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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
On systems with Cisco ASR 9000 High-Density 100GE Ethernet line cards and fifth-generation line cards, ensure that you configure the minimum and maximum threshold values that are greater than the default minimum and maximum threshold values. If you apply a policy that has lesser than default values to a bundle that has both these line cards, the show policy-map interface command displays a mismatch in statistics bag size.
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.
|
|
Feature Name |
Release Information |
Feature Description |
|---|---|---|
|
Low Queue Limit and WRED Values for Low Access Speeds |
Release 7.11.1 |
You can now design your infrastructure for low access speeds, allowing your customers to use low-speed Layer 3 VPN services. Your customers can thus deliver these low-speed services to their end-customers, who can use them to connect their branch offices, home-workers, or any other business scenario requiring low-speed services. To achieve these services, we have enabled the configuration of lower minimum values for queue limit and WRED on the fourth and fifth generations of the ASR 9000 Series High Density Ethernet line cards. You can configure lower queue limit values only for 1 Gbps, 10 Gbps, and 400 Gbps interfaces and lower WRED values for 1 Gbps and 10 Gbps interfaces. There’s no action required for you to enable this functionality, and there are no changes to command options. |
Previously, the minimum value for both queue limit and Weighted Random Early Detection (WRED) was 100 KB. These values meant that when you deployed lower-capacity devices in your networks for low access speeds, you couldn’t configure any value lower than 100 KB for queue limit and WRED. If you did, the hardware would round off that value to 100 KB.
From Release 7.11.1, you can configure lower values for queue limit (using the queue-limit command) and WRED (using the random-detect command) to effectively deploy and operate lower-capacity devices in your network for lower access speeds. These new values also apply to 1 Gbps and 10 Gbps interfaces when they’re part of a bundle interface, while other interfaces that are part of the bundle will continue to have the earlier minimum values. For 400 Gbps interfaces, the new queue limit value is 400 KB, which also applies when they’re part of a bundle interface.
The table displays the queue limit and WRED values for different interfaces before and from Release 7.11.1.
|
Interface Speed |
Minimum Queue Limit Value Before Release 7.11.1 |
Minimum Queue Limit Value After Release 7.11.1 |
Minimum WRED Value Before Release 7.11.1 | Minimum WRED Value After Release 7.11.1 |
|---|---|---|---|---|
|
1 Gbps |
100 KB |
10 KB |
100 KB |
5 KB |
|
10 Gbps |
100 KB |
10 KB |
100 KB |
5 KB |
|
400 Gbps |
100 KB |
400 KB |
100 KB |
100 KB (no change) |
|
All other interfaces |
100 KB |
100 KB (no change) |
100 KB |
100 KB (no change) |
The minimum queue limit value is only modified for 1 Gbps, 10 Gbps, and 400 Gbps interfaces, and the minimum WRED is only modified for 1 Gbps and 10 Gbps interfaces.
This feature is supported on the fourth and fifth generations of the ASR 9000 Series High Density Ethernet line cards.
This feature isn’t supported on the third generation of the ASR 9000 Series High Density Ethernet line cards.
These new minimum queue limit and WRED values apply to 1 Gbps and 10 Gbps interfaces when they’re part of a bundle interface, while other interfaces that are part of the bundle will continue to have the earlier minimum values. For 400 Gbps interfaces, you can configure a WRED value, which also applies when they’re part of a bundle interface.
Let’s say you configure a queue limit of 10 KB for a policy map named test-qlimit and class map match-vlan1 , which you attach to a 10 Gbps interface.
Router(config)#policy-map test-qlimit
Router(config-pmap)#class match-vlan1
Router(config-pmap-c)#queue-limit 10 kybtes
Router(config-pmap-c)#priority level 3
Router(config-pmap)#commit
Router(config)#int tenGigE 0/0/0/3/3
Router(config-if)#service-policy output test-qlimit
Router(config-if)#commit
Run the show qos interface command to view the lower queue limit value that you configured for class map match-vlan1 .
