TNETX15VE datasheet(58/113 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)

Figure 7. Example of Lookup-Number Algorithm (Adding 0xB0)

0xB0 follows exactly the same thread as 0xB2 and the only difference between the two is in the last table. 0xB0

matches offset 00b while 0xB2 matches offset 10b. There are now two numbers being represented, but there

are still the same number of tables allocated (four).

Continuing this example, we can add 0xB1 and 0xB2 with the same number of tables allocated. This is called

the best-case scenario since the maximum amount of addresses can be stored in the minimum amount of

memory. Now add 0x22 (00.10.00.10b) to a lookup table containing only 0xB2. Figure 8 shows the results.

Figure 8. Example of Lookup-Number Algorithm (Adding 0x22)

Adding 0x22 requires allocating three additional tables. Seven tables are now required to hold two addresses.

Compared to numbers that differ in their least-significant bits, numbers that differ in their most-significant bits

require more tables. Continuing the example, adding 0x2A requires an additional three tables, as would 0xE2.

This is the worst-case scenario, and it is the least-efficient way of storing addresses.

The TNETX15VE is designed to handle the worst-case address distribution. The worst-case address

distribution requires a separate thread per address. A purely random distribution creates multiple threads at the

top levels. The most-significant bits of a network card address are the vendor bits. However (in most networks)

there are only a couple of vendors used. These cards do not have a purely random address distribution, and

they all share a common set of bits that identifies the vendor. This configuration requires fewer pointers for the

same number of addresses. In such a network, the tables look more like those in Figure 9.

TNETX15VE 数据表(PDF) 58 Page - Texas Instruments