TNETX15VE datasheet(59/113 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

a graphical example of the lookup algorithm (continued)

Table

Figure 9. Address Distribution Table Hierarchy (Worst Case)

There is a need to allocate for worst case, but since the worst case is not likely to happen in a real system, the

opportunity exists to include more addresses than indicated by the worst-case rating. The actual number of

addresses supported depends on the nature of the nodes in the network. TNETX15VE devices in networks with

nodes from one or a small number of manufacturers are able to recognize more addresses than those in a purely

random-address network.

This algorithm has the primary advantage that the lookup time is independent of the number of addresses stored

in the lookup table. Whether the number of addresses present is one or a million, the lookup time depends on

the number of levels required to match the address.

Another side effect of this algorithm is that the table order is already sorted (smallest to largest).

The choice of N (the number of bits to consider each cycle) is a balance of:

The size of each table — it must be 2N words.

The clocks required to process a whole address (in this case 48 IEEE addresses plus two VLAN bits) are

÷ N.

TNETX15VE 数据表(PDF) 59 Page - Texas Instruments