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Root port
The root port is the port in a non-root bridge device that has the smallest path cost from the
bridge to the root bridge, responsible for communication with the root bridge. There is only
one root port on the non-root bridge device and no root port on the root bridge device.
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Designated bridge and designated port
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Designated bridge: For a switch, designated bridge is the device that connects to and
forwards BPDUs to the switch. For the LAN, it is the device that forwards BPDUs in
the same network segment.
In each network segment, the device with the least path cost to the root bridge is the
designated bridge. If more than one switch has the same path cost to the root bridge,
the one with the smallest bridge ID is the designated bridge.
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Designated port: As for a device, the designated port is the port that forwards BPDUs
to the host. As for a LAN, it is the port that forwards BPDUs in the same network
segment.
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Path cost
It is a parameter for choosing the link path by STP. By calculating the path cost, STP chooses the
better links and blocks the redundant links, so as to disbranch the loop-network to form a tree-
topological loop-free network.
The basic network diagram of STP is shown as the following figure. The switch A, B and C are
connected successively.
Switch A
Port A1
Port A2
Port C6
Port B3
Port B4
Port C5
Switch B
Switch C
After calculation, switch A is selected as the root bridge, and the link between ports A2 and C6
is blocked.
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Bridges: Switch A is the root bridge of the network, while switch B is the designated
bridge of switch C.
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Ports: Port B3 and port C5 are the root ports of switch B and switch C respectively.
Port A1 and port B4 are the designated ports of switch A and switch B respectively.
Port C6 is the blocking port of switch C.
Chapter 9 Device management 34