Datasheet

KSZ8873MLL/FLL/RLL
DS00002348A-page 28  2017 Microchip Technology Inc.
3.4.5 802.1P-BASED PRIORITY
For 802.1p-based priority, the KSZ8873MLL/FLL/RLL 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 12 and 13. The “priority mapping” value is programmable.
Figure 3-6 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 registers 16, 32, and 48 for ports 1, 2, and 3, respectively.
The KSZ8873MLL/FLL/RLL provides the option to insert or remove the priority tagged frame's header at each individual
egress port. This header, consisting of the 2 bytes VLAN Protocol ID (VPID) and the 2-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 port registers control 0 and the register 194 to select which source port (ingress
port) PVID can be inserted on the egress port for ports 1, 2, and 3, respectively. At the egress port, untagged packets
are tagged with the ingress port’s default tag. The default tags are programmed in register sets {19,20}, {35,36}, and
{51,52} for ports 1, 2, and 3, respectively, and the source port VID has to be inserted at selected egress ports by bit[5:0]
of register 194. The KSZ8873MLL/FLL/RLL will not add tags to already tagged packets.
Tag Removal is enabled by bit [1] of registers 16, 32, and 48 for ports 1, 2, and 3, respectively. At the egress port, tagged
packets will have their 802.1Q VLAN Tags removed. The KSZ8873MLL/FLL/RLL 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 KSZ8873MLL/FLL/RLL to set the “User Priority Ceil-
ing” at any ingress port. 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.
3.4.6 DIFFSERV-BASED PRIORITY
DiffServ-based priority uses the ToS registers (registers 96 to 111) in the Advanced Control Registers section. The ToS
priority control registers implement a fully decoded, 64-bit Differentiated Services Code Point (DSCP) register to deter-
mine packet priority from the 6-bit ToS field in the IP header. When the most significant 6 bits of the ToS field are fully
decoded, the resultant of the 64 possibilities is compared with the corresponding bits in the DSCP register to determine
priority.
3.5 Spanning Tree Support
To support spanning tree, port 3 is designated as the processor port.
The other ports (port 1 and port 2) can be configured in one of the five spanning tree states via “transmit enable”, “receive
enable” and “learning disable” register settings in registers 18 and 34 for ports 1 and 2, respectively. The following table
shows the port setting and software actions taken for each of the five spanning tree states.
FIGURE 3-6: 802.1P PRIORITY FIELD FORMAT
Preamble DA TCI
866 2
length LLC Data FCS
2 46-1500 4
1
Tagged Packet Type
(8100 for Ethernet)
802.1p
CFI
VLAN ID
Bytes
Bits
16 3 12
802.1q VLAN Tag
2
SA VPID