Datasheet

 2017 Microchip Technology Inc. DS00002348A-page 29
KSZ8873MLL/FLL/RLL
3.6 Rapid Spanning Tree Support
There are three operational states of the Discarding, Learning, and Forwarding assigned to each port for RSTP:
Discarding ports do not participate in the active topology and do not learn MAC addresses.
Discarding state: the state includes three states of the disable, blocking and listening of STP.
Port setting: "transmit enable = 0, receive enable = 0, learning disable = 1."
Software action: the processor should not send any packets to the port. The switch may still send specific packets to
the processor (packets that match some entries in the static table with “overriding bit” set) and the processor should
discard those packets. When disable the port’s learning capability (learning disable=’1’), set the register 2 bit 5 and bit
4 will flush rapidly the port related entries in the dynamic MAC table and static MAC table.
Note: processor is connected to port 3 via MII interface. Address learning is disabled on the port in this state.
Ports in Learning states learn MAC addresses, but do not forward user traffic.
TABLE 3-12: SPANNING TREE STATES
Disable State Port Setting Software Action
The port should not forward or
receive any packets. Learn-
ing is disabled.
“transmit enable = 0,
receive enable = 0,
learning disable =1”
The processor should not send any packets to the port. The
switch may still send specific packets to the processor
(packets that match some entries in the “static MAC table”
with “overriding bit” set) and the processor should discard
those packets. Address learning is disabled on the port in
this state.
Blocking State Port Setting Software Action
Only packets to the processor
are forwarded. Learning is
disabled.
“transmit enable = 0,
receive enable = 0,
learning disable =1”
The processor should not send any packets to the port(s) in
this state. The processor should program the “Static MAC
table” with the entries that it needs to receive (for example,
BPDU packets). The “overriding” bit should also be set so
that the switch will forward those specific packets to the pro-
cessor. Address learning is disabled on the port in this state.
Listening State Port Setting Software Action
Only packets to and from the
processor are forwarded.
Learning is disabled.
“transmit enable = 0,
receive enable = 0,
learning disable =1”
The processor should program the “Static MAC table” with
the entries that it needs to receive (for example, BPDU
packets). The “overriding” bit should be set so that the
switch will forward those specific packets to the processor.
The processor may send packets to the port(s) in this state.
See “Tail Tagging Mode” for details. Address learning is dis-
abled on the port in this state.
Learning State Port Setting Software Action
Only packets to and from the
processor are forwarded.
Learning is enabled.
“transmit enable = 0,
receive enable = 0,
learning disable = 0”
The processor should program the “Static MAC table” with
the entries that it needs to receive (for example, BPDU
packets). The “overriding” bit should be set so that the
switch will forward those specific packets to the processor.
The processor may send packets to the port(s) in this state.
See “Tail Tagging Mode” for details. Address learning is
enabled on the port in this state.
Forwarding State Port Setting Software Action
Packets are forwarded and
received normally. Learning is
enabled.
“transmit enable = 1,
receive enable = 1,
learning disable = 0”
The processor programs the “Static MAC table” with the
entries that it needs to receive (for example, BPDU pack-
ets). The “overriding” bit is set so that the switch forwards
those specific packets to the processor. The processor can
send packets to the port(s) in this state. See “Tail Tagging
Mode” for details. Address learning is enabled on the port in
this state.