PM3350-SW datasheet(82/224 Pages) PMC

82 / 224 page

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

Type:

This 2-bit field is used to initialize the 2-bit frame Type subfield in the Flags

fields of data descriptors corresponding to Ethernet frames received on this

MAC channel. Normally set to 01 binary, corresponding to unicast frames

received on a local MAC channel.

TXBlkd:

If set, indicates that the MAC channel is blocked to transmit; i.e., no frames

can be transmitted out this MAC channel. Normally used by the spanning tree

algorithm.

RXBlkd:

If set, indicates that the MAC channel is blocked to received; i.e., frames

received on this MAC channel must be discarded. Normally used by the

spanning tree algorithm.

Special:

If set, indicates that the data descriptors created for Ethernet frames received

on this MAC channel should have the Special flag bits set in their Flags fields,

causing them to be specially handled by the switching firmware. Normally set

to zero; usually set to a '1' by the spanning tree algorithm.

The remainder of the local port descriptor holds per-port counters that are maintained

and updated by the switching firmware in order to support the SNMP and RMON MIB

statistics. A total of 11 32-bit counters are implemented. Note that the TXDeferred

counter tracks the number of frames whose transmission was delayed due to the need

to defer to received frames as stipulated by the Carrier Sense Multiple Access protocol

of IEEE 802.3. (This counter is part of the standard Ethernet MIB, and is typically used

to track the utilization of the link.)

The Switch Processor caches all of the eight local port descriptors during normal

operation. The in-memory copy of the local port descriptors, therefore, is likely to be

'stale' (i.e., outdated by information in the Switch Processor's data cache). The local

port descriptors should preferably be read, if desired, by using the message interface

maintained by the Switch Processor. If the contents of the local port descriptors must

be directly read over the PCI bus, the Switch Processor data cache must be flushed

prior to the read, again via the message interface. (The operation of the message

interface is described below.)

Transmit Queue

Each local port descriptor maintains, as already described, a queue of frames that are

awaiting transmission out the MAC channel. These frames have typically been received

on some other port, and are directed to the given MAC channel during the switching

process.

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