Router#show qos interface tenGigE 0/0/0/3/3 output location 0/0/0/3/3
Interface: TenGigE0_0_0_3_3 output
Bandwidth configured: 10000000 kbps Bandwidth programed: 10000000 kbps
ANCP user configured: 0 kbps ANCP programed in HW: 0 kbps
Port Shaper programed in HW: 0 kbps
Policy: test-q-limit Total number of classes: 2
----------------------------------------------------------------------
Level: 0 Policy: test-q-limit Class: match-vlan1
QueueID: 0x9032 (Priority 3)
Queue Limit: 10 kbytes (10 kbytes) Abs-Index: 0 Template: 0 Curve: 0
----------------------------------------------------------------------
Level: 0 Policy: test-qlimit Class: class-default
QueueID: 0x9037 (Priority Normal)
Queue Limit: 1250 kbytes Abs-Index: 0 Template: 0 Curve: 0
WFQ Profile: 0/0 Committed Weight: 0 Excess Weight: 1
Bandwidth: 0 kbps, BW sum for Level 0: 0 kbps, Excess Ratio: 1
WFQ HW CIR Token refill: 0 Saturation num: 0 Excess wt: 0
Let’s say you configure the minimum and maximum values for random-detect (WRED) value as 5 KB for policy map test-wred and class map match-vlan1 .
Note |
Per the new functionality, any WRED value you configure below 5 KB gets rounded to 5 KB. Also, you can configure these values in units other than KBs (such as packets or ms), but the router hardware converts them to KB and rounds off the value to 5 KB if the converted value in KB is less than 5 KB. |
Router(config)#policy-map test-wred
Router(config-pmap)#class match-vlan1
Router(config-pmap-c)#queue-limit 10 kbytes
Router(config-pmap-c)#bandwidth percent 10
Router(config-pmap-c)#random-detect dscp 30 5 kbytes 5 kbytes
Router(config-pmap-c)#commit
Router(config)#int tenGigE 0/0/0/3/3
Router(config-if)#service-policy output test-wred
Router(config-if)#commit
Run the show qos interface command to view the lower WRED value (random-detect ) that you configured for class map match-vlan1 .
Router#sh qos interface tenGigE 0/0/0/3/3 output location 0/0/0/3/3
Interface: TenGigE0_0_0_3_3 output
Bandwidth configured: 10000000 kbps Bandwidth programed: 10000000 kbps
ANCP user configured: 0 kbps ANCP programed in HW: 0 kbps
Port Shaper programed in HW: 0 kbps
Policy: test-wred Total number of classes: 2
----------------------------------------------------------------------
Level: 0 Policy: test-wred Class: match-vlan1
QueueID: 0x9032 (Priority Normal)
Queue Limit: 10 kbytes (10 kbytes) Abs-Index: 0 Template: 0 Curve: 0
WFQ Profile: 0/0 Committed Weight: 1000000 Excess Weight: 1
Bandwidth: 1000000 kbps, BW sum for Level 0: 1000000 kbps, Excess Ratio: 1
WFQ HW CIR Token refill: 0 Saturation num: 0 Excess wt: 655
WRED Type: DSCP and EXP based Curves: 2
Default RED Curve Profile: 0/0/0 Thresholds Min : 10 (10) kbytes Max: 10 (10) kbytes
Hardware Output: 5000000a00000000
segment_size = 10 (0xa)
wred_index = 0 (0x0)
min_threshold = 10 (0xa)
first_segment = 0 (0x0)
max_minus_min_threshold = 0 (0x0)
WRED Curve: 1 Profile: 0/0/0 Thresholds Min : 5 (5) kbytes Max: 5 (5) kbytes
Match: 30
Hardware Output: 5000000500000000
segment_size = 10 (0xa)
wred_index = 0 (0x0)
min_threshold = 5 (0x5)
first_segment = 0 (0x0)
max_minus_min_threshold = 0 (0x0)
----------------------------------------------------------------------
Level: 0 Policy: test-wred Class: class-default
QueueID: 0x9033 (Priority Normal)
Queue Limit: 1250 kbytes Abs-Index: 0 Template: 0 Curve: 0
WFQ Profile: 0/0 Committed Weight: 0 Excess Weight: 1
Bandwidth: 0 kbps, BW sum for Level 0: 1000000 kbps, Excess Ratio: 1
WFQ HW CIR Token refill: 0 Saturation num: 0 Excess wt: 16
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 |
|---|---|
|
The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content. |
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