KSZ8893FQL datasheet(35/109 Pages) MICROCHIP

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KSZ8893FQL

DS00003038A-page 34

 2019 Microchip Technology Inc.

3.4.2

SPECIAL TAGGING MODE

Special Tagging Mode is designed for spanning tree protocol IGMP snooping and is flexible for use in other applications.

Special Tagging, similar to 802.1Q Tagging, requires software to change network drivers to insert/modify/strip/interpret

the special tag. This mode is enabled by setting both register 11 bit [0] and register 48 bit [2] to ‘1’.

TABLE 3-16:

SPECIAL TAGGING MODE FORMAT

802.1Q Tag Format

Special Tag Format

TPID (tag protocol identifier, 0x8100) + TCI

STPID (special tag identifier, 0x810 + 4 bit for “port mask”) +

TCI

The STPID is only seen and used by the port 3 interface, which should be connected to a processor. Packets from the

processor to the switch’s port 3 should be tagged with the STPID and the port mask, defined as follows:

• “0001”, forward packet to port 1 only

• “0010”, forward packet to port 2 only

• “0011”, broadcast packet to port 1 and port 2

Packets with normal tags (“0000” port masks) will use KSZ8893FQL internal MAC table look-up to determine the for-

warding port(s). Also, if packets from the processor are not tagged, the KSZ8893FQL will treat them as normal packets

and use internal MAC table lookup to determine the forwarding port(s).

The KSZ8893FQL uses a non-zero “port mask” to bypass the internal MAC table lookup result, and override any port

setting, regardless of port states (disable, blocking, listening, and learning). Table 3-17 below shows the processor to

switch egress rules when dealing with STPID.

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 Special Tagging Mode 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 (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 Special Tagging Mode 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 programs the “Static MAC table” with the entries that it

needs to receive (for example, BPDU packets). The “overriding” bit is

set so that the switch forwards those specific packets to the proces-

sor. The processor can send packets to the port(s) in this state. See

Special Tagging Mode for details. Address learning is enabled on the

port in this state.

TABLE 3-17:

STPID EGRESS RULES (PROCESSOR TO SWITCH PORT 3)

Ingress Tag Field

TX Port “Tag

Insertion”

TX Port “Tag

Removal”

Egress Action to Tag Field

(0x810 + port mask)

- Modify tag field to 0x8100

- Recalculate CRC

- No change to TCI if not null VID

- Replace VID with ingress (port 3) port VID if null VID

(0x810 + port mask)

- (STPID + TCI) will be removed

- Padding to 64 bytes if necessary

- Recalculate CRC

TABLE 3-15:

SPANNING TREE STATES (CONTINUED)

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