Downloads |
Table Of Contents
Distributed Weighted Random Early Detection
random-detect exponential-weighting-constant
Distributed Weighted Random Early Detection
Feature Summary
Random Early Detection (RED) is a congestion avoidance mechanism that takes advantage of TCP's congestion control mechanism. 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 will decrease its transmission rate until all the packets reach their destination, indicating that the congestion is cleared.
Weighted RED (WRED) generally drops packets selectively based on IP precedence. Packets with a higher IP precedence are less likely to be dropped than packets with a lower precedence. Thus, higher priority traffic is delivered with a higher probability than lower priority traffic. However, you can also configure WRED to ignore IP precedence when making drop decisions so that non-weighted RED behavior is achieved.
WRED is useful on any output interface where you expect to have congestion. However, WRED is usually used in the core routers of a network, rather than the edge. Edge routers assign IP precedences to packets as they enter the network. WRED uses these precedences to determine how it treats different types of traffic.
The Distributed WRED (DWRED) feature uses the VIP rather than the RSP to perform the queuing; therefore, it requires a Cisco 7500 series router or Cisco 7000 series router with RSP7000.
The Release 11.1 CC version of DWRED differs from the previously existing RSP-based version of WRED. This document describes only the Release 11.1 CC version of DWRED. Refer to the Cisco IOS Release 11.3 Configuration Fundamentals Configuration Guide and Configuration Fundamentals Command Reference for information on the RSP version of WRED.
Benefits
When RED is not configured, output buffers fill during periods of congestion. When the buffers are full, tail drop occurs; all additional packets are dropped. Since the packets are dropped all at once, global synchronization of TCP hosts can occur as multiple TCP hosts reduce their transmission rates. The congestion clears, and the TCP hosts increase their transmissions rates, resulting in waves of congestion followed by periods where the transmission link is not fully used.
RED reduces the chances of tail drop by selectively dropping packets when the output interface begins to show signs of congestion. By dropping some packets early rather than waiting until the buffer is full, RED avoids dropping large numbers of packets at once and minimizes the chances of global synchronization. Thus, RED allows the transmission line to be used fully at all times.
In addition, RED statistically drops more packets from large users than small. Therefore, traffic sources that generate the most traffic are more likely to be slowed down than traffic sources that generate little traffic.
WRED provides separate thresholds and weights for different IP precedences, allowing you to provide different qualities of service for different traffic. Standard traffic may be dropped more frequently than premium traffic during periods of congestion.
List of Terms
DWRED—Distributed WRED, an implementation of WRED. DWRED performs all of the functions of WRED, but all of the processing takes place on the VIP.
RED—Random Early Detection.
VIP—Versatile Interface Processor.
WRED—Weighted RED.
Restrictions
DWRED has the following restrictions:
•
WRED is only useful when the bulk of the traffic is TCP/IP traffic. With TCP, dropped packets indicate congestion, so the packet source will reduce its transmission rate. With other protocols, packet sources may not respond or may resend dropped packets at the same rate. Thus, dropping packets does not decrease congestion.
•
•
•
•
•
Platforms
This feature is supported on the following routers with a VIP2-40 card or better:
•
•
A VIP2-50 card is strongly recommended when the aggregate line rate of the port adapters on the VIP is greater than DS-3. A VIP2-50 card is required for OC-3 rates.
Prerequisites
In order to use DWRED, Distributed Cisco Express Forwarding switching must be enabled on the interface. Refer to the Cisco Express Forwarding feature documentation for configuration information.
Supported MIBs and RFCs
This feature supports the CISCO-WRED-MIB.
For descriptions of supported MIBs and how to use MIBs, see Cisco's MIB website on CCO at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
No RFCs are supported by this feature.
Functional Description
When a packet arrives, the following events occur:
•
•
•
•
Average Queue Size
The average queue size is based on the previous average and the current size of the queue. The formula is
average = (old_average * (1-1/2^n)) + (current_queue_size * 1/2^n)where n is the exponential weight factor, a user-configurable value.
Note
For high values of n, the previous average becomes more important. A large factor smooths out the peaks and lows in queue length. The average queue size is unlikely to change very quickly, avoiding drastic swings in size. The WRED process will be slow to start dropping packets, but it may continue dropping packets for a time after the actual queue size has fallen below the minimum threshold. The slow-moving average will accommodate temporary bursts in traffic.
If the value of n gets too high, WRED will not react to congestion. Packets will be transmitted or dropped as if WRED were not in effect.
