TNETX3150 datasheet(59/78 Pages) TI

59 / 78 page

TNETX15AE

ADDRESS-LOOKUP DEVICE

SPWS041A – AUGUST 1997 – REVISED OCTOBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

aging (AGE logic operation) (continued)

D The operation AGE scans the table for the oldest address (state = find oldest state). AGE determines the

oldest address by finding the address in the lookup table with the lowest time stamp. If more than one

address has the same oldest time stamp, AGE picks the first address. The AGE scanning process omits

all multicast addresses. This process also omits unicast addresses that have been secured by setting the

SECURE flag. These addresses can be deleted only by a DIO-delete command.

D Once the oldest address is found, AGE keeps this address, enters a waiting state (state = wait for condition),

until one of two conditions occurs:

If the address table is changed by either the ADD logic operation performing an address

addition/time-stamp update or by deleting (DEL) an address, AGE checks the address that ADD or DEL

is working on. If ADD has changed the time stamp for the current oldest address, it must scan the whole

table for a new oldest address (state = find oldest). If DEL has deleted the current oldest address, it again

must rescan the whole table for a new oldest address (state = find oldest). If neither ADD nor DEL touched

the current oldest address, it still remains the oldest address and AGE returns to the wait state

(state = wait for condition).

The aging condition is met. In this case, AGE calls on the DEL logic operation to delete the node from the

table. After a successful deletion, AGE rescans the whole table for the next node to age (state = find oldest)

and then gives an interrupt to the host to indicate the previous oldest address was deleted from the table.

D During the find-oldest process, and if AGE is in a time-threshold age, AGE deletes all addresses that are

over the threshold value and reports them via the interrupt.

The aging condition is different for time-threshold aging and table-full aging, and both are described in

the following paragraphs.

time-threshold aging

In time-threshold aging, the aging condition occurs when the address age is larger than the time threshold

entered in the agingtimer (0x2) registers. The address age is not the time stamp written in the SRAM but the

difference between current time and each time stamp. When this value becomes greater than agingtimer value,

the address is deleted.

Example:

Time threshold value

= 192d seconds (0x00C0)

Current timer value

= 256d seconds (0x0100)

Address time-stamp value = 80d seconds (0x0050)

Address aging time

= 256d – 80d = 176d seconds (0x00B0)

Conclusion: The address is not aged yet since the address aging time (176d seconds) is less than the time

threshold value (192d seconds) by 16d seconds. It takes an additional 16d seconds (0x0010) for

the address aging time to equal the time threshold (192d seconds) (0x00C0) and age (delete)

the address.

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