TNETX15VE datasheet(86/113 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

adding unicasts and multicast addresses

There is no significant change in procedure between adding unicast and multicast addresses. There is one

difference that the programmer must observe. The TNETX15VE stores different information for multicast

addresses than for unicast addresses. Unicast addresses use a 4-bit code for the port number and three flag

bits. Multicast addresses store a 15-bit port vector code.

Both data formats are added through the addport register. The format for this register changes, depending on

bit 40 in the addnode register. If set to 1, the addport data is interpreted as multicast (if not, it is interpreted as

unicast).

deleting an address

The TNETX15VE has two ways to delete addresses from the lookup table — the internal aging algorithm and

a host request through the DIO interface. The DEL state machine is responsible for deleting addresses from

the lookup table. DEL takes its information from the DIO registers for DIO deletes and from the AGE logic

operation for aging deletes.

The TNETX15VE implements a 16-bit timer, incrementing every second for the aging process. This timer is used

to write the time stamp during ADDs and for comparing ages.

aging (AGE logic operation)

The AGE logic operation is responsible for automatic address deletes. The TNETX15VE implements two styles

of aging: time-threshold aging and table-full aging. The aging style is selected through the agingtimer register.

A value of 0x0000 or 0xFFFF in the agingtimer register selects table-full aging. Any other value selects

time-threshold aging. The AGE logic operation is disabled when the TNETX15VE is placed in NAUTO mode.

The aging algorithm works as shown in Figure 16 and described in the following:

Find Oldest

Start

Wait for

Condition

Delete

Scan

Figure 16. Aging Algorithm Block Diagram

• 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.

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