User's Manual

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entering the Forwarding state, the port must be in the Listening state and Learning state.
The same with BPDU Guard, BPDU Filter characteristic can be configured in global configuration mode or in
port configuration mode. In global configuration mode, run the command spanning-tree portfast bpdufilter to
block all ports to send BPDU out. The port, however, can still receive and process BPDU.
37.1.4 Uplink Fast
The characteristic Uplink Fast enables new root ports to rapidly enter the Forwarding state when the
connection between the switch and the root bridge is disconnected.
A complex network always contains multiple layers of devices, as shown in figure 1.2. Both aggregation layer
and the access layer of the switch have redundancy connections with the upper layer. These redundancy
connections are normally blocked by the STP to avoid loops.
Figure 1.2 Switching network topology
Suppose the connection between a switch and the upper layer is disconnected (called as Direct Link Failure),
the STP chooses the Alternate port on the redundancy line as the root port. Before entering the Forwarding
state, the Alternate port must be in the Listening state and Learning state. If the Uplink Fast feature is
configured by running the command spanning-tree uplinkfast in global configuration mode, new root port can
directly enter the forwarding state, resuming the connection between the switch and the upper layer.
Figure 1.3 shows the working principle of the Uplink Fast feature. The port for device C to connect device B is
the standby port when the port is in the original state. When the connection between device C and root device
A is disconnected, the previous Alternate port is selected as new root port and immediately starts forwarding.