FTOS Configuration Guide for the C-Series - Force10 Networks

All ports support eight queues, 4 for data traffic and 4 for control traffic. Only the 4 data traffic queues are
tunable. Physical memory is organized into cells of 128 bytes. The cells are organized into two buffer
pools—dedicated buffers and dynamic buffers.
• Dedicated buffers refer to reserved memory that cannot be used by other interfaces on the same ASIC
or by other queues on the same interface. These buffers are always allocated, and no dynamic
recarving takes place based on changes in interface status.
• Dedicated buffers introduce a tradeoff. They provide each interface with a guaranteed minimum set of
buffers to prevent an overused and congested interface from completely starving all other interfaces.
However, this minimum guarantee means the buffer manager will not reallocate the buffer to an
adjacent congested interface. Effectively, in some cases, memory may be underused.
Dynamic buffers consist of shared memory allocated as needed, up to a configured limit. Using dynamic
buffers provides the benefit of statistical buffer sharing.
An interface requests dynamic buffers when its dedicated buffer pool becomes exhausted. The buffer
manager grants the request based on three conditions:
• Number of used and available dynamic buffers
• Maximum number of cells that an interface can occupy.
• Available cell pointers (2k per interface). Each packet/frame is managed in the buffer using a unique
cell pointer. Thus, each interface can manage up to 2k packets.
You can configure dynamic buffers per port on both 1G and 10G FPs and per queue on CSFs. By default,
the FP dynamic buffer allocation is oversubscribed by 10 times.
For 48-port 1G card:
• Dynamic Pool= Total Available Pool(16384 cells) – Total Dedicated Pool =5904 cells
• Oversubscription ratio = 10
• Dynamic Cell Limit Per port= 59040/29 =2036 cells
Deciding Whether to Tune Buffers
Force10 recommends exercising caution when configuring any non-default buffer settings as tuning can
significantly affect system performance. The default values work for most cases.
In rare cases, persistent packet loss affects network performance. Such cases may be a candidate for buffer
tuning.
The typical network experiences two kinds of traffic bursts:
• Slow burst – Sufficient time should be available for the buffer manager to create new buffers.
Generally, no buffer tuning is required.
712 C-Series Debugging and Diagnostics