TNETX15VE datasheet(57/113 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

lookup algorithm (continued)

Since each table needs to compare 2N possible combinations, it requires 2N pointers. Each table has the

following format, assuming 16-bit wide memory:

(Bits to be compared)

16-BIT POINTERS

00000

00001

00010

2N – 1

OFFSET

2N – 1

To table x

To table y

To table z

Only the pointers column of the above table occupies memory locations.

Each pointer points to a table in the next level. The TNETX15VE uses N bits in the address as an offset to this

table and, if a pointer is found, it uses it to go to the next level. A zero pointer indicates that the entry was not

found (in this case the search fails).

a graphical example of the lookup algorithm

This example uses the following method to look up the number 0xB2 (10.11.00.10b), two bits at a time (N = 2).

Graphically, this number is represented in Figure 6.

Figure 6. Example of Lookup-Number Algorithm (Matching 0xB2)

The first table is offset 0x10b points to the second level (see Figure 6). The second level uses the second set

of bits (0x11b) and points to the third table. This process continues until the last two bits are matched. Matching

0xB2, two bits at a time, uses four tables with each containing four possible pointers. Not all locations in the

tables are used, which potentially can lead to unused memory. Now, add 0xB0 (10.11.00.00b) to the table.

Figure 7 illustrates the results.

TNETX15VE データシート(PDF) 57 Page - Texas Instruments