KSZ8895MQ-EVAL Datasheet, PDF (35/118 Pages) Micrel Semiconductor – Integrated 5-Port 10/100 Managed Ethernet Switch with MII/RMII Interface

Micrel, Inc.
KSZ8895MQ/RQ/FMQ
The KSZ8895MQ/RQ/FMQ will not forward the following packets:
1. Error packets
These include framing errors, Frame Check Sequence (FCS) errors, alignment errors, and illegal size packet
errors.
2. IEEE802.3x PAUSE frames
KSZ8895MQ/RQ/FMQ intercepts these packets and performs full duplex flow control accordingly.
3. "Local" packets
Based on destination address (DA) lookup, if the destination port from the lookup table matches the port from
which the packet originated, the packet is defined as "local."
Half-Duplex Back Pressure
The KSZ8895MQ/RQ/FMQ also provides a half-duplex back pressure option (note: this is not in IEEE 802.3
standards). The activation and deactivation conditions are the same as the ones given for full-duplex mode. If back
pressure is required, the KSZ8895MQ/RQ/FMQ sends preambles to defer the other station's transmission (carrier
sense deference). To avoid jabber and excessive deference as defined in IEEE 802.3 standards, after a certain
period of time, the KSZ8895MQ/RQ/FMQ discontinues carrier sense but raises it quickly after it drops packets to
inhibit other transmissions. This short silent time (no carrier sense) is to prevent other stations from sending out
packets and keeps other stations in a carrier sense-deferred state. If the port has packets to send during a back
pressure situation, the carrier sense-type back pressure is interrupted and those packets are transmitted instead. If
there are no more packets to send, carrier sense-type back pressure becomes active again until switch resources
are free. If a collision occurs, the binary exponential backoff algorithm is skipped and carrier sense is generated
immediately, reducing the chance of further colliding and maintaining carrier sense to prevent reception of packets.
To ensure no packet loss in 10BASE-T or 100BASE-TX half-duplex modes, the user must enable the following:
• Aggressive backoff (Register 3, bit 0)
• No excessive collision drop (Register 4, bit 3)
• Back pressure (Register 4, bit 5)
These bits are not set as the default because this is not the IEEE standard.
Broadcast Storm Protection
The KSZ8895MQ/RQ/FMQ has an intelligent option to protect the switch system from receiving too many broadcast
packets. Broadcast packets are normally forwarded to all ports except the source port and thus use too many switch
resources (bandwidth and available space in transmit queues). The KSZ8895MQ/RQ/FMQ 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 50ms (0.05s) interval for 100BT and a 500ms (0.5s)
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 Registers 6 and 7. The default
setting for Registers 6 and 7 is 0x4A (74 decimal). This is equal to a rate of 1%, calculated as follows:
148,80 frames/sec X 50ms (0.05s)/interval X 1% = 74 frames/interval (approx.) = 0x4A
January 22, 2013
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Revision 1.6