Datasheet

KSZ8873MLL/FLL/RLL
DS00002348A-page 22  2017 Microchip Technology Inc.
3.3.7.3 Late Collision
If a transmit packet experiences collisions after 512 bit times of the transmission, the packet is dropped.
3.3.7.4 Illegal Frames
The KSZ8873MLL/FLL/RLL discards frames less than 64 bytes and can be programmed to accept frames up to1518
bytes, 1536 bytes, or 1916 bytes. These maximum frame size settings are programmed in register 4 (0x04). Because
the KSZ8873MLL/FLL/RLL supports VLAN tags, the maximum sizing is adjusted when these tags are present.
3.3.7.5 Full-Duplex Flow Control
The KSZ8873MLL/FLL/RLL supports standard IEEE 802.3x flow control frames on both transmit and receive sides.
On the receive side, if the KSZ8873MLL/FLL/RLL receives a pause control frame, the KSZ8873MLL/FLL/RLL will not
transmit the next normal frame until the timer, specified in the pause control frame, expires. If another pause frame is
received before the current timer expires, the timer will be updated with the new value in the second pause frame. During
this period (while it is flow controlled), only flow control packets from the KSZ8873MLL/FLL/RLL are transmitted.
On the transmit side, the KSZ8873MLL/FLL/RLL has intelligent and efficient ways to determine when to invoke flow con-
trol. The flow control is based on availability of the system resources, including available buffers, available transmit
queues, and available receive queues.
The KSZ8873MLL/FLL/RLL will flow control a port that has just received a packet if the destination port resource is busy.
The KSZ8873MLL/FLL/RLL issues a flow control frame (XOFF), containing the maximum pause time defined by the
IEEE 802.3x standard. Once the resource is freed up, the KSZ8873MLL/FLL/RLL sends out the other flow control frame
(XON) with zero pause time to turn off the flow control (turn on transmission to the port). A hysteresis feature is provided
to prevent the flow control mechanism from being constantly activated and deactivated.
The KSZ8873MLL/FLL/RLL flow controls all ports if the receive queue becomes full.
3.3.7.6 Half-Duplex Backpressure
A half-duplex backpressure option (not in IEEE 802.3 standards) is also provided. The activation and deactivation con-
ditions are the same as full-duplex flow control. If backpressure is required, the KSZ8873MLL/FLL/RLL sends pream-
bles to defer the other stations' transmission (carrier sense deference).
To avoid jabber and excessive deference (as defined in the 802.3 standard), after a certain time, the KSZ8873MLL/FLL/
RLL discontinues the carrier sense and then raises it again quickly. This short silent time (no carrier sense) prevents
other stations from sending out packets thus keeping other stations in a carrier sense deferred state. If the port has pack-
ets to send during a backpressure situation, the carrier sense type backpressure is interrupted and those packets are
transmitted instead. If there are no additional packets to send, carrier sense type backpressure is reactivated again until
switch resources free up. If a collision occurs, the binary exponential back-off algorithm is skipped and carrier sense is
generated immediately, thus reducing the chance of further collisions and carrier sense is maintained to prevent packet
reception.
To ensure no packet loss in 10BASE-T or 100BASE-TX half-duplex modes, the user must enable the following:
• Aggressive back-off (register 3 (0x03), bit [0])
• No excessive collision drop (register 4 (0x04), bit [3])
Note that these bits are not set as defaults because this is not the IEEE standard.
3.3.7.7 Broadcast Storm Protection
The KSZ8873MLL/FLL/RLL has an intelligent option to protect the switch system from receiving too many broadcast
packets. As the broadcast packets are forwarded to all ports except the source port, an excessive number of switch
resources (bandwidth and available space in transmit queues) may be utilized. The KSZ8873MLL/FLL/RLL has the
option to include “multicast packets” for storm control. The broadcast storm rate parameters are programmed globally,
and can be enabled or disabled on a per port basis. The rate is based on a 67 ms interval for 100BT and a 500 ms
interval for 10BT. At the beginning of each interval, the counter is cleared to zero, and the rate limit mechanism starts
to count the number of bytes during the interval. The rate definition is described in register 6 (0x06) and 7 (0x07). The
default setting is 0x63 (99 decimal). This is equal to a rate of 1%, calculated as follows:
148,800 frames/sec × 67 ms/interval × 1% = 99 frames/interval (approx.) = 0x63
Note: 148,800 frames/sec is based on 64-byte block of packets in 100BASE-TX with 12 bytes of IPG and 8 bytes of
preamble between two packets.