PM3351 Datasheet, PDF (137/268 Pages) PMC-Sierra, Inc – Single-Port 10/100 Mbit/s Ethernet Switch

DATA SHEET
PMC-970113
ISSUE 3
PM3351 ELAN 1X100
SINGLE PORT FAST ETHERNET SWITCH
that is used at various points during the switching process must always point to a valid
hash bucket data structure, as this pointer is utilized by the switching firmware to
update statistics and so on. In addition, the local port descriptor pointer within the hash
bucket must also be valid, and pointing to the correct local port descriptor
corresponding to the physical port upon which the frame arrived.
To deal with the above case, the initialization process creates one dummy (default)
hash bucket within the Switch Processor operating environment at system start-up. This
hash bucket is set up to correspond to the single physical port of the ELAN 1x100. This
invalid hash bucket is not placed into the address table, and its MAC address field is
invalid. The dummy hash bucket 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 to point to the default hash bucket.
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 1x100 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 ports on different ELAN 1x100 devices) 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 ReceiveTimeStamp field in the hash bucket
data structures. This field is updated with an absolute time in microseconds by the
Switch Processor whenever a valid frame has been received from the corresponding
source entity. The ReceiveTimeStamp field thus indicates the last time at which the
source was active. The aging process periodically scans the entire address table,
comparing the ReceiveTimeStamp 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
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