KSZ8795CLX
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. Table 3-13 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
The port should not "Transmit enable = 0, The processor should not send any packets to the port. The switch
forward or receive Receive enable = 0, may still send specific packets to the processor (packets that match
any packets. Learn- Learning disable = 1." some entries in the static table with “overriding bit” set) and the pro-
Port Setting
Software Action
ing is disabled.
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 "Transmit enable = 0, The processor should not send any packets to the port(s) in this state.
processor are for- Receive enable = 0, The processor should program the static MAC table with the entries
warded. Learning is Learning disable = 1" that it needs to receive (e.g., BPDU packets). The “overriding” bit
disabled.
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 "Transmit enable = 0, The processor should program the static MAC table with the entries
from the processor
are forwarded.
Learning is disabled.
Receive enable = 0, that it needs to receive (e.g. BPDU packets). The “overriding” bit
Learning disable = 1. 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 “Transmit enable = 0, The processor should program the static MAC table with the entries
from the processor
are forwarded.
Learning is enabled.
Receive enable = 0, that it needs to receive (e.g., BPDU packets). The “overriding” bit
Learning disable = 0.” 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-
“Transmit enable = 1, The processor should program the static MAC table with the entries
warded and received Receive enable = 1, that it needs to receive (e.g., BPDU packets). The “overriding” bit
normally. Learning is Learning disable = 0.” should be set so that the switch will forward those specific packets to
enabled.
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.
2016 Microchip Technology Inc.
DS00002112A-page 35
MICREL [ MICREL SEMICONDUCTOR ]