For low values of n, the average queue size closely tracks the current queue size. The resulting average may fluctuate with changes in the traffic levels. In this case, the WRED process responds quickly to long queues. Once the queue falls below the minimum threshold, the process will stop dropping packets.
If the value of n gets too low, WRED will overreact to temporary traffic bursts and drop traffic unnecessarily.
Packet-Drop Probability
The packet drop probability is based on the minimum threshold, maximum threshold, and mark probability denominator.
When the average queue depth is above the minimum threshold, RED starts dropping packets. The rate of packet drop increases linearly as the average queue size increases until the average queue size reaches the maximum threshold.
The mark probability denominator is the fraction of packets dropped when the average queue depth is at the maximum threshold. For example, if the denominator is 512, one out of every 512 packets is dropped when the average queue is at the maximum threshold.
When the average queue size is above the maximum threshold, all packets are dropped.
summarizes the packet drop probability.
Figure 1 WRED Packet Drop Probability
The minimum threshold value should be set high enough to maximize the link utilization. If the minimum threshold is too low, packets may be dropped unnecessarily, and the transmission link will not be fully used.
The difference between the maximum threshold and the minimum threshold should be large enough to avoid global synchronization. If the difference is too small, many packets may be dropped at once, resulting in global synchronization.
Configuration Tasks
To configure DWRED on an interface, perform the following tasks. The first task is required; the second is optional.
Enable WRED
To enable WRED, perform the following task in interface configuration mode:
You do not need to specify any other commands or parameters in order to configure WRED on the interface. WRED will use the default parameter values.
Use the show interfaces random-detect EXEC command to view the current WRED configuration and status.
Change WRED Parameters
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.
The default minimum threshold depends on the precedence. The minimum threshold for IP Precedence 0 corresponds to one half of the maximum threshold. The values for the remaining precedences fall between one half the maximum threshold and the maximum threshold at evenly spaced intervals.
Note
To change WRED parameters, perform one of the following tasks in interface configuration mode:
Use the show interfaces random-detect EXEC command to view the current WRED configuration and status.
Configuration Examples
This section contains the following examples:
Default DWRED Example
The following example enables DWRED with the default parameter values:
interface Hssi0/0/0description 45Mbps to R1ip address 200.200.14.250 255.255.255.252random-detectUse the show interfaces random-detect command to verify the configuration and view the default settings for the different precedences.
Router# show interfaces random-detectHssi0/0/0 queue size 0packets output 29692, drops 0WRED: queue average 0weight 1/512Precedence 0: 109 min threshold, 218 max threshold, 1/10 mark weight1 packets output, drops: 0 random, 0 thresholdPrecedence 1: 122 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 2: 135 min threshold, 218 max threshold, 1/10 mark weight14845 packets output, drops: 0 random, 0 thresholdPrecedence 3: 148 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 4: 161 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 5: 174 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 6: 187 min threshold, 218 max threshold, 1/10 mark weight14846 packets output, drops: 0 random, 0 thresholdPrecedence 7: 200 min threshold, 218 max threshold, 1/10 mark weight(no traffic)The following is sample show interfaces command output for this configuration. Notice that the Packet Drop Strategy is listed as "VIP-based weighted RED."
Router# show interfaces hssi0/0/0Hssi0/0/0 is up, line protocol is upHardware is cyBus HSSIDescription: 45Mbps to R1Internet address is 200.200.14.250/30MTU 4470 bytes, BW 45045 Kbit, DLY 200 usec, rely 255/255, load 1/255Encapsulation HDLC, loopback not set, keepalive set (10 sec)Last input 00:00:02, output 00:00:03, output hang neverLast clearing of "show interface" counters neverQueueing strategy: fifoPacket Drop strategy: VIP-based weighted REDOutput queue 0/40, 0 drops; input queue 0/75, 0 drops5 minute input rate 0 bits/sec, 0 packets/sec5 minute output rate 0 bits/sec, 0 packets/sec1976 packets input, 131263 bytes, 0 no bufferReceived 1577 broadcasts, 0 runts, 0 giants0 parity4 input errors, 4 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort1939 packets output, 130910 bytes, 0 underruns0 output errors, 0 applique, 3 interface resets0 output buffers copied, 0 interrupts, 0 failuresDRED Example
The following example configures DRED, rather than DWRED, by specifying the same parameters for each IP precedence. Thus, all IP precedences receive the same treatment. Start by enabling DRED:
interface FastEthernet1/0/0ip address 200.200.14.250 255.255.255.252random-detectNext, use the show interfaces random-detect command to determine reasonable values to use for the precedence-specific parameters:
Router# show interfaces random-detectFastEthernet1/0/0 queue size 0packets output 29692, drops 0WRED: queue average 0weight 1/512Precedence 0: 109 min threshold, 218 max threshold, 1/10 mark weight1 packets output, drops: 0 random, 0 thresholdPrecedence 1: 122 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 2: 135 min threshold, 218 max threshold, 1/10 mark weight14845 packets output, drops: 0 random, 0 thresholdPrecedence 3: 148 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 4: 161 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 5: 174 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 6: 187 min threshold, 218 max threshold, 1/10 mark weight14846 packets output, drops: 0 random, 0 thresholdPrecedence 7: 200 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Complete the configuration by assigning the same parameter values to each precedence. Use the values obtained from the show interfaces random-detect command output to pick reasonable parameter values.
