ZL50405GDC ZARLINK [Zarlink Semiconductor Inc], ZL50405GDC Datasheet - Page 45

• Manufacturer Part Number: ZL50405GDC
• Description: Managed5-Port 10/100 M Ethernet Switch
• Manufacturer: ZARLINK [Zarlink Semiconductor Inc]

be temporarily postponed. This ensures that every frame can be received first before subjecting them to the frame
drop discipline after classifying.
Three priority sections, one for each pair of the first six priority classes, ensure a programmable number of FDB
slots per class. The lowest two classes do not receive any buffer reservation. Furthermore, a frame is stored in the
region of the FDB corresponding to its class. As we have indicated, the eight classes use only two transmission
scheduling queues for RMAC ports (four queues for the MMAC & CPU ports), but as far as buffer usage is
concerned, there are still eight distinguishable classes.
Another segment of the FDB reserves space for each of the 6 ports — 5 ports for Ethernet and one CPU port (port
number 8). Each port has it’s own programmable source port reservation. These 6 reserved regions make sure that
no well-behaved source port can be blocked by another misbehaving source port.
In addition, there is a shared pool, which can store any type of frame. The frame engine allocates the frames first in
the three priority sections. When the priority section is full or the packet has priority 1 or 0, the frame is allocated in
the shared pool. Once the shared pool is full the frames are allocated in the section reserved for the source port.
The following registers define the size of each section of the Frame data Buffer:
See Buffer Allocation application note, ZLAN-47, for more information.
7.6.1
As already discussed, the WRED mechanism may drop frames on output queue status. In addition to these
reasons for dropping, we also drop frames when global buffer space becomes scarce. The function of buffer
management is to make sure that such dropping causes as little blocking as possible. If a received frame is
dispatched to the best effort queue, the buffer management will check on the overall buffer situation plus the output
queue status to decide the frame drop condition. If the source port has not enough buffer for it, the frame will be
dropped. If the output queue reach the UCC (unicast congest control) and the shared buffer has run out, the frame
will be dropped by b%. If the output queue reach the UCC and the source port reservation is lower than the buffer
low threshold, the frame will be dropped. All the dropping functions are disabled if the source port is flow control
capable.
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-
-
-
-
-
-
Per Source Port
PR100_N - Port Reservation for RMAC Ports
PR100_CPU - Port Reservation for CPU Port
PRM - Port Reservation for MMAC Port
SFCB - Share FCB Size
C1RS - Class 1 Reserve Size (priority 2 & 3)
C2RS - Class 2 Reserve Size (priority 4 & 5)
C3RS - Class 3 Reserve Size (priority 6 & 7)
Reservation
Reservation
Dropping When Buffers Are Scarce
Per Class
R
R
p0
pri1
Figure 11 - Buffer Partition Scheme
R
p1
R
Zarlink Semiconductor Inc.
pri2
R
ZL50405
p2
45
R
R
pri3
Temporary reservation
p3
Shared Pool S
(CPU)
R
p8
Data Sheet
R
p9