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TNETX15VE

VLAN-ENGINE ADDRESS-LOOKUP DEVICE

SPWS028B – APRIL 1997 – REVISED SEPTEMBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

VLAN registers

The VLAN registers are used to link individual ports together into VLANs. With VLANs active, if a packet needs

to be broadcast, the state machine must know which ports are included in the sourcing port’s VLAN (VLAN

routing mask registers). To enforce VLANs after setup, it is important to identify the VLAN of the incoming source

address because the VLAN is considered part of the address during lookup (port VLAN assignment registers).

To do this, the state machines need a table entry for each port that identifies which VLAN it is part of, and a table

of vectors (or masks of bits) that indicates which stations are included in that VLAN. The other table in this group

is the port 00 tag VLAN array. This array supports VLANs across a cascaded port 00 by allowing the state

machines to identify individual ports across the cascade to a second TNETX3150/TNETX3150A device.

With these tables set up and coordinated, nonleaky VLANs are created. Broadcast/multicast packets are

delivered only to the stations in the same VLAN as the originating station. The vector associated with the

broadcast/multicast address is ANDed with its VLANID mask, has the source port removed, and is ANDed with

the inverse of the TX block register (no forwarding to blocked ports) to determine which ports transmit the

packet. Broadcast/multicast traffic can even be confined to ports known to contain servers by excluding bits in

the VLAN routing mask registers that contain only clients. Unicast traffic is confined to the destination station

address on the same VLAN. It is the designer’s task to ensure that these tables are coordinated if nonleaky

VLANs are needed. No checks are made to see that VLAN routing mask registers and the port VLAN

assignment registers are essentially reciprocals of each other.

If VLANs are not supported, leave the port VLAN assignment registers, and the port 00 tag VLAN array set to

their default of 0x0, and the VLAN routing mask registers set to their default of 0x7FFF. This effectively creates

one VLAN with all stations included.

If VLANs are supported to other stations in systems linked through port 00 of the TNETX3150/TNETX3150A

(to which this TNETX15VE is connected), the index into the port 00 tag VLAN array is the pretag field on the

inbound packet on the TNETX3150/TNETX3150A port 00. This value is passed to the TNETX15VE as the frame

data is stored to DRAM. If VLANs are supported across port 00 of the attached TNETX3150/TNETX3150A, the

presence of the pretag on port 00 is required. If VLANs are not supported, the pretag is unimportant only if all

the entries in the port 00 tag VLAN array are left at their default value of 0. Thus, any value that is clocked off

the front of the packet yields an index at 0 to the routing mask register.

TNETX15VE データシート(PDF) 43 Page - Texas Instruments