TNETX15VE datasheet(87/113 Pages) TI1

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TNETX15VE

VLAN-ENGINE ADDRESS-LOOKUP DEVICE

SPWS028B – APRIL 1997 – REVISED SEPTEMBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

aging (AGE logic operation) (continued)

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

address, it again must rescan the 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 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.

• Duringthefind-oldestprocessandifAGEisinatime-thresholdage,AGEdeletesalladdressesthatare

over the threshold value.

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

following:

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.

table-full aging

Table-full aging is implemented for applications that do not want to use aging based on time, but still require

aging. As its name implies, aging in this mode only happens when the lookup table is full and needs additional

room to add a new address. The ADD logic operation initiates an aging request when it determines that it does

not have enough tables to add the address it is working on. The time behaves differently in this mode. In table-full

aging, the age timer does not increment every second, but rather when a new address is added. There must

be a time difference between addresses to decide which is oldest, but a single count is adequate. Since ADD

time stamps every time it sees a node come through the bus, nodes that are actively transmitting between adds

quickly move up to the same (new) age level. Those nodes that do not transmit remain at the lower age stamps.

It is these nodes that are deleted in table-full aging when the table is full.

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