Classbased configuration of WRED is identical to standalone WRED

Congestion avoidance techniques monitor the network interface load in an effort to anticipate and avoid congestion at common network bottlenecks. Congestion avoidance is achieved through intelligent packet dropping techniques. Traditionally, Cisco IOS software used standalone RED and WRED mechanisms to avoid congestion on an interface. Those mechanisms can perform a differentiated drop based on the IP precedence or DSCP value.

The class-based weighted fair queuing (CBWFQ) system supports the use of WRED inside the queuing system, thereby implementing CB-WRED. Each class is queued in its separate queue and has a queue limit, performing tail drop by default. WRED can be configured as the preferred dropping method in a queue, implementing a differentiated drop based on traffic class, and further, on the IP precedence or DSCP value.

Note The combination of CBWFQ and WRED on a single device is currently the only way to implement the DiffServ Assured Forwarding per-hop behavior (AF PHB) using Cisco IOS software.

WRED Building Blocks

The figure shows how WRED is implemented, and the parameters that are used by WRED to influence packet-drop decisions.

The router constantly updates the WRED algorithm with the calculated average queue length, which is based on the recent history of queue lengths.

Configured in the traffic profile are the parameters that define the drop characteristics used by WRED (minimum threshold, maximum threshold, and mark probability denominator). These parameters define the WRED probability slopes.

When a packet arrives at the output queue, the IP precedence or DSCP value is used to select the correct WRED profile for the packet. The packet is then passed to WRED for processing. Based on the selected traffic profile and the average queue length, WRED calculates the probability for dropping the current packet and either drops the packet or passes it to the output queue.

If the queue is already full, the packet is tail-dropped. Otherwise, the packet will eventually be transmitted out onto the interface. If the average queue length is greater than the minimum threshold but less than the maximum threshold, based on the drop probability, WRED will either queue the packet or perform a random drop.

WRED Profiles

This topic describes the different traffic profiles that are used in WRED implementations.

WRED Profiles

WRED profiles can be manually set.

WRED has 8 default value sets for precedence-based WRED. WRED has 64 default value sets for DSCP-based WRED.

WRED profiles can be manually set.

WRED has 8 default value sets for precedence-based WRED. WRED has 64 default value sets for DSCP-based WRED.

This figure shows two different WRED profiles that are used for traffic of two different QoS classes ("BE" class and "Premium" class).

The BE traffic class has a much lower minimum (10) and maximum threshold (30). As a result, traffic belonging to the BE class will be dropped much earlier and more aggressively than traffic from the Premium class. When heavy congestion occurs, traffic belonging to the BE class will ultimately be tail dropped.

The Premium traffic class has been configured with higher minimum (20) and maximum thresholds (40). Therefore packet drop as a result of congestion will occur later (longer average queue size) and is less likely, compared to the BE class. The differences in these traffic profiles, as defined in the figure, maintain differentiated levels of service in the event of congestion.

To avoid the need for setting all WRED parameters in a router, 8 default values are already defined for precedence-based WRED, and 64 DiffServ aligned values are defined for DSCP-based WRED. Therefore, the default settings should suffice in the vast majority of deployments.

By default, the maximum threshold for all DSCP values is 40. The default mark probability denominator for all DSCP values is 10.

IP Precedence and Class Selector Profiles

A PHB is the externally observable forwarding behavior applied at a DiffServ-compliant node to a DiffServ behavior aggregate (BA). With the ability of the system to mark packets according to DSCP setting, collections of packets (each with the same DSCP setting and sent in a particular direction) can be grouped into a DiffServ BA. Packets from multiple sources or applications can belong to the same DiffServ BA.

The class selector BA is used for backward compatibility with non-DiffServ-compliant devices (RFC 1812-compliant devices and, optionally, RFC 791-compliant devices). Therefore, the class selector range of DSCP values is used for backward compatibility with IP precedence. The same WRED profiles are applied to equal IP precedence and class selector values:

IP Precedence and Class Selector Profiles

IP Precedence

DSCP (Class Selector)

Default Minimum Threshold

0 (000)

Default (0)

20

1 (001)

CS1 (8) (001000)

22

2 (010)

CS2 (16) (010000)

24

3 (011)

CS3 (24) (011000)

26

4 (100)

CS4 (32) (100000)

28

5 (101)

CS5 (40) (101000)

30

6 (110)

CS6 (48) (110000)

32

7 (111)

CS7 (56) (111000)

34

RSVP

RSVP

37

DSCP-Based WRED (Expedited Forwarding)

In DSCP, the Expedited Forwarding (EF) PHB is identified based on these parameters:

■ Ensures a minimum departure rate to provide the lowest possible delay to delay-sensitive applications.

■ Guarantees bandwidth to prevent starvation of the application if there are multiple applications using EF PHB.

■ Polices bandwidth to prevent starvation of other applications or classes that are not using this PHB.

■ Packets requiring EF should be marked with DSCP binary value "101110" (46 or 0x2E).

For the EF DiffServ traffic class, Cisco IOS DSCP-based WRED configures itself by default so that the minimum threshold is very high, increasing the probability of no drops being applied to that traffic class. It is expected that EF traffic will be dropped very late, compared to other traffic classes, and the EF traffic is therefore prioritized in the event of congestion.

Expedited Forwarding Profile

DSCP (Six Bits)

Default Minimum Threshold

EF (101110)

Drop Probability

100%

AF Low Drop AF Medium Drop

AF High Droo

AF Low Drop AF Medium Drop

20 24 28 32

Average Queue J Size g

In DSCP, the Assured Forwarding (AF) PHB is identified based on these parameters:

■ 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 "aaaddO" where "aaa" is the number of the class and "dd" is the 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 each AF DiffServ traffic class, Cisco IOS DSCP-based WRED configures itself by default for three different profiles, depending on the drop preference bits. All AF classes are initially marked with Drop Preference 1 (lowest drop preference), but in transit they may be marked down by policers to Drop Preference 2 or Drop Preference 3, depending if they are exceeding or violating administratively defined traffic rates.

Assured Forwarding Profiles

Assured Forwarding Class

Drop Probability

(AF Class) DSCP

Default Minimum Threshold

AF Class 1

Low Drop Prob

(AF11) 001010

32

Medium Drop Prob

(AF12) 001100

28

High Drop Prob

(AF13) 001110

24

AF Class 2

Low Drop Prob

(AF21) 010010

32

Medium Drop Prob

(AF22) 010100

28

High Drop Prob

(AF23) 010110

24

AF Class 3

Low Drop Prob

(AF31) 011010

32

Medium Drop Prob

(AF32) 011100

28

High Drop Prob

(AF33) 011110

24

AF Class 4

Low Drop Prob

(AF41) 100010

32

Medium Drop Prob

(AF42) 100100

28

High Drop Prob

(AF43) 100110

24

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