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This chapter describes the tasks for configuring Weighted Random Early Detection (WRED), VIP-Distributed WRED (DWRED), and flow-based WRED on a router.
For complete conceptual information, see the section "Weighted Random Early Detection" in the chapter "Congestion Avoidance Overview" in this book.
For a complete description of the WRED and DWRED commands in this chapter, refer to the Cisco IOS Quality of Service Solutions Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index, or search online.
The RSVP-ATM QoS Interworking and IP to ATM Class of Service features also use WRED. For information on how to configure these features with WRED, see the chapters "Configuring RSVP-ATM QoS Interworking" and "Configuring IP to ATM Class of Service" in this book.
The WRED feature is supported on the following Cisco router platforms:
The DWRED feature is only supported on Cisco 7000 series routers with an RSP-based RSP7000 interface processor and Cisco 7500 series routers with a Versatile Interface Processor-based VIP2-40 or greater interface processor. A VIP2-50 interface processor is strongly recommended when the aggregate line rate of the port adapters on the VIP is greater than DS3. A VIP2-50 interface processor is required for OC-3 rates.
To use DWRED, distributed Cisco Express Forwarding (dCEF) switching must first be enabled on the interface. For more information on dCEF, refer to the Cisco IOS Switching Services Configuration Guide and the Cisco IOS Switching Services Command Reference.
Random Early Detection (RED) is a congestion avoidance mechanism that 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. WRED drops packets selectively based on IP Precedence. Edge routers assign IP Precedences to packets as they enter the network. 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 at the edge. WRED uses these precedences to determine how it treats different types of traffic.
When a packet arrives, the following events occur:
1. The average queue size is calculated.
2. If the average is less than the minimum queue threshold, the arriving packet is queued.
3. If the average 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.
4. If the average queue size is greater than the maximum threshold, the packet is dropped.
See the section "About WRED" in the chapter "Congestion Avoidance Overview" in this book for more details on the queue calculations and how WRED works.
To configure WRED or DWRED on an interface, perform the tasks in the following sections. The first section is required; the remaining sections are optional.
To use DWRED, dCEF switching must first be enabled on the interface.
See the end of this chapter for the section "WRED and DWRED Configuration Examples."
To enable WRED, use the following command in interface configuration mode:
Command | Purpose |
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rando m-detect |
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You need not specify any other commands or parameters in order to configure WRED on the interface. WRED will use the default parameter values.
To change WRED parameters, use one of the following commands in interface configuration mode:
Command | Purpose |
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random-detect exponential-weighting-constant exponent |
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random-detect precedence precedence min-threshold max-threshold mark-prob-denominator |
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When you enable WRED with the random-detect interface configuration 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 half of the maximum threshold. The values for the remaining precedences fall between half the maximum threshold and the maximum threshold at evenly spaced intervals.
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Note The default WRED parameter values are based on the best available data. We recommend that you do not change the parameters from their default values unless you have determined that your applications will benefit from the changed values. |
To monitor WRED and DWRED services in your network, use one or more of the following commands in EXEC mode:
To configure flow-based WRED on an interface, perform the tasks in the following section:
See the end of this chapter for the section "Flow-Based WRED Configuration Example."
Before you can configure flow-based WRED, you must enable WRED and configure it. For information on how to configure WRED, see the section "Weighted Random Early Detection Configuration Task List" in this chapter.
To configure an interface for flow-based WRED, use the following commands in interface configuration mode:
| Command | Purpose | |
|---|---|---|
Step 1 | random-detect flow | |
Step 2 | random-detect flow average-depth-factor scaling-factor | |
Step 3 | random-detect flow count number |
The following sections provide WRED and DWRED configuration examples:
The following example enables WRED or DWRED with default parameter values:
interface Serial5/0 description to qos1-75a ip address 200.200.14.250 255.255.255.252 random-detect
Use the show interfaces command output to verify the configuration. Notice that the "Queueing strategy" report lists "random early detection (RED)."
router# show interfaces serial 5/0
Serial5/0 is up, line protocol is up
Hardware is M4T
Description: to qos1-75a
Internet address is 200.200.14.250/30
MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 237/255
Encapsulation HDLC, crc 16, loopback not set
Keepalive not set
Last input 00:00:15, output 00:00:00, output hang never
Last clearing of "show interface" counters 00:05:08
Input queue: 0/75/0 (size/max/drops); Total output drops: 1036
Queueing strategy: random early detection(RED)
5 minutes input rate 0 bits/sec, 2 packets/sec
5 minutes output rate 119000 bits/sec, 126 packets/sec
594 packets input, 37115 bytes, 0 no buffer
Received 5 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
37525 packets output, 4428684 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out
0 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up
Use the show queue command output to view the current contents of the interface queue. Notice that there is only a single queue into which packets from all IP precedences are placed after dropping has taken place. The output has been truncated to show only three of the five packets.