interface FastEthernet1/0/0random-detect precedence 0 100 218 10random-detect precedence 1 100 218 10random-detect precedence 2 100 218 10random-detect precedence 3 100 218 10random-detect precedence 4 100 218 10random-detect precedence 5 100 218 10random-detect precedence 6 100 218 10random-detect precedence 7 100 218 10Command Reference
This section documents new or modified commands. All other commands used with this feature are documented in the Cisco IOS Release 11.1 command references and the Cisco Express Forwarding feature documentation.
•
•
random-detect
To enable Weighted Random Early Detection (WRED) on an interface, use the random-detect interface configuration command. The no form of this command disables WRED.
random-detect
no random-detectSyntax Description
This command has no arguments or keywords.
Default
Disabled
Command Mode
Interface configuration
Usage Guidelines
This command first appeared in Cisco IOS Release 11.1 CC.
WRED is a congestion avoidance mechanism that slows traffic by randomly dropping packets when there is congestion. WRED is only useful with protocols like TCP, which respond to dropped packets by decreasing the transmission rate.
The router automatically determines parameters to use in the WRED calculations. If you wish to change these parameters, use the random-detect exponential-weighting-constant or random-detect precedence command.
Example
The following example configures WRED on the HSSI 0/0/0 interface:
interface Hssi0/0/0random-detectRelated Commands
random-detect exponential-weighting-constant
random-detect precedence
show interfaces random-detectrandom-detect exponential-weighting-constant
To configure the exponential weight factor for the average queue size calculation, use the random-detect exponential-weighting-constant interface configuration command. The no form of this command returns the value to the default.
random-detect exponential-weighting-constant exponent
no random-detect exponential-weighting-constantSyntax Description
Default
The weight factor is 9.
Command Mode
Interface configuration
Usage Guidelines
This command first appeared in Cisco IOS Release 11.1 CC.
Use this command to change the exponent used in the average queue size calculation. The average queue size is based on the previous average and the current size of the queue. The formula is
average = (old_average * (1-1/2^n)) + (current_queue_size * 1/2^n)where n is the exponential weight factor specified in this command. Thus, the higher the factor, the more dependent the average is on the previous average.
Note
For high values of n, the previous average becomes more important. A large factor smooths out the peaks and lows in queue length. The average queue size is unlikely to change very quickly, avoiding drastic swings in size. The WRED process will be slow to start dropping packets, but it may continue dropping packets for a time after the actual queue size has fallen below the minimum threshold. The slow-moving average will accommodate temporary bursts in traffic.
If the value of n gets too high, WRED will not react to congestion. Packets will be transmitted or dropped as if WRED were not in effect.
For low values of n, the average queue size closely tracks the current queue size. The resulting average may fluctuate with changes in the traffic levels. In this case, the WRED process responds quickly to long queues. Once the queue falls below the minimum threshold, the process will stop dropping packets.
If the value of n gets too low, WRED will overreact to temporary traffic bursts and drop traffic unnecessarily.
Example
The following example configures WRED on an interface with weight factor of 10:
interface Hssi0/0/0description 45Mbps to R1ip address 200.200.14.250 255.255.255.252random-detectrandom-detect exponential-weighting-constant 10Related Commands
random-detect
random-detect precedence
show interfaces random-detectrandom-detect precedence
To configure WRED parameters for a particular IP precedence, use the random-detect precedence interface configuration command. The no form of this command returns the values to the default for the precedence.
random-detect precedence precedence min-threshold max-threshold mark-prob-denominator
no random-detect precedence precedence min-threshold max-threshold
mark-prob-denominatorSyntax Description
Default
For all precedences, the mark-prob-denominator is 10, and the max-threshold is based on the output buffering capacity and the transmission speed for the interface.
