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Congestion avoidance techniques monitor traffic flow in an effort to anticipate and avoid congestion at common network bottlenecks. Avoidance techniques are implemented before congestion occurs as compared with congestion management techniques that control congestion after it has occurred.
Congestion avoidance is achieved through packet dropping. Cisco IOS XR software supports these quality of service (QoS) congestion avoidance techniques that drop packets:
Random early detection (RED)
Weighted random early detection (WRED)
Tail drop
The module describes the concepts and tasks related to these congestion avoidance techniques.
|
Feature |
ASR 9000 Ethernet Line Cards |
SIP 700 for the ASR 9000 |
|---|---|---|
|
Random Early Detection |
yes |
yes |
|
Weighted Random Early Detection |
yes |
yes |
|
Tail Drop |
yes |
yes |
|
Release |
Modification |
|---|---|
|
Release 3.7.2 |
The Congestion Avoidance feature was introduced on ASR 9000 Ethernet Line Cards. The Random Early Detection, Weighted Random Early Detection, and Tail Drop features were introduced on ASR 9000 Ethernet Line Cards. |
|
Release 3.9.0 |
The Random Early Detection, Weighted Random Early Detection, and Tail Drop features were supported on the SIP 700 for the ASR 9000. |
This prerequisite is required for configuring QoS congestion avoidance on your network:
You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance.
The Random Early Detection (RED) congestion avoidance technique takes advantage of the congestion control mechanism of TCP. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. Assuming the packet source is using TCP, it decreases its transmission rate until all packets reach their destination, indicating that the congestion is cleared. You can use RED as a way to cause TCP to slow transmission of packets. TCP not only pauses, but it also restarts quickly and adapts its transmission rate to the rate that the network can support.
RED distributes losses in time and maintains normally low queue depth while absorbing traffic bursts. When enabled on an interface, RED begins dropping packets when congestion occurs at a rate you select during configuration.
Queue-limit is used to fine-tune the number of buffers available for each queue. It can only be used on a queuing class. Default queue limit is 100 ms of the service rate for the given queue. The service rate is the sum of minimum guaranteed bandwidth and bandwidth remaining assigned to a given class either implicitly or explicitly.
The queue-limit is rounded up to the nearest power of 2, and depending on the line cards on your system, the queue-limit values vary. To check the current queue-limit for class-default, use the show qos interface command.
Because WRED needs a queue to operate on, the class that WRED is applied on must have either a bandwidth statement or a parent policy with a shaper if WRED is applied only on a class default queue.
The following policy configuration does not use the queue limit because the policy is flat and doesn’t have a designated queue on which it operates.
policy-map incorrect-flat
class class-default
random-detect dscp 16 250 packets 500 packets
queue-limit 158000 kbytes
The following policy configuration can use the queue limit because it uses a parent policy map with the shape average command.
policy-map parent
class class-default
shape average 100 mbps
service-policy child
policy-map child
class class-default
random-detect dscp 16 250 packets 500 packets
queue-limit 158000 kbytes
The following policy configuration can use the queue limit because it provides a flat policy with a shaped queue through the bandwidth command for the class-default.
policy-map correct-flat
class class-default
bandwidth 100 mbps
random-detect dscp 16 250 packets 500 packets
queue-limit 158000 kbytes
Tail drop is a congestion avoidance technique that drops packets when an output queue is full until congestion is eliminated. Tail drop treats all traffic flow equally and does not differentiate between classes of service. It manages the packets that are unclassified, placed into a first-in, first-out (FIFO) queue, and forwarded at a rate determined by the available underlying link bandwidth.
See the “Default Traffic Class” section of the “Configuring Modular Quality of Service Packet Classification and Marking on Cisco ASR 9000 Series Routers”.
This configuration task is similar to that used for WRED except that the random-detect precedence command is not configured and the random-detect command with the default keyword must be used to enable RED.
