
connected to root switch is named root port. In the above picture, port of SWITCH C con-
nected to SWITCH A as Root switch is root port. There can be only one root port on
equipment.
When root path-costs are same, bridge ID is compared.
Designated Port and Root Port
A root port is the port in the active topology that provides connectivity from the designated
switch toward the root. A designated port is a port in the active topology used to forward
traffic away from the root onto the link for which this switch is the designated switch. That
is; except root port in each switch, the selected port to communicate is a designated port.
Port Priority
Meanwhile, when the path-cost of two paths are same, port-priority is compared. As the
below picture, suppose that two switches are connected. Since the path-costs of two
paths are 100, same, their port priorities are compared and port with smaller port priority
is selected to transmit packet.
All these functions are automatically performed by BPDU, which is the bridge information
exchanged between switches to activate or disable a specific port. It is also possible to
configure BPDU to change a root switch or path manually.
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