LAN9303 datasheet(82/366 Pages) SMSC

82 / 366 page

Small Form Factor Three Port 10/100 Managed Ethernet Switch with Single MII/RMII/Turbo MII

Datasheet

Revision 1.3 (08-27-09)

SMSC LAN9303/LAN9303i

DATASHEET

6.4.11

Counters

A counter is maintained per port that contains the number of MAC address that were not learned or

were overwritten by a different address due to MAC Address Table space limitations. These counters

are accessible via the following registers:

Switch Engine Port 0 Learn Discard Count Register (SWE_LRN_DISCRD_CNT_0)

Switch Engine Port 1 Learn Discard Count Register (SWE_LRN_DISCRD_CNT_1)

Switch Engine Port 2 Learn Discard Count Register (SWE_LRN_DISCRD_CNT_2)

A counter is maintained per port that contains the number of packets filtered at ingress. This count

includes packets filtered due to broadcast throttling, but does not include packets dropped due to

ingress rate limiting. These counters are accessible via the following registers:

Switch Engine Port 0 Ingress Filtered Count Register (SWE_FILTERED_CNT_0)

Switch Engine Port 1 Ingress Filtered Count Register (SWE_FILTERED_CNT_1)

Switch Engine Port 2 Ingress Filtered Count Register (SWE_FILTERED_CNT_2)

6.5

Buffer Manager (BM)

The Buffer Manager (BM) provides control of the free buffer space, the multiple priority transmit

queues, transmission scheduling, and packet dropping. VLAN tag insertion and removal is also

performed by the Buffer Manager. The following sections detail the various features of the Buffer

Manager.

6.5.1

Packet Buffer Allocation

The packet buffer consists of 32KB of RAM that is dynamically allocated in 128 byte blocks as packets

are received. Up to 16 blocks may be used per packet, depending on the packet length. The blocks

are linked together as the packet is received. If a packet is filtered, dropped, or contains a receive

error, the buffers are reclaimed.

6.5.1.1

Buffer Limits and Flow Control Levels

The BM keeps track of the amount of buffers used per each ingress port. These counts are used to

generate flow control (half-duplex backpressure or full-duplex pause frames) and to limit the amount

of buffer space that can be used by any individual receiver (hard drop limit). The flow control and drop

limit thresholds are dynamic and adapt based on the current buffer usage. Based on the number of

active receiving ports, the drop level and flow control pause and resume thresholds adjust between

fixed settings and two user programmable levels via the Buffer Manager Drop Level Register

(BM_DROP_LVL), Buffer Manager Flow Control Pause Level Register (BM_FC_PAUSE_LVL), and

Buffer Manager Flow Control Resume Level Register (BM_FC_RESUME_LVL) respectively.

The BM also keeps a count of the number of buffers that are queued for multiple ports (broadcast

queue). This count is compared against the Buffer Manager Broadcast Buffer Level Register

(BM_BCST_LVL), and if the configured drop level is reached or exceeded, subsequent packets are

dropped.

6.5.2

Random Early Discard (RED)

Based on the ingress flow monitoring detailed in Section 6.4.6, "Ingress Flow Metering and Coloring,"

on page 75, packets are colored as Green, Yellow, or Red. Packets colored Red are always discarded

if the Drop on Red bit in the Buffer Manager Configuration Register (BM_CFG) is set. If the Drop on

Yellow bit in the Buffer Manager Configuration Register (BM_CFG) is set, packets colored Yellow are

randomly discarded based on the moving average number of buffers used by the ingress port.

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