Drop Precedence - Extreme Networks BlackDiamond 6800 Installation And User Manual

Configuring the ATM Module
By default, the minimum threshold for ATM ports is 10%, or 100 buffers; thus, the
default maximum threshold is 300 buffers.
You can use the command to display the settings of the
show ports info detail
minimum and maximum thresholds, displayed in terms of the number of buffers.
Use the keyword to configure the threshold parameter on specific ATM ports.
ports
Support for Standard Per-Hop Behaviors
The per-hop behavior (PHB) describes the externally observable packet forwarding
handling (or "behavior") to be applied by the receiving network element when there are
competing requests for resources such as bandwidth and buffer space. In the packet
forwarding path, differentiated services are identified by mapping the differentiated
services code point (DSCP) contained in the IP packet header to a specific forwarding
behavior at each network element along its path. The DSCP is 6 bits wide, and takes the
form xxxxxx, where x can be either 0 or 1. The DSCP field is capable of identifying one
of 64 distinct code points. For purposes of code point allocation and management, the
code point space is divided into three pools: one pool of 32 code points (pool 1)
constitutes the recommended code points to be allocated as standards; a second pool of
16 code points (pool 2) is set aside for experimental or local use; a third pool of 16 code
points (pool 3) that are initially set aside for experimental or local use, but that might be
used for standard assignments if pool 1 is ever exhausted. The mapping of DSCPs to
PHBs is a user-configurable function, as described below.
The current standards call for two PHBs: Assured Forwarding (AF) and Expedited
Forwarding (EF). The EF PHB describes the required behavior for voice-over-IP service.
The AF PHB consists of four independently forwarded AF classes: AF1, AF2, AF3, and
AF4. Within each of these classes, an IP packet can be assigned to different levels of
drop precedence (used to determine drop probability) depending on how many levels
of drop precedence the implementation supports. RFC 2597 describes two schemes for
drop-precedence levels: a three-level scheme (see Table 3-10) and a two-level scheme
(see Table 3-11). The three-level scheme supports low, medium, and high
drop-precedence levels for the AF classes; the two-level scheme supports low and high
drop-precedence levels (and groups the medium drop-precedence code-points with the
high drop-precedence code-points). The Extreme implementation for the ATM module
supports the two-level drop-precedence scheme.
3-40 Asynchronous Transfer Mode (ATM) Module Installation and User Guide