MVTX2601
Data Sheet
13
Zarlink Semiconductor Inc.
3.2 Multicast Data Frame Forwarding
After receiving the switch response, the TxQ manager has to make the dropping decision. A global decision to drop
can be made, based on global FDB utilization and reservations. If so, then the FCB is released and the frame is
dropped. In addition, a selective decision to drop can be made, based on the TxQ occupancy at some subset of the
multicast packet’s destinations. If so, then the frame is dropped at some destinations but not others and the FCB is
not released.
If the frame is not dropped at a particular destination port, then the TxQ manager formats an entry in the multicast
queue for that port and class. Multicast queues are physical queues (unlike the linked lists for unicast frames).
There are 2 multicast queues for each of the 24 10/100 ports. The queue with higher priority has room for 32 entries
and the queue with lower priority has room for 64 entries. There is one multicast queue for every two priority
classes. For the 10/100 ports to map the 8 transmit priorities into 2 multicast queues, the 2 LSB are discarded.
During scheduling, the TxQ manager treats the unicast queue and the multicast queue of the same class as one
logical queue. The older head of line of the two queues is forwarded first.
The port control requests a FCB release only after the EOF for the multicast frame has been read by all ports to
which the frame is destined.
4.0 Memory Interface
4.1 Overview
The MVTX2601 provides a 64-bit wide SRAM bank. Each DMA can read and write from the SRAM bank. The
following figure provides an overview of the MVTX2601 SRAM bank.
Figure 3 - MVTX2601 SRAM Interface Block Diagram (DMAs for 10/1000 Ports Only)
4.2 Detailed Memory Information
Because the bus for each bank is 64 bits wide, frames are broken into 8-byte granules, written to and read from
memory.
4.3 Memory Requirements
To support 64 K MAC address, 2 MB memory is required. When VLAN support is enabled, 512 entries of the MAC
address table are used for storing the VLAN ID at VLAN Index Mapping Table.
SRAM
TX DMA
0-7
TX DMA
8-15
TX DMA
16-23
RX DMA
0-7
RX DMA
8-15
RX DMA
16-23