Restrictions
If you configure the random-detect default command on any class including class-default, you must configure one of the following commands:
shape average
bandwidth
bandwidth remaining
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
configure Example: |
Enters global configuration mode. |
|
Step 2 |
policy-map policy-map-name Example: |
Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy and enters the policy map configuration mode. |
|
Step 3 |
class class-name Example: |
Specifies the name of the class whose policy you want to create or change and enters the policy map class configuration mode. |
|
Step 4 |
random-detect {cos value | default | discard-class value | dscp value | exp value | precedence value | min-threshold [units] max-threshold [units] } Example: |
Enables RED with default minimum and maximum thresholds. |
|
Step 5 |
bandwidth {bandwidth [units] | percent value} or bandwidth remaining [percent value | ratio ratio-value Example:or |
(Optional) Specifies the bandwidth allocated for a class belonging to a policy map. or (Optional) Specifies how to allocate leftover bandwidth to various classes. |
|
Step 6 |
shape average {percent percentage | value [units]} Example: |
(Optional) Shapes traffic to the specified bit rate or a percentage of the available bandwidth. |
|
Step 7 |
exit Example: |
Returns the router to policy map configuration mode. |
|
Step 8 |
exit Example: |
Returns the router to global configuration mode. |
|
Step 9 |
interface type interface-path-id Example: |
Enters the configuration mode and configures an interface. |
|
Step 10 |
service-policy {input | output} policy-map Example: |
Attaches a policy map to an input or output interface to be used as the service policy for that interface. In this example, the traffic policy evaluates all traffic leaving that interface. |
|
Step 11 |
Use the commit or end command. |
commit —Saves the configuration changes, and remains within the configuration session.
|
WRED drops packets selectively based on any specified criteria, such as CoS, DSCP, EXP, discard-class, or precedence . WRED uses these matching criteria to determine how to treat different types of traffic.
Configure WRED using the random-detect command and different CoS, DSCP, EXP, and discard-class values. The value can be range or a list of values that are valid for that field. You can also use minimum and maximum queue thresholds to determine the dropping point.
When a packet arrives, the following actions occur:
If the queue size is less than the minimum queue threshold, the arriving packet is queued.
If the queue size is between the minimum queue threshold for that type of traffic and the maximum threshold for the interface, the packet is either dropped or queued, depending on the packet drop probability for that type of traffic.
If the queue size is greater than the maximum threshold, the packet is dropped.
Restrictions
When configuring the random-detect dscp command, you must configure one of the following commands: shape average, bandwidth, and bandwidth remaining.
 Note |
The Cisco ASR 9000 Series ATM SPA supports only time-based WRED thresholds. Therefore, if you try to configure the WRED threshold using the random-detect default command with bytes or packet as the threshold units, the "Unsupported WRED unit on ATM interface" error occurs. |
Only two minimum and maximum thresholds (each with different match criteria) can be configured per class.
| Command or Action | Purpose | |||
|---|---|---|---|---|
|
Step 1 |
configure Example: |
Enters global configuration mode. |
||
|
Step 2 |
policy-map policy-name Example: |
Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy and enters the policy map configuration mode. |
||
|
Step 3 |
class class-name Example: |
Specifies the name of the class whose policy you want to create or change and enters the policy map class configuration mode. |
||
|
Step 4 |
random-detect dscp dscp-value min-threshold [units] max-threshold [units] Example: |
Modifies the minimum and maximum packet thresholds for the DSCP value.
|
||
|
Step 5 |
bandwidth {bandwidth [units] | percent value} or bandwidth remaining [percent value | ratio ratio-value] Example:or |
(Optional) Specifies the bandwidth allocated for a class belonging to a policy map. or (Optional) Specifies how to allocate leftover bandwidth to various classes. |
||
|
Step 6 |
bandwidth {bandwidth [units] | percent value} Example: |
(Optional) Specifies the bandwidth allocated for a class belonging to a policy map. This example guarantees 30 percent of the interface bandwidth to class class1. |
||
|
Step 7 |
bandwidth remaining percent value Example: |
(Optional) Specifies how to allocate leftover bandwidth to various classes.
|
||
|
Step 8 |
shape average {percent percentage | value [units]} Example: |
(Optional) Shapes traffic to the specified bit rate or a percentage of the available bandwidth. |
||
|
Step 9 |
queue-limit value [units] Example: |
(Optional) Changes queue-limit to fine-tune the amount of buffers available for each queue. The default queue-limit is 100 ms of the service rate for a non-priority class and 10ms of the service rate for a priority class.
|
||
|
Step 10 |
exit Example: |
Returns the router to global configuration mode. |
||
|
Step 11 |
interface type interface-path-id Example: |
Enters the configuration mode and configures an interface. |
||
|
Step 12 |
service-policy {input | output} policy-map Example: |
Attaches a policy map to an input or output interface to be used as the service policy for that interface.
|
||
|
Step 13 |
Use the commit or end command. |
commit —Saves the configuration changes and remains within the configuration session.
|
Packets satisfying the match criteria for a class accumulate in the queue reserved for the class until they are serviced. The queue-limit command is used to define the maximum threshold for a class. When the maximum threshold is reached, enqueued packets to the class queue result in tail drop (packet drop).