The default min-threshold depends on the precedence. The min-threshold for IP precedence 0 corresponds to one half of the max-threshold. The values for the remaining precedences fall between one half the max-threshold and the max-threshold at evenly spaced intervals. lists the default minimum value for each precedence.
Command Mode
Interface configuration
Usage Guidelines
This command first appeared in Cisco IOS Release 11.1 CC.
When you configure the random-detect command on an interface, packets are given preferential treatment based on the IP precedence of the packet. Use the random-detect precedence command to adjust the treatment for different IP precedences.
If you want WRED to ignore the precedence when determining which packets to drop, enter this command with the same parameters for each precedence. Remember to use reasonable values for the minimum and maximum thresholds.
Note
Example
The following example enables WRED on the interface and specifies parameters for the different IP precedences:
interface Hssi0/0/0description 45Mbps to R1ip address 200.200.14.250 255.255.255.252random-detectrandom-detect precedence 0 32 256 100random-detect precedence 1 64 256 100random-detect precedence 2 96 256 100random-detect precedence 3 128 256 100random-detect precedence 4 160 256 100random-detect precedence 5 192 256 100random-detect precedence 6 224 256 100random-detect precedence 7 256 256 100Related Commands
random-detect
random-detect exponential-weighting-constant
show interfaces random-detectshow interfaces
Use the show interfaces EXEC command to display statistics for all interfaces. The resulting output varies depending on the network for which an interface has been configured.
show interfaces [type slot/port-adapter/port] (for ports on VIP cards in the
Cisco 7500 series routers)Syntax Description
Command Mode
EXEC
Usage Guidelines
This command first appeared in Cisco IOS Release 10.0.
The show interfaces command displays statistics for the network interfaces.
The output of the show interfaces command varied based in the interface type and configuration. This document describes the output related to the DWRED feature. Refer to the Cisco IOS Release 11.1 command references for a complete description of the entire output.
Sample Display for DWRED
The following is sample output from the show interfaces command when DWRED is enabled on an interface. Notice that the packet drop strategy is listed as "VIP-based weighted RED."
Router# show interfaces hssi0/0/0Hssi0/0/0 is up, line protocol is upHardware is cyBus HSSIDescription: 45Mbps to R1Internet address is 200.200.14.250/30MTU 4470 bytes, BW 45045 Kbit, DLY 200 usec, rely 255/255, load 1/255Encapsulation HDLC, loopback not set, keepalive set (10 sec)Last input 00:00:02, output 00:00:03, output hang neverLast clearing of "show interface" counters neverQueueing strategy: fifoPacket Drop strategy: VIP-based weighted REDOutput queue 0/40, 0 drops; input queue 0/75, 0 drops5 minute input rate 0 bits/sec, 0 packets/sec5 minute output rate 0 bits/sec, 0 packets/sec1976 packets input, 131263 bytes, 0 no bufferReceived 1577 broadcasts, 0 runts, 0 giants0 parity4 input errors, 4 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort1939 packets output, 130910 bytes, 0 underruns0 output errors, 0 applique, 3 interface resets0 output buffers copied, 0 interrupts, 0 failuresshow interfaces random-detect
To display information about WRED for an interface, use the show interfaces random-detect EXEC command.
show interfaces [interface] random-detect
Syntax Description
Command Mode
EXEC
Usage Guidelines
This command first appeared in Cisco IOS Release 11.1 CC.
Sample Display
The following is sample output from the show interfaces random-detect command:
Router# show interfaces random-detectFastEthernet1/0/0 queue size 0packets output 29692, drops 0WRED: queue average 0weight 1/512Precedence 0: 109 min threshold, 218 max threshold, 1/10 mark weight1 packets output, drops: 0 random, 0 thresholdPrecedence 1: 122 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 2: 135 min threshold, 218 max threshold, 1/10 mark weight14845 packets output, drops: 0 random, 0 thresholdPrecedence 3: 148 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 4: 161 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 5: 174 min threshold, 218 max threshold, 1/10 mark weight(no traffic)Precedence 6: 187 min threshold, 218 max threshold, 1/10 mark weight14846 packets output, drops: 0 random, 0 thresholdPrecedence 7: 200 min threshold, 218 max threshold, 1/10 mark weight(no traffic)describes the fields shown in this display.
Related Commands
random-detect
random-detect exponential-weighting-constant
random-detect precedence
