Cisco Systems 12000/10700 V3.1.1 CAR and WRED Overview

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Cisco 12000/10700 v3.1.1 Router Manager User Guide

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Chapter 11 Layer 3 QoS CAR and WRED Overview

CAR and WRED Overview

Access lists en hance the abilit ies of a CAR poli cy. Fo r ex ample, access lists a llo w yo u to specify c ertain

types of traffic, or certain locations where the traffic is coming from.

CAR is a policing mechanism that allows you to partition your network into multiple priority levels or

classes of service. You set the IP precedence for packets entering the network. Networking devices

(within your ne twork) can then use the conf igured IP prece dence in the packets to deter mine how to treat

the traffic. CAR services, limit the input or output transmission rate on an interface or sub-interface,

based on a flexible set o f criteria. CAR is often config ured on interface s at the edge of a network to limit

traffic in to or out of the ne twork. CAR , can rate limit traffic based on certain matching criteria, such as

incoming interface, incoming and outgoing traffic, IP precedence, or IP access list. You can configure

the actions CAR will take when traffic conforms to or exceeds the rate limit. Each interface can have

multiple CAR pol ic ies, corresponding t o different types of traffic. For example, low priority traffic can

be limited to a lower rate than high priority traffic.

Note In Cisc o 12000/ 10 720 Ro uter M anag er, you can curr en tly appl y on l y one CAR pol icy to a mo dul e.

WRED is a conge stion avoidan ce mech anis m that takes ad vantage of Transmis sio n Contro l Prot ocol

(TCP) conge stion cont rol mec hanism. WRED drops pa ckets sele ctively, prior to pe riods of high

congest ion, based on IP pre ceden ce. Packets with a high er IP preceden ce are less likely to be drop ped

than packets with a lower precedence. WRED is normally used in the core routers of a network, rather

than on the edge. Edge routers assign IP precedence to packets as they enter the network. WRED uses

these precedents to determine how it treats different types of traffic.

Yo u can have a symm et ric as wel l a s a sym met ric da ta ra te for ne twork tra nsmis sions. I n o rder to

smoothen th e data traffic at transmit as well as th e receive end, the W RED QoS parameters can be

configured for the transmitting and receiving traffic. The WRED ToFabric (henceforth referred as

WRED T oF ab, see Towards the F abric (ToF ab) for more details) fe ature enables the user to conf igure the

WRED at the receive side in conjunction with the transmit side.

All WRE D processin g tak es place on the line ca rd, rather than on th e GRP mana gement c ard. No def ault

configurati on values are s uppli ed. You must provide values for a ll c onfigurab le fields. WR ED al so

incorp orates Modi fied Deficit Round Rob in (MDRR ).

You can see from the descriptions where these two mechanisms differ . CAR focuses more on classifying

traffic according to QoS paramet ers, whi le WRED fu nctions to ease network tra ffi c and prior itize

specified traffic.