TNETX15VE datasheet(110/113 Pages) TI1

110 / 113 page

TNETX15VE

VLAN-ENGINE ADDRESS-LOOKUP DEVICE

SPWS028B – APRIL 1997 – REVISED SEPTEMBER 1997

110

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

spanning-tree support

The TNETX15VE is designed to simplify the spanning-tree protocol. The TNETX15VE uses registers that affect

routing and learning; these are useful for the spanning-tree states.

learning state

A port in the learning state does not forward frames that are received on this port to the rest of the network. It

does not transmit frames that are ordinarily routed to this port. Learning is enabled for this port and the

addresses on the port are added to the lookup table.

To place a port in a learning state, its corresponding bit in TXblock and RXblock are set to 1. Its corresponding

bit in NLRNport is cleared, which enables learning.

blocking/listening/disabled state

A port in this state does not transmit frames that are sourced by this port to the rest of the network. It does not

forward frames that are ordinarily routed to this port. Learning is disabled for this port.

To place a port in a blocking/listening/disabled state, its corresponding bit in TXblock, RXblock, and NLRNport

is set to 1.

forwarding state

A port in the forwarding state does not block traffic through the port. Frames that are sourced by this port are

routed to the rest of the network. Frames that are routed to this port are transmitted. Learning is enabled for this

port and the addresses on the port are added to the lookup table.

To place a port in a learning state, its corresponding bit in TXblock, RXblock, and NLRNports is cleared.

setting up the TNETX15VE to accept BPDUs (determining source port for the BPDUs)

The NBLCK bit in the TNETX15VE is implemented to allow for easy BPDU reception and for determining the

source port that originated the frame. To set up the system (to accept BPDUs), the programmer must:

• Program the BPDU addresses into the TNETX15VE with the NBLCK bit set to 1 and the destination

address pointing to the port that contains the MAC for the management CPU.

• All BPDU frames are forwarded to the inband CPU. Since the NBLCK bit is set to 1, the TNETX15VE

places the source address of this frame in the NBLCK RX FIFO. The NBLCK bit allows the reception of

BPDUs when a port is RXblocked.

• On reception of BPDU frames, the source port is read for this frame from the NBLCK RX FIFO. The

FIND logic operation is used for this task.

transmitting BPDU frames

The CPUTX register is implemented in the TNETX15VE to allow for easy transmission of BPDU frames. The

CPUTX register overrides the LKUP logic operation and the TXblock register. In this way, transmissions to ports

that have been TXblocked are performed. To transmit a BPDU frame to a specific port, the programmer must:

• ProgramtheCPUTXregistertopointtotheportwheretheBPDUframecomesfrom(usuallytheinband

CPU is connected to this port).

• Program the CPUTX register to point to the port where the BPDU frame is to be transmitted.

• Transmitstheframe.TheframeenterstheTNETX3150/TNETX3150A,goesoutontheDRAMbus,and

the TNETX15VE matches the source port. When the source port is matched, the TNETX15VE directs

the frame to the destination port in CPUTX. It then disables the CPUTX register.

• Repeat the procedure for additional frames.

TNETX15VE 数据表(PDF) 110 Page - Texas Instruments

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