KSZ8893-MQL Micrel Semiconductor, Inc., KSZ8893-MQL Datasheet - Page 42

• Manufacturer Part Number: KSZ8893-MQL
• Description: Integrated 3-Port 10/100 Managed Switch with PHYs
• Manufacturer: Micrel Semiconductor, Inc.

802.1p-based priority is enabled by bit [5] of registers 16, 32 and 48 for ports 1, 2 and 3, respectively.
The KSZ8893MQL 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 registers 16, 32 and 48 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. The KSZ8893MQL 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 KSZ8893MQL 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 KSZ8893MQL to set the “User Priority
Ceiling” 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. The “User
Priority Ceiling” is enabled by bit [3] of registers 17, 33 and 49 for ports 1, 2 and 3, respectively.
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 determine 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.
Rate Limiting Support
The KSZ8893MQL supports hardware rate limiting from 64 Kbps to 88 Mbps, independently on the “receive side”
and on the “transmit side” on a per port basis. For 10BASE-T, a rate setting above 10 Mbps means the rate is not
limited. On the receive side, the data receive rate for each priority at each port can be limited by setting up
Ingress Rate Control Registers. On the transmit side, the data transmit rate for each priority queue at each port
can be limited by setting up Egress Rate Control Registers. The size of each frame has options to include
minimum IFG (Inter Frame Gap) or Preamble byte, in addition to the data field (from packet DA to FCS).
For ingress rate limiting, KSZ8893MQL provides options to selectively choose frames from all types, multicast,
broadcast, and flooded unicast frames. The KSZ8893MQL counts the data rate from those selected type of
frames. Packets are dropped at the ingress port when the data rate exceeds the specified rate limit.
For egress rate limiting, the Leaky Bucket algorithm is applied to each output priority queue for shaping output
traffic. Inter frame gap is stretched on a per frame base to generate smooth, non-burst egress traffic. The
throughput of each output priority queue is limited by the egress rate specified.
If any egress queue receives more traffic than the specified egress rate throughput, packets may be accumulated
in the output queue and packet memory. After the memory of the queue or the port is used up, packet dropping or
flow control will be triggered. As a result of congestion, the actual egress rate may be dominated by flow
control/dropping at the ingress end, and may be therefore slightly less than the specified egress rate.
To reduce congestion, it is a good practice to make sure the egress bandwidth exceeds the ingress bandwidth.
Unicast MAC Address Filtering
The unicast MAC address filtering function works in conjunction with the static MAC address table. First, the
static MAC address table is used to assign a dedicated MAC address to a specific port. If a unicast MAC address
is not recorded in the static table, it is also not learned in the dynamic MAC table. The KSZ8893MQL is then
configured with the option to either filter or forward unicast packets for an unknown MAC address. This option is
enabled and configured in register 14.
Micrel
November 2005
42
KSZ8893MQL/MQLI
M9999-111705