The queue-limit value uses the guaranteed service rate (GSR) of the queue as the reference value for the queue_bandwidth . If the class has bandwidth percent associated with it, the queue-limit is set to a proportion of the bandwidth reserved for that class.
If the GSR for a queue is zero, use the following to compute the default queue-limit :
1 percent of the interface bandwidth for queues in a nonhierarchical policy.
1 percent of parent maximum reference rate for hierarchical policy.
The parent maximum reference rate is the minimum of parent shape, policer maximum rate, and the interface bandwidth.
Note |
The default queue-limit is set to bytes of 100 ms of queue bandwidth. The following formula is used to calculate the default queue limit (in bytes):??bytes = (100 ms / 1000 ms) * queue_bandwidth kbps)) / 8 |
Restrictions
When configuring the queue-limit command in a class, you must configure one of the following commands: priority , shape average , bandwidth , or bandwidth remaining , except for the default class.
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
configure Example: |
Enters global configuration mode. |
|
Step 2 |
policy-map policy-name Example: |
Creates or modifies a policy map that can be attached to one or more interfaces to specify a service policy and also enters the policy map configuration mode. |
|
Step 3 |
class class-name Example: |
Specifies the name of the class whose policy you want to create or change and enters the policy map class configuration mode. |
|
Step 4 |
queue-limit value [units] Example: |
Specifies or modifies the maximum the queue can hold for a class policy configured in a policy map. The default value of the units argument is packets . In this example, when the queue limit reaches 1,000,000 bytes, enqueued packets to the class queue are dropped. |
|
Step 5 |
priority[ level priority-level ] Example: |
Specifies priority to a class of traffic belonging to a policy map. |
|
Step 6 |
police rate percent percentage Example: |
Configures traffic policing. |
|
Step 7 |
class class-name Example: |
Specifies the name of the class whose policy you want to create or change. In this example, class2 is configured. |
|
Step 8 |
bandwidth {bandwidth [units] | percent value} Example: |
(Optional) Specifies the bandwidth allocated for a class belonging to a policy map. This example guarantees 30 percent of the interface bandwidth to class class2. |
|
Step 9 |
bandwidth remaining percent value Example: |
(Optional) Specifies how to allocate leftover bandwidth to various classes. This example allocates 20 percent of the leftover interface bandwidth to class class2. |
|
Step 10 |
exit Example: |
Returns the router to policy map configuration mode. |
|
Step 11 |
exit Example: |
Returns the router to global configuration mode. |
|
Step 12 |
interface type interface-path-id Example: |
Enters the configuration mode and configures an interface. |
|
Step 13 |
service-policy {input | output} policy-map Example: |
Attaches a policy map to an input or output interface to be used as the service policy for that interface. In this example, the traffic policy evaluates all traffic leaving that interface. |
|
Step 14 |
Use the commit or end command. |
commit —Saves the configuration changes and remains within the configuration session.
|
These sections provide references related to implementing QoS congestion avoidance.
|
Related Topic |
Document Title |
|---|---|
|
Initial system bootup and configuration |
Cisco ASR 9000 Series Aggregation Services Router Getting Started Guide |
|
QoS commands |
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Command Reference |
|
User groups and task IDs |
“Configuring AAA Services on Cisco ASR 9000 Series Router” module of Cisco Cisco ASR 9000 Series Aggregation Services Router System Security Configuration Guide |
|
Standards |
Title |
|---|---|
|
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature. |
— |
|
MIBs |
MIBs Link |
|---|---|
|
— |
To locate and download MIBs using Cisco IOS XR software, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml |
|
RFCs |
Title |
|---|---|
|
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature. |
— |
|
Description |
Link |
|---|---|
|
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