PM3350 Datasheet, PDF (138/224 Pages) PMC-Sierra, Inc – 8-Port 10 Mbit/s Switch

ELAN 8X10
DATA SHEET
PMC-970109
ISSUE 3
PM3350 ELAN 8 X10
8 PORT ETHERNET SWITCH
and processed by the system per second. It should also be noted that multicasts are
not subject to this limit.
The ELAN 8x10 implements broadcast rate limiting via firmware running on the Switch
Processor, using a combination of operations performed by the frame switching tasks
coupled with a periodic background process. The broadcast rate limit is actually
implemented by the 16-bit BcstRem variable in the Switch Processor firmware data
space. BcstRem is initially set to a programmable value, defined by the 16-bit BcstLimit
variable (which in turn is initialized to a default during system boot time, and which may
be altered during system operation to modify the permissible broadcast rate).
Whenever a broadcast frame (i.e., one with the destination MAC address set to all-
ones) is being processed by the switching firmware, the Switch Processor decrements
the BcstRem variable by 1 and checks the result. If the result is less than or equal to
zero, the Switch Processor simply discards the frame, as the broadcast rate limit has
been exceeded; otherwise, it updates the BcstRem variable with the decremented
value, and accepts the broadcast frame for normal processing and eventual
transmission. The BcstRem variable is never incremented by the switching firmware.
Hence, when it goes to zero, no more broadcast frames will be accepted by the ELAN
8x10 device.
A background task is periodically executed every ten milliseconds by the ELAN 8x10
Switch Processor under timer control. The purpose of this background task is to reset
the BcstRem variable to the contents of BcstLimit, thus permitting broadcasts to
continue for the next ten millisecond period. The value of the BcstLimit variable is
therefore directly equal to the number of broadcast frames that will be forwarded by the
system every ten milliseconds. The BcstLimit variable may be set to any value between
0 and 32,767. Setting this variable to 100, for example, results in a maximum average
broadcast rate of 10,000 frames per second; setting it to zero shuts off broadcasts
completely, while setting it to its maximum value (32,767) effectively removes the
broadcast rate limiting feature.
It should be noted that the broadcast rate limiting feature allows a short burst of
broadcast every 10 milliseconds until the limit is reached, at which point all further
broadcast frames are discarded.
Statistics Collection
The ELAN 8x10 device collects a number of SNMP and RMON MIB statistics during
normal operation. These statistics are maintained in the local port descriptors, the port
descriptor counter structures, and the hash buckets. Some internal statistics are also
held in the global memory area.
The locations and sizes of the individual statistics counters are described in the
sections above that deal with the various data structures. A summary of all of the
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