Document ID: 10485
Contents
Introduction
Prerequisites
Requirements
Components Used
Conventions
Configure
Network Diagram
Configuration
Verify
Troubleshoot
Related Information
Introduction
Random Early Discard (RED) is an "intelligent" mechanism which randomly drops packets to avoid link congestion. RED also achieves a better link utilization. These drops occur when the number of packets in the queue are between specific thresholds. These thresholds are configured by the user. Use RED with TCP traffic only (FTP, Telnet, and so on).
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
Per-Virtual Circuit (VC) Distributed Weighted RED (DWRED) is available in Cisco IOS Software Release 11.1(22)CC and Cisco IOS Software Release 12.0(3)T and later.
The information in this document is based on these software and hardware versions:
-
Cisco IOS Software Release 11.1(22)CC or later.
Supported only on Cisco 7500, VIP2-50, and PA-A3 hardware platforms. Distributed Cisco Express Forwarding (DCEF) must be enabled.
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Cisco IOS Software Release 12.0(3)T or later.
Cisco 7500: VIP2-50 and all PA-A3 versions. Only one PA-A3 per VIP2-50.
Cisco 7200: NPE200 or later and all PA-A3 versions.
Note: Distributed mechanisms such as DCEF or DWRED are specific to a 7500/VIP architecture. These mechanisms are not handled by the Route Switch Processor (RSP) CPU. These mechanisms ARE handled by the CPU that exists on the VIP module.
The PA-A3 uses Per-VC queues in hardware and the VIP/7200 shared memory stores a queue per Per-VC queue that exists on the PA-A3. These Per-VC queues on the VIP/7200 only exist if you run one of the Cisco IOS Software Releases mentioned in this section.
When you use Per-VC DWRED, the PA-A3 no longer drops packets. When congestion occurs on a queue, the PA-A3 refuses any packets coming from the IOS on that queue until the congestion stops.
Note: Congestion can occur on a queue because there are no more buffers or because cells exceed the provided contract.
The VIP/shared memory only sends packets to the PA-A3 VC queue if there are buffers available. Discards no longer occur on the PA-A3, but do occur on the VIP/shared memory. It is possible to implement intelligent drop mechanisms on the VIP/shared memory queues (such as WRED) to preserve sensitive traffic.
WRED implements Random Early Discard, but takes into account the IP precedence when frames are dropped. The higher the precedence, the less likely that packets are dropped. The VIP2-50/7200 also implements this drop mechanism independently on each VC queue.
WRED can be configured at the interface level, at the PVC level, and at the class level (class applied to a policy). Since Cisco IOS Software Release12.2(8)T, a new feature WRED Explicit Congestion Notification (ECN) is supported.
When you use WRED, the PA-A3 shapes the traffic according to the contract provided while the drop mechanism is implemented intelligently on the VIP/PCI memory.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Conventions
For more information on document conventions, refer to Cisco Technical Tips Conventions.
Configure
In this section, you are presented with the information to configure the features described in this document.
Note: To find additional information on the commands used in this document, use the Command Lookup Tool (registered customers only) .
Network Diagram
This document uses this network setup:
The 7507 runs Cisco IOS Software Release 12.3(10) and uses a PA-A3 in a VIP2-50. The traffic generator sends two streams of IP frames, one with precedence 3 and the other one with precedence 5. DWRED is configured on the PA-A3 and different WRED parameters are defined for each precedence value.
Note: If you configure the same WRED parameters for each precedence, you are actually using RED and not WRED.
Configuration
|
Sample Configuration |
|---|
!
ip cef distributed
!
class-map match-all twred
!
policy-map twred
class twred
bandwidth percent 60
random-detect
random-detect ecn
!
random-detect-group testWRED
exponential-weighting-constant 2
precedence 3 300 1000 3
precedence 5 2000 4000 5
!
interface ATM2/0/0
ip route-cache distributed
ip route-cache cef
pvc 0/5 qsaal
!
pvc 0/16 ilmi
!
interface ATM2/0/0.116 multipoint
ip address 10.10.10.1 255.255.255.0
no ip directed-broadcast
pvc 1/116
random-detect attach testWRED
protocol ip 10.0.0.2 broadcast
abr 8000 1000
oam-pvc manage
!
interface ATM5/0/0
no ip address
load-interval 30
no atm ilmi-keepalive
pvc 0/5 qsaal
!
pvc 0/16 ilmi
!
!
interface ATM5/0/0.1 point-to-point
ip address 10.60.60.2 255.255.255.0
pvc 0/60
vbr-nrt 3200 3050 420
service-policy out twred
!
|
Note these items for the random-detect-group configuration.
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precedence precedence-value min-threshold max-threshold 1/mark-probability
When the average queue size (in number of packets) that corresponds to the IP precedence value is between the minimum and maximum threshold value, there is a probability of mark-probability that a new packet will be dropped.
If the average queue size is greater than the maximum threshold, a new packet is automatically dropped.
