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Either the CPU or the external INCR pin can increment
both counters simultaneously. Whenever STPURG is
incremented, a purge operation is initiated. The counters
roll-over so the times should be thought of as slots to be
used and reused in a round-robin fashion.
The time stamping of LANCAM entries and the procedure
required to initiate a purge is explained as follows:
1. Incoming SAs to be learned are associated with the
most recent STCURR value. The time stamps of each
SA already in the CAM database is updated to
STCURR, each time a packet with that SA is
processed.
The existence of two counters (time stamps) allows the
data-aging rate to be varied according to network traffic
density. When the difference between the counters is large
(default), the address data is purged less frequently;
shrinking the counter difference causes the data to age
sooner. Incoming SAs are time stamped or updated with
the current value of STCURR. Older entries time stamped
with the same value as STPURG are purged upon the
increment of STPURG. The permanent address database
built using the Add routine is not affected by time stamps.
2. STPURG and STCURR are advanced as described
earlier to purge entries that have the same time stamp
value as STCURR.
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This example begins with the initial defaults, STCURR =
00H and STPURG = 01H. As packets arrive, learned or
refreshed, SAs are labeled with STCURR = 00H. (At that
moment STPURG = 01H). Increment, either hardware or
software initiated, results in STCURR = 01H and
STPURG=02H.
The data age gap is effectively the length of time an entry
will exist in the LANCAM database if it is not updated.
This gap is the difference between the STCURR and
STPURG counter. When network traffic is low, STCURR
may be increased in order to increase the length of time an
entry will exist. When network traffic is high, STPURG
may be increased in order to decrease the length of time an
entry will exist. When STPURG is incremented older
entries are also purged from the database if their time
stamp matches STPURG.
A purge operation is initiated that eliminates all CAM
entries with time stamp = 02H. The oldest entries (SAs)
that have not been updated in 255 increment times are
purged automatically without further involvement.
If the CAM Full flag is asserted, an interrupt (if
configured) to the CPU is generated. Assume that
STCURR = F0H, and STPURG = F1H. The CPU may
initiate an increment STPURG operation so that older
entries may be purged. This increases the value of
STPURG to F2H. A purge operation is initiated that will
eliminate all CAM entries with time stamp = F2H.
STCURR and STPURG may be incremented
simultaneously to keep the data age gap constant and to
purge the older entries from the database.
To maintain "current" time, STCURR is advanced in any
one of the three ways:
The CPU should monitor the System Status register, and if
the CAM is still full, the operation can be repeated until
entries are purged and the CAM Full flag is de-asserted.
1. The CPU issues an increment STCURR command.
Only the STCURR counter is increased.
Assume that STPURG was incremented 128 times. This
would purge the oldest half of the time stamp values and
thus, reduce the maximum age to half the previous 255.
This is accomplished without changing STCURR.
2. The CPU issues an increment STCURR and STPURG
command. Both counters are increased simultaneously.
3. The INCR pin is asserted. Both counters are increased
simultaneously.
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To maintain "purge" time and to purge aged CAM entries,
STPURG is advanced in any one of the three ways:
For 10Base-X packets, a 32-bit cyclic redundancy check is
calculated from the data frame (exclusive of the preamble
and start frame delimiter) and compared to the frame
check sequence (FCS). This check is only performed if the
PCFG register for the appropriate port is set accordingly to
enable the facility.
1. The CPU issues an increment STPURG command. Only
the STPURG counter is increased.
2. The CPU issues an increment STCURR and STPURG
command. Both counters are increased simultaneously.
Also, according to the MII interface specifications, the
RX_ER, CRS, and COL signals are monitored and error
conditions are recognized. If any error is identified, the
source address is not processed. This is intended to
maintain the integrity of the LANCAM database.
3. The INCR pin is asserted. Both counters are increased
simultaneously.
If the STPURG value was incremented, the MU9C8358L
initiates a purge operation using the new STPURG value.
STPURG should never be incremented to equal STCURR.
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