router# show queue serial 5/0
Output queue for Serial5/0 is 5/0 Packet 1, linktype: ip, length: 118, flags: 0x288 source: 190.1.3.4, destination: 190.1.2.2, id: 0x0001, ttl: 254, TOS: 128 prot: 17, source port 11111, destination port 22222 data: 0x2B67 0x56CE 0x005E 0xE89A 0xCBA9 0x8765 0x4321 0x0FED 0xCBA9 0x8765 0x4321 0x0FED 0xCBA9 0x8765 Packet 2, linktype: ip, length: 118, flags: 0x288 source: 190.1.3.5, destination: 190.1.2.2, id: 0x0001, ttl: 254, TOS: 160 prot: 17, source port 11111, destination port 22222 data: 0x2B67 0x56CE 0x005E 0xE89A 0xCBA9 0x8765 0x4321 0x0FED 0xCBA9 0x8765 0x4321 0x0FED 0xCBA9 0x8765 Packet 3, linktype: ip, length: 118, flags: 0x280 source: 190.1.3.6, destination: 190.1.2.2, id: 0x0001, ttl: 254, TOS: 192 prot: 17, source port 11111, destination port 22222 data: 0x2B67 0x56CE 0x005E 0xE89A 0xCBA9 0x8765 0x4321 0x0FED 0xCBA9 0x8765 0x4321 0x0FED 0xCBA9 0x8765
Use the show queueing command output to view the current settings for each of the precedences. Also notice that the default minimum thresholds are spaced evenly between half and the entire maximum threshold. Thresholds are specified in terms of packet count.
router# show queueing
Current random-detect configuration:
Serial5/0
Queueing strategy:random early detection (WRED)
Exp-weight-constant:9 (1/512)
Mean queue depth:28
Class Random Tail Minimum Maximum Mark
drop drop threshold threshold probability
0 330 0 20 40 1/10
1 267 0 22 40 1/10
2 217 0 24 40 1/10
3 156 0 26 40 1/10
4 61 0 28 40 1/10
5 6 0 31 40 1/10
6 0 0 33 40 1/10
7 0 0 35 40 1/10
rsvp 0 0 37 40 1/10
The following example specifies the same parameters for each IP Precedence. Thus, all IP precedences receive the same treatment. Start by enabling DWRED.
interface FastEthernet1/0/0 ip address 200.200.14.250 255.255.255.252 random-detect
Next, enter the show queueing random-detect command to determine reasonable values to use for the precedence-specific parameters:
router# show queueing random-detect
Current random-detect configuration:
FastEthernet2/0/0
Queueing strategy:fifo
Packet drop strategy:VIP-based random early detection (DWRED)
Exp-weight-constant:9 (1/512)
Mean queue depth:0
Queue size:0 Maximum available buffers:6308
Output packets:5 WRED drops:0 No buffer:0
Class Random Tail Minimum Maximum Mark Output
drop drop threshold threshold probability Packets
0 0 0 109 218 1/10 5
1 0 0 122 218 1/10 0
2 0 0 135 218 1/10 0
3 0 0 148 218 1/10 0
4 0 0 161 218 1/10 0
5 0 0 174 218 1/10 0
6 0 0 187 218 1/10 0
7 0 0 200 218 1/10 0
Complete the configuration by assigning the same parameter values to each precedence. Use the values obtained from the show queueing random-detect command output to choose reasonable parameter values.
interface FastEthernet1/0/0 random-detect precedence 0 100 218 10 random-detect precedence 1 100 218 10 random-detect precedence 2 100 218 10 random-detect precedence 3 100 218 10 random-detect precedence 4 100 218 10 random-detect precedence 5 100 218 10 random-detect precedence 6 100 218 10 random-detect precedence 7 100 218 10
The following example enables WRED on the interface and specifies parameters for the different IP precedences:
interface Hssi0/0/0 description 45Mbps to R1 ip address 10.200.14.250 255.255.255.252 random-detect random-detect precedence 0 32 256 100 random-detect precedence 1 64 256 100 random-detect precedence 2 96 256 100 random-detect precedence 3 120 256 100 random-detect precedence 4 140 256 100 random-detect precedence 5 170 256 100 random-detect precedence 6 290 256 100 random-detect precedence 7 210 256 100 random-detect precedence rsvp 230 256 100
The following example enables WRED on the Serial1 interface and configures flow-based WRED. The random-detect interface configuration command is used to enable WRED. Once WRED is enabled, the random-detect flow command is used to enable flow-based WRED.
After flow-based WRED is turned on, the random-detect flow average-depth-factor command is used to set the scaling factor to 8 and the random-detect flow count command is used to set the flow count to 16. The scaling factor is used to scale the number of buffers available per flow and to determine the number of packets allowed in the output queue for each active flow.
configure terminal interface Serial1 random-detect random-detect flow random-detect flow average-depth-factor 8 random-detect flow count 16 end
The following part of the example shows a sample configuration file after the above flow-based WRED commands are issued:
Router# more system:running-config Building configuration... Current configuration: ! version 12.0 service timestamps debug datetime msec localtime service timestamps log uptime no service password-encryption service tcp-small-servers ! no logging console enable password lab ! clock timezone PST -8 clock summer-time PDT recurring ip subnet-zero no ip domain-lookup ! interface Ethernet0 no ip address no ip directed-broadcast no ip mroute-cache shutdown ! interface Serial0 no ip address no ip directed-broadcast no ip mroute-cache no keepalive shutdown ! interface Serial1 ip address 190.1.2.1 255.255.255.0 no ip directed-broadcast load-interval 30 no keepalive random-detect random-detect flow random-detect flow count 16 random-detect flow average-depth-factor 8 ! router igrp 8 network 190.1.0.0 ! ip classless no ip http server ! line con 0 transport input none line 1 16 transport input all line aux 0 transport input all line vty 0 4 password lab login ! end
Posted: Mon Aug 21 21:39:59 PDT 2000
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