-
average queue size = (old average * (1-1/2^n)) + (current_queue_size * 1/2^n), where n is the exponential-weighting-constant.
Verify
This section provides information you can use to confirm your configuration is working properly.
Certain show commands are supported by the Output Interpreter Tool (registered customers only) , which allows you to view an analysis of show command output.
-
show random-detect-group
7507#show random-detect-group random-detect group testWRED: exponential weight 2 class min-threshold max-threshold mark-probablity ---------------------------------------------------------- 0 - - 1/10 1 - - 1/10 2 - - 1/10 3 300 1000 1/3 4 - - 1/10 5 2000 4000 1/5 6 - - 1/10 7 - - 1/10 rsvp - - 1/10
-
show queuing interface atm
7507#show queuing interface atm 2/0/0.116 Interface ATM2/0/0.116 VC 1/116 Exp-weight-constant: 2 (1/4) Mean queue depth: 1221 Queue size: 0 Maximum available buffers: 2628 Output packets: 6504236 WRED drops: 5713943 No buffer: 2985579 Class Random Tail Minimum Maximum Mark Output drop drop threshold threshold probability Packets 0 0 0 20 40 1/10 0 1 0 0 22 40 1/10 0 2 0 0 100 600 1/2 0 3 222489 5327260 300 1000 1/3 667469 4 0 0 28 40 1/10 0 5 164194 0 2000 4000 1/5 5834704 6 0 0 32 40 1/10 0 7 0 0 34 40 1/10 0
-
show policy-map interface atm
7507#show policy-map interface atm 5/0/0.1 ATM5/0/0.1: VC 0/60 - Service-policy output: twred Class-map: twred (match-all) 315503 packets, 393747744 bytes 30 second offered rate 3234000 bps, drop rate 739000 bps Match: dscp cs3 queue size 47, queue limit 89 packets output 284473, packet drops 34384 tail/random drops 34384, no buffer drops 0, other drops 0 Bandwidth: 60.0%, kbps 1830 Random-detect: Exp-weight-constant: 9 (1/512) Mean queue depth: 44 Class ECN Random Tail Minimum Maximum Mark Output marked drop drop threshold threshold probability packets 0 0 0 0 22 44 1/10 0 1 0 0 0 24 44 1/10 0 2 0 0 0 27 44 1/10 0 3 0 12418 21938 30 44 1/10 238814 4 0 0 0 33 44 1/10 0 5 0 0 0 35 44 1/10 0 6 0 0 0 38 44 1/10 0 7 0 0 0 41 44 1/10 0 Class-map: class-default (match-any) 488 packets, 25848 bytes 30 second offered rate 0 bps, drop rate 0 bps Match: any queue size 0, queue limit 59 packets output 488, packet drops 0 tail/random drops 0, no buffer drops 0, other drops 0 -
show atm pvc
7507#show atm pvc 1/116 VC 1/116 doesn't exist on interface ATM1/0 - cannot display ATM2/0/0.116: VCD: 3, VPI: 1, VCI: 116 ABR, PeakRate: 8000, Minimum Rate: 1000, Initial Rate: 8000, Current Rate: 0 RIF: 16, RDF: 16 FRM cells received: 0, BRM cells received: 0 RM cells sent: 45824 AAL5-LLC/SNAP, etype:0x0, Flags: 0x110020, VCmode: 0x0 OAM frequency: 10 second(s), OAM retry frequency: 1 second(s), OAM retry frequency: 1 second(s) OAM up retry count: 3, OAM down retry count: 5 OAM Loopback status: OAM Received OAM VC state: Verified ILMI VC state: Not Managed VC is managed by OAM. InARP frequency: 15 minutes(s) Transmit priority 3 InPkts: 104, OutPkts: 290804, InBytes: 6192, OutBytes: 70834672 InPRoc: 89, OutPRoc: 0, Broadcasts: 3 InFast: 0, OutFast: 0, InAS: 15, OutAS: 290801 InPktDrops: 14476, OutPktDrops: 0 CrcErrors: 0, SarTimeOuts: 0, OverSizedSDUs: 0 OAM cells received: 9 F5 InEndloop: 9, F5 InSegloop: 0, F5 InAIS: 0, F5 InRDI: 0 F4 InEndloop: 0, F4 InSegloop: 0, F4 InAIS: 0, F4 InRDI: 0 OAM cells sent: 9 F5 OutEndloop: 9, F5 OutSegloop: 0, F5 OutRDI: 0 F4 OutEndloop: 0, F4 OutSegloop: 0, F4 OutRDI: 0 OAM cell drops: 0 Status: UP
The PA-A3 no longer drops packets. When you use DWRED, this process occurs at the Cisco IOS queuing level.
Troubleshoot
There is currently no specific troubleshooting information available for this configuration.
Related Information
- ATM Technology Support Pages
- Configuring Weighted Random Early Detection
- WRED — Explicit Congestion Notification
- Technical Support - Cisco Systems
| Updated: Nov 15, 2007 | Document ID: 10485 |