PM3350-SW datasheet(129/224 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

127

These hash buckets are set up to correspond to the eight physical ports, respectively

(i.e., their local port descriptor pointers are set up to point to the various ports). These

hash buckets are not placed into the address table, and their MAC address fields are

invalid. They will never be found by the hash lookup mechanism during the source and

destination MAC address resolution process.

When a particular source MAC address is not found in the address table, and cannot

be learned due to some constraint, the Switch Processor instead sets up the source

hash bucket pointer placed into the data descriptor and used during the switching

process to point to the appropriate default hash bucket corresponding to the physical

port upon which the frame arrived. This source hash bucket pointer hence indicates a

valid hash bucket (even though it is one with the wrong MAC address, and invalid

statistics fields); thus the remainder of the switching and frame forwarding firmware will

continue to operate as normal. Statistics updates may be done to the default hash

bucket; as the statistics fields within the hash bucket are invalid anyway, these statistics

updates will be irrelevant. In this manner, the Switch Processor can ensure that the

source hash bucket pointer passed to the remainder of the system will always point to a

valid hash bucket, even if learning is disabled or not possible.

Address Aging Process

To reclaim the address table resources that have been allocated to network entities that

are currently inactive or non-existent, the ELAN 8x10 implements an automatic address

table aging mechanism. In essence, this mechanism causes a given address table

entry to be deleted if a frame has not been received from the corresponding host

computer (or other originator of traffic) for a specified period of time. The mechanism

also implements a means whereby a topology change in the network (i.e., the

movement of one or more hosts between ELAN 8x10 ports) can result in the removal of

the outdated address table entry or entries and the substitution of new ones to reflect

the new state of the network.

In general, aging is carried out using the AgeControl field in the hash bucket data

structures. This field is updated with the current value of the 32-bit real-time system

clock (represented by the CLOCK register pair in the Switch Processor) whenever a

valid frame has been received from the corresponding source entity. The AgeControl

field thus indicates the last time at which the source was active. The aging process

periodically scans the entire address table, comparing the AgeControl fields in the

various hash buckets with the current value of the real-time clock; if the difference

exceeds a pre-set threshold, then the hash bucket is removed and returned to the free

hash bucket pool. In a system containing more than one ELAN 8x10 device, an

additional process is followed to ensure that the forwarding tags on all of the other

ELAN 8x10 devices corresponding to the deleted hash bucket are removed before the

hash bucket is itself eliminated. This is necessary to prevent system corruption due to

inconsistent data structures.

PM3350-SW Datasheet(PDF) 129 Page - PMC-Sierra, Inc

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