Datasheet

ManualsBrandsMicrochip ManualsIntegrated Circuits (ICs)5-Port Managed Ethernet Switch Ethernet/SPI/MAC/PHY/MII/RMII/RGMII/GMII LQFP-80

 2016-2017 Microchip Technology Inc. DS00002112B-page 37

KSZ8795CLX

3.6.3 RAPID SPANNING TREE SUPPORT

There are three operational states of the discarding, learning, and forwarding assigned to each port for RSTP. Discard-

ing ports do not participate in the active topology and do not learn MAC addresses. Ports in the learning states learn

MAC addresses, but do not forward user traffic. Ports in the forwarding states fully participate in both data forwarding

and MAC learning. RSTP uses only one type of BPDU called RSTP BPDUs. They are similar to STP configuration

BPDUs with the exception of a type field set to “version 2” for RSTP and “version 0” for STP, and flag field carrying addi-

tional information.

3.6.4 TAIL TAGGING MODE

The tail tag is only seen and used by the Port 5 interface, which should be connected to a processor by the SW5-GMII,

RGMII, MII, or RMII interfaces. One byte tail tagging is used to indicate the source/destination port on Port 5. Only bits

[3:0] are used for the destination in the tail tagging byte. Other bits are not used. The tail tag feature is enabled by setting

TABLE 3-14: PORT SETTING AND SOFTWARE ACTIONS FOR RAPID SPANNING TREE

Disable State Port Setting Software Action

The state includes

three states of the

disable, blocking and

listening of STP.

"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. When disable the port’s learning

capability (learning disable = ’1’), set the Register 1 Bit[5] and Bit[4]

will flush rapidly with the port-related entries in the dynamic MAC

table and static MAC table.

Note: Processor is connected to Port 5 via MII interface. 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,