IBM PowerPRS Q-64G
Packet Routing Switch
Preliminary
Figure 3-9. Best-Effort Discard Drop Filters
Destination
Bitmap
Packet Priority
0
Input
Controller
Data
Best-Effort Bit
31
Discard Filters
Priority 0 Priority 1 Priority 2 Priority 3
Shared Memory Input
AND
0
0
0
0
I
31
31
31
31
AND
I
1
Convert to
Filter Mask
1
32
32
32
32
1
1
1
Priority to Be
Discarded
Drop Counters
3.5 Egress Flow Control
Egress traffic flow from the PowerPRS Q-64G is controlled by a variety of mechanisms, including send
grants, the send grant antistreaming function, and the credit table.
3.5.1 Send Grants
Egress traffic flow from the PowerPRS Q-64G to an attached device is controlled primarily by a grant mecha-
nism. The PowerPRS Q-64G transmits a packet on an output port when the attached device issues a send
grant for that port and priority. The attached device inserts the send grant status for a port into the ingress idle
packet payload and ingress data packet headers (see Section 3.3 Packet Format According to Packet Type
on page 28). The PowerPRS Q-64G extracts the send grant status from the ingress packets and stores it in
internal tables. When the send grant is removed, the PowerPRS Q-64G generates idle packets on the port.
Note: The PowerPRS Q-64G sends control packets to an output port if the send grant status for that port is
active for at least one priority. It sends service packets to an output port regardless of the send grant status.
3.5.2 Send Grant Antistreaming
The optional send grant antistreaming function is intended to prevent a defective attached device from indefi-
nitely removing the send grant. This function is enabled with the send grant antistreaming enable bit in the
Configuration 0 Register (page 110). If the send grant is not issued for any priority for a given number of con-
tiguous packet cycles, the antistreaming function (if enabled) internally forces the send grant to active for all
priorities until the attached device issues another send grant. The number of contiguous packet cycles (from
Functional Description
Page 52 of 199
prsq-64g.01.fm
December 20, 2001
IBM [ IBM ]
