KSZ9563R datasheet(37/226 Pages) MICROCHIP

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DS00002419D-page 37

KSZ9563R

4.4.8

MULTIPLE SPANNING TREE SUPPORT

Multiple Spanning Tree Protocol (MSTP) is an extension of RSTP that allows different VLANs to have different spanning

tree configurations. The VLAN Table, Address Lookup Table and Static Address Table all contain a 3-bit field which can

be used to specify one of eight spanning trees. Each port contains state registers for specifying unique states for each

of the spanning trees.

4.4.9

TAIL TAGGING MODE

Tail tagging is a method to communicate ingress and egress port information between the host processor and the switch.

It us useful for spanning tree protocol, IGMP/MLD snooping, IEEE 1588, and other applications. As shown in Figure 4-

5, the tail tag is inserted at the end of the packet, between the payload and the 4-byte CRC / FCS.

When the switch forwards a received packet to the host port, one tail tagging byte is added to the packet by the switch

to indicate to the host processor which port the packet was received on. The format is shown in Table 4-13.

In the opposite direction, the host processor must add one tail tag byte to each packet that it sends to the switch to indi-

cate the intended egress ports. When multiple priority queues are enabled, the tail tag is also used to indicate the priority

queue. The format is shown in Table 4-14. This tail tag is removed by the switch before the packet leaves the switch. If

the Lookup bit (bit 6) is set, packet forwarding follows the standard forwarding process, and bits [5:0] are ignored. When

the Lookup bit is not set, bits [4:0] determine the forwarding ports and priority queue, while the Override bit (bit 5) deter-

mines whether port blocking is overridden.

Tail tagging applies only to the host port, never to any other ports of the switch.

FIGURE 4-5:

TAIL TAG FRAME FORMAT

TABLE 4-13:

RECEIVED TAIL TAG FORMAT (FROM SWITCH TO HOST)

Bits

Description

PTP Message Indication

0 = Is not a PTP message. A 4-byte receive timestamp has not been added.

1 = Is a PTP message. A 4-byte timestamp has been added before the tail tag.

6:2

RESERVED

1:0

Received Port

00 = Packet received at Port 1

01 = Packet received at Port 2

10 = Packet received at Port 3

11 = RESERVED

TABLE 4-14:

TRANSMIT TAIL TAG FORMAT (FROM HOST TO SWITCH)

Bits

Description

RESERVED

Lookup

0 = Port forwarding is determined by tail tag bits [5:0] below.

1 = Tail tag bits [5:0] are ignored and port forwarding is determined by the standard

switch forwarding process (address lookup, VLAN, etc.)

Port Blocking Override

When set, packets will be sent from the specified port9s0 regardless, and any port

blocking (see Port Transmit Enable in Port MSTP State Register) is ignored.

4:3

Egress priority (0 to 3)

DESTINATION

ADDRESS

SOURCE

ADDRESS

802.1Q

TAG

(4)

ETYPE or

LENGTH

TAIL

TAG

PAYLOAD

46 (42) - 1500

FCS

BYTES

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