Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring QoS (Rev. 03)
Overview
© 2008 Foundry Networks, Inc.
Page 12 of 62
value of 100). These Class-Selector PHBs ensure that DS-compliant nodes can coexist with IP
Precedence-based nodes.
For more information about Class-Selector PHB, refer to RFC 2474,
Definition of the
Differentiated Services Field in IPv4 and IPv6 Headers.
Assured Forwarding PHB
Assured Forwarding (AF) PHB is nearly equivalent to the Controlled Load Service available in the
integrated services model. The AF PHB defines a method by which BAs can be given different
forwarding assurances.
For example, network traffic can be divided into the following classes:
•
Gold: Traffic in this category is allocated 50 percent of the available bandwidth.
•
Silver: Traffic in this category is allocated 30 percent of the available bandwidth.
•
Bronze: Traffic in this category is allocated 20 percent of the available bandwidth.
Further, the AF PHB defines four AF classes: AF1, AF2, AF3, and AF4. Each class is assigned a
specific amount of buffer space and interface bandwidth, according to the SLA with the service
provider or policy map.
Within each AF class, the user can specify three drop precedence (dP) values: 1, 2, and 3.
Assured Forwarding PHB can be expressed as follows:
AF
ny
In this example,
n
represents the AF class number (1, 2, or 3) and y represents the dP value (1, 2, or
3) within the AFn class.
In instances of network traffic congestion, if packets in a particular AF class (for example, AF1)
need to be dropped, packets in the AF1 class will be dropped according to the following guideline:
dP(AF
ny
) >= dP(AF
nz
) >= dP(AF
nx
)
where dP (AF
ny
) is the probability that packets of the AF
ny
class will be dropped. The variable
y
denotes the dP within an AFn class.
In the following example, packets in the AF13 class will be dropped before packets in the AF12
class, which in turn will be dropped before packets in the AF11 class:
dP(AF13) >= dP (AF12) >= dP(AF11)
The dP method penalizes traffic flows within a particular BA that exceed the assigned bandwidth.
Packets in these offending flows could be remarked by an ACL to higher drop precedence.
An AF
x
class can be denoted by the DSCP value,
xyz
ab0, where
xyz
can be 001, 010, 011, or 100,
and
ab
represents the dP value.
Table 3
lists the DSCP value and corresponding dP value for each AF PHB class.
Table 3: DSCP Values and Corresponding Drop Precedence, by AF PHB Class Drop
Precedence
AF PHB Class
Class 1
Class 2
Class 3
Class 4
Low
drop
precedence
001010 010010 011010 100010
Medium drop precedence
001100 010100 011100 100100
High drop precedence
001110 010110 011110 100110