Datasheet

KSZ8795CLX
DS00002112B-page 36  2016-2017 Microchip Technology Inc.
The KSZ8795CLX supports common spanning tree (CST). To support spanning tree, the host port (Port 5) is the des-
ignated port for the processor. The other ports can be configured in one of the five spanning tree states via “transmit
enable”, “receive enable” and “learning disable” register settings in: Port Control 2 Registers. Tab le 3-1 3 shows the port
setting and software actions taken for each of the five spanning tree states.
TABLE 3-13: PORT SETTING AND SOFTWARE ACTIONS FOR SPANNING TREE
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 table with “overriding bit” set) and the pro-
cessor should discard those packets.
Note: Processor is connected to Port 5 via MII interface. Address
learning is disabled on the port in this state.
Blocking State Port Setting Software Action
Only packets to the
processor are for-
warded. 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 (e.g., BPDU packets). The “overriding” bit
should also be set so that the switch will forward those specific pack-
ets to the processor. 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 (e.g. 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” section for details). Address learning is
disabled 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 (e.g., 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” section for details). Address learning is
enabled on the port in this state.
Forwarding State Port Setting Software Action
Packets are for-
warded and received
normally. Learning is
enabled.
“Transmit enable = 1,
Receive enable = 1,
Learning disable = 0.”
The processor should program the static MAC table with the entries
that it needs to receive (e.g., 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” section for details). Address learning is
enabled on the port in this state.