Datasheet

KSZ8895MQX/RQX/FQX/MLX
DS00002246A-page 36  2016 Microchip Technology Inc.
Please see the descriptions of Register 130 bits [7:6] for detail.
3.5.1.1 Port-Based Priority
With port-based priority, each ingress port is individually classified as a priority 0-3 receiving port. All packets received
at the priority 3 receiving port are marked as high priority and are sent to the high-priority transmit queue if the corre-
sponding transmit queue is split. The Port Registers Control 0 bits [4:3] is used to enable port-based priority for ports 1,
2, 3, 4 and 5, respectively.
3.5.1.2 802.1p-Based Priority
For 802.1p-based priority, the KSZ8895MQX/RQX/FQX/MLX examines the ingress (incoming) packets to determine
whether they are tagged. If tagged, the 3-bit priority field in the VLAN tag is retrieved and compared against the “priority
mapping” value, as specified by the Registers 128 and 129, both Register 128/129 can map 3-bit priority field of 0-7
value to 2-bit result of 0-3 priority levels. The “priority mapping” value is programmable.
Figure 3-8 illustrates how the 802.1p priority field is embedded in the 802.1Q VLAN tag.
802.1p-based priority is enabled by bit [5] of the Registers Port Control 0 for ports 1, 2, 3, 4 and 5, respectively.
The KSZ8895MQX/RQX/FQX/MLX provides the option to insert or remove the priority tagged frame's header at each
individual egress port. This header, consisting of the two-byte VLAN Protocol ID (VPID) and the two-byte Tag Control
Information field (TCI), is also referred to as the IEEE 802.1Q VLAN tag.
Tag Insertion is enabled by bit [2] of the Registers Port Control 0 and the Register Port Control 8 to select which source
port (ingress port) PVID can be inserted on the egress port for ports 1, 2, 3, 4 and 5, respectively. At the egress port,
untagged packets are tagged with the ingress port’s default tag. The default tags are programmed in the Registers Port
Control 3 and control 4 for ports 1, 2, 3, 4 and 5, respectively. The KSZ8895MQX/RQX/FQX/MLX will not add tags to
already tagged packets.
Tag Removal is enabled by bit [1] of the Registers Port Control 0 for ports 1, 2, 3, 4 and 5, respectively. At the egress
port, tagged packets will have their 802.1Q VLAN tags removed. The KSZ8895MQX/RQX/FQX/MLX will not modify
untagged packets.
The CRC is recalculated for both tag insertion and tag removal.
802.1p Priority Field Re-Mapping is a QoS feature that allows the KSZ8895MQX/RQX/FQX/MLX to set the “User Pri-
ority Ceiling” at any ingress port by the Register Port Control 2 bit 7. If the ingress packet’s priority field has a higher
priority value than the default tag’s priority field of the ingress port, the packet’s priority field is replaced with the default
tag’s priority field.
FIGURE 3-8: 802.1P PRIORITY FIELD FORMAT