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19.12 Description of common Spanning Tree functionality
19.12.1 PortFast
(1) Overview
PortFast is functionality for ports for which a terminal is connected and loops are known in
advance not to occur. PortFast is not subject to Spanning Tree topology calculations,
allowing communication immediately after link-up.
PortFast operates according to the PortFast settings and the port type. The following table
describes the operating conditions of the PortFast functionality.
Table 19-20 Operating conditions of the PortFast functionality
Configuration definition
Configuration for each
port
(spanning-tree portfast)
PortFast is set (trunk)
PortFast disabled (disable)
Parameter omitted
No command specified
Legend:
Y: Supported, --: Not supported
(2) BPDU reception when PortFast is applied
PortFast is set for ports for which no BPDUs are expected to be received, but when a
BPDU is received on a port for which PortFast is set, a switch might exist ahead, meaning
that a loop is possible. Therefore, PortFast functionality is stopped, and operation is started
as a normal port subject to Spanning Tree operations including topology calculations and
BPDU sending and reception.
After operation starts as a port subject to Spanning Tree operation, PortFast functionality is
enabled again by links being brought up or down.
Use this in combination with BPDU filter functionality to prevent PortFast functionality from
stopping when a BPDU is received.
(3) BPDU transmission when PortFast is applied
Because Spanning Tree Protocols cannot be run on ports for which PortFast is set, BPDUs
are not sent.
However, to detect whether ports with PortFast set are mistakenly connected, BPDUs are
sent for only 10 frames immediately after communication becomes possible due to
PortFast functionality.
(4) BPDU guard
Functionality applied to PortFast includes the BPDU guard functionality. On ports for which
Configuration for each
switch
(spanning-tree portfast
default)
(Priority is given to the
configuration for each port.)
Command specified
No command specified
19 Spanning Tree Protocol
Port type
Access port
Trunk port
Protocol port
MAC port
Y
Y
--
--
Y
--
Y
--
--
--
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