PM9313 datasheet(52/343 Pages) PMC

52 / 343 page

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

PM9311/2/3/5 ETT1™ CHIP SET

PMC-2000164

ISSUE 3

ENHANCED TT1™ SWITCH FABRIC

Released

Data Sheet

PMC-Sierra, Inc.

In order to reduce the blocking effects of one OC-48c channel over another channel on the same port,

there is a special multicast output queue for each priority. The multicast cells forwarded through the

Crossbar are still queued in the order they arrive. This special queue enables cells to be sent out-of-order,

but will always be sent in-order for a given egress subport. Therefore, a multicast cell waiting in the output

queue memory can only be blocked by cells destined to the same egress subport. Additional output queue

memory becomes available whenever a cell is dequeued.

If backpressure is asserted by one of the OC-48c channels to the output multicast queue, then subsequent

cells going to other channels on the same port may be blocked if the entire queue is occupied by cells

destined to the blocked OC-48c channel. This is a consequence of sharing the output queue memory

allocation for multicast cells at a given priority. This can cause multicast backpressure to the central

Scheduler which can block multicast cells destined to other ports at the same priority if multicast

backpressure is enabled.

1.3.4.1

Multicast Tables (OC-192c and Quad OC-48c Modes)

The input multicast fanout table (EITIBM in the EPP) is always used for multicast cells. Every EPP has its

own EITIBM table with 4096 entries. The EPP uses the 12-bit multicast tag field from the LCS label field to

determine which entry in the table should be used for each multicast cell, as shown in Figure 21. Each

entry is simply a 32 bit vector where a 1 means that the cell should go to that port. So if an entry has a 1 at

bit 12, for example, then a multicast cell that uses that entry will be sent to port 12 (as well as any other

ports that have their bit set to 1).

Figure 21. EITIBM structure

Multicast tag [11:0]

iEPP

Input Multicast

Fanout Table EITIBM

To Scheduler

4095

1 bit for each output OC-192c port

PM9313 Datasheet(PDF) 52 Page - PMC-Sierra, Inc