Avoidance 4.6.7 WRED Profiles:
DSCP-Based WRED (AF) In DSCP, the following parameters
identify Assured Forwarding (AF) PHB based on: -
Guarantees a certain amount of bandwidth to an AF class.
- Allows access to extra bandwidth, if available.
Packets requiring AF PHB should be marked with DSCP value
aaadd0, where aaa is the number of the class and dd is the drop
probability, or drop preference, of the traffic class. There
are four defined AF classes. Each class should be treated
independently and have bandwidth allocated that is based on the
QoS policy. For the AF DiffServ traffic class, WRED configures
itself by default for three different profiles, depending on
the drop preference DSCP marking bits. Therefore, AF traffic
should be classified into the three possible classes, such as
AF high drop, AF medium drop, and AF low drop. These three
classes are based on the sensitivity to packet drops of the
application or applications represented by the class. This
would mean that the mission-critical class would have an AF low
drop, an AF medium drop, and an AF high drop. Configuring
DSCP-Based CBWRED
The random-detect dscp-based
command is used to enable DSCP-based WRED on an interface.
Changing WRED weighting to values based on DSCP increases the
number of WRED traffic profiles to 64 (as compared to 8
profiles for precedence-based WRED). Figure shows the command
syntax. You can configure WRED as part of the policy for a
standard class or the default class. The WRED
random-detect command and the WFQ queue-limit
command are mutually exclusive for class policy. If you
configure WRED, its packet-drop capability is used to manage
the queue when packets exceeding the configured maximum count
are enqueued. If you configure the WFQ queue-limit
command for class policy, tail drop is used. Changing the
WRED Traffic Profile
When DSCP-based WRED is enabled,
default values are selected for each traffic profile based on
DSCP. Network administrators can then modify these default
values to match their specific administrative QoS policy goals.
Figure shows the command syntax. When you are modifying the
default WRED profile for DSCP, the same values described
previously for IP precedence are configurable, except that the
minimum threshold, the maximum threshold, and the mark
probability denominator will be applied to DSCP classified
packets. CBWRED Using DSCP: Example
This example in
Figure shows DSCP-based CBWRED. Remember that the DiffServ
model itself provides defined traffic classes and their
associated PHBs. DiffServ-based classification is used in this
example as follows: - Mission-critical class:
This class is marked using DSCP AF class 2 and should get 30
percent of an interface bandwidth.
- Bulk
class: This class is marked using DSCP AF class 1 and
should get 20 percent of the interface bandwidth.
-
Best-effort class: This traffic should get the remaining
bandwidth share and should be fair-queued.
To
enforce this service policy, a router uses CBWFQ to perform
bandwidth sharing and uses WRED within service classes to
perform differentiated drop. Figure shows sample drop profile
graphs and outputs. The configuration example shows how traffic
classification is performed using DSCP-based classes,
representing the mission-critical class as the AF1 class, and
the bulk class as the AF2 class. WRED DSCP-based parameters are
set reflecting the class-dependent drop strategy: - The
mission-critical class is guaranteed at least 30 percent of
bandwidth, with a custom WRED profile that establishes three
different drop probabilities for AF class 2. The minimum
threshold for DSCP AF21 packets is 32. The minimum threshold
for DSCP AF22 packets is 28. The minimum threshold for DSCP
AF23 and CS2 packets is 24. The maximum threshold for all
Mission-critical traffic is 40 - the default maximum threshold
is equal to the default hold queue size (40) on an interface.
The (default) mark probability is 10 for the Mission-critical
traffic (one out of every 10 packets is dropped when the
average queue depth is at the maximum threshold – the average
queue size changes with time as a function of the previously
calculated average queue size and the current queue
size).
- The bulk class is guaranteed at least 20
percent of bandwidth, is configured with three different drop
probabilities for AF class 1, and has a somewhat lower WRED
maximum threshold. As a result, bulk-class traffic is more
likely to be dropped than the mission-critical class if
interface congestion occurs. The minimum threshold for DSCP
AF11 packets is 32. The minimum threshold for DSCP AF12 packets
is 28. The minimum threshold for DSCP AF13 packets is 24. The
minimum threshold for DSCP CS1 packets is 24. The maximum
threshold for all Mission-critical traffic is 36. The (default)
mark probability is 10 for the Bulk traffic.
All
other traffic is part of the default class and is fair-queued,
with default WRED parameters. Note
When you enable
WRED with the random-detect command, the parameters are
set to their default values. The weight factor is 9. For all
precedences, the mark probability denominator is 10, and
maximum threshold is based on the output buffering capacity and
the transmission speed for the interface.
Content
4.6 Congestion Avoidance 4.6.8
Monitoring CBWRED The show policy-map interface
command displays the configuration of all classes configured
for all service policies on the specified interface. This
includes all WRED parameters implementing the drop policy on
the specified interface. Figure shows the command syntax.The
“Key Fields in the show policy-map interface Command
Output” table explains some of the key fields of the output of
the show policy-map interface command. Key Fields in
the show policy-map interface Command Output
Field Description Service-policy output Name of the
output service policy applied to the specified interface or
virtual circuit (VC). Class-map Class of traffic being
displayed. Output is displayed for each configured class in the
policy. The choice for implementing class matches (for example,
match-all or match-any) can also appear next to the traffic
class. Match Match criteria specified for the class of traffic.
Choices include criteria such as IP precedence, IP DSCP value,
Multiprotocol Label Switching (MPLS) experimental (EXP) value,
access groups, and QoS groups. exponential weight Exponent used
in the average queue size calculation for a WRED parameter
group. mean queue depth Average queue depth based on the actual
queue depth on the interface and the exponential weighting
constant. It is a fluctuating average. The minimum and maximum
thresholds are compared against this value to determine drop
decisions.
Content 4.7 Introducing
Traffic Policing and Shaping 4.7.1 Traffic
Policing and Shaping Overview Both traffic shaping and
policing mechanisms are traffic-conditioning mechanisms that
are used in a network to control the traffic rate. Both
mechanisms use classification so that they can differentiate
traffic. They both measure the rate of traffic and compare that
rate to the configured traffic-shaping or traffic-policing
policy. Figure illustrates these mechanisms. The difference
between traffic shaping and policing can be described in terms
of their implementation. Traffic policing drops excess traffic
to control traffic flow within specified rate limits. Traffic
policing does not introduce any delay to traffic that conforms
to traffic policies. Traffic policing can cause more TCP
retransmissions, because traffic in excess of specified limits
is dropped. Traffic-policing mechanisms such as class-based
policing or committed access rate (CAR) also have marking
capabilities in addition to rate-limiting capabilities. Instead
of dropping the excess traffic, traffic policing can mark and
then send the excess traffic. This feature allows the excess