PM3350-SW datasheet(134/224 Pages) PMC

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ELAN 8X10

PM3350 ELAN 8 X10

DATA SHEET

PMC-970109

ISSUE 3

8 PORT ETHERNET SWITCH

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND PMC-SIERRA, INC. CUSTOMERS ONLY

132

Buffer Management

As already mentioned, two forms of buffer management are performed by the ELAN

8x10 device. The first mechanism attempts to distribute the available frame buffer

space equitably across the eight ports in the device, such that no one port will be

permitted to starve the others of buffer space to hold incoming frames. The second

mechanism places a limit on the number of broadcast or multicast frames that will be

buffered by the system at any one time; this limit prevents unicast traffic from being

unfairly affected due over-consumption of system resources by large amounts of

broadcasts or multicasts.

Per-port Buffer Allocation Limits

Per-port buffer allocation is controlled using the 16-bit MaxBuffers fields in the local port

descriptor structures. This field is set up at initialization time with the maximum number

of (fixed-size) packet buffers that the given local port is allowed to consume. Whenever

a valid frame is received on a MAC channel, the Switch Processor firmware subtracts

the number of packet buffers required to hold the frame from the MaxBuffers field in the

corresponding local port descriptor. If the result is less than or equal to zero, it indicates

that the port does not have sufficient buffer capacity to hold the frame; the frame is then

discarded. If the result is greater than zero, the frame is accepted, and the MaxBuffers

field is updated to reflect this fact. The MaxBuffers field is subsequently incremented

(again, by the number of packet buffers used to contain the frame data) when the frame

is either transmitted, transferred to another device across the PCI bus, or discarded.

This method of limiting buffer allocation has the advantage that there is no need to

physically reserve portions of memory for specific ports. All of the available free packet

buffers are placed into a single, common, global free pool, and allocated by the various

ports as required. The MaxBuffers fields in the local port descriptor structures serve as

'soft' limits, preventing any port or group of ports with high incoming traffic from starving

the others. It is not necessary that all ports be assigned the same buffer limit; the

MaxBuffers fields may, for example, be set higher for ports that are expected to carry

high traffic, and lower for the others. It is required, however, that the sum of the

MaxBuffers fields of all of the local ports be no greater than the total number of packet

buffers in the system (minus approximately 200 buffers to compensate for system

overheads and temporary overruns). The permissible range of the MaxBuffers fields is

between 44 and 32767, depending on the available packet buffer memory.

Buffer allocation limits are also maintained separately for each external ELAN 8x10 (or

other) device in the system that is connected to this device via the PCI bus. These

buffer allocation limits are located in the MaxBuffers fields of the expansion port

descriptors, and operate in substantially the same way as the corresponding fields in

the local port descriptors, but control frame transfers from other devices to this ELAN

8x10 device. One significant difference is that frames are not discarded when the

MaxBuffers field in an expansion port descriptor becomes less than or equal to zero;

PM3350-SW Datasheet(PDF) 134 Page - PMC-Sierra, Inc

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