Car and wred overview, Access lists, Committed access rate (car) – Cisco 12000/10700 V3.1.1 User Manual
Page 289: Weighted random early detection (wred)
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Cisco 12000/10700 v3.1.1 Router Manager User Guide
OL-4455-01
Chapter 11 Layer 3 QoS
CAR and WRED Overview
CAR and WRED Overview
Access Lists
Access lists enhance the abilities of a CAR policy. For example, access lists allow you to specify certain
types of traffic, or certain locations where the traffic is coming from.
Committed Access Rate (CAR)
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 network) can then use the configured IP precedence in the packets to determine 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 of criteria. CAR is often configured on interfaces at the edge of a network to limit
traffic into or out of the network. 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 policies, corresponding to different types of traffic. For example, low priority traffic can
be limited to a lower rate than high priority traffic.
Note
In Cisco 12000/10720 Router Manager, you can currently apply only one CAR policy to a module.
Weighted Random Early Detection (WRED)
WRED is a congestion avoidance mechanism that takes advantage of Transmission Control Protocol
(TCP) congestion control mechanism. WRED drops packets selectively, prior to periods of high
congestion, based on IP precedence. Packets with a higher IP precedence are less likely to be dropped
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.
You can have a symmetric as well as asymmetric data rate for network transmissions. In order to
smoothen the data traffic at transmit as well as the receive end, the WRED QoS parameters can be
configured for the transmitting and receiving traffic. The WRED ToFabric (henceforth referred as
WRED ToFab, see
for more details) feature enables the user to configure the
WRED at the receive side in conjunction with the transmit side.
All WRED processing takes place on the line card, rather than on the GRP management card. No default
configuration values are supplied. You must provide values for all configurable fields. WRED also
incorporates Modified Deficit Round Robin (MDRR).
You can see from the descriptions where these two mechanisms differ. CAR focuses more on classifying
traffic according to QoS parameters, while WRED functions to ease network traffic and prioritize
specified traffic.