CCIE R&S Written Exam Overview: QoS

Quality of Service (QoS)

QoS Overview

• QoS Components

– Classification & Marking

– Queueing & Congestion Management

– Shaping & Policing

Classification & Marking

• Differentiated Services (DiffServ) Model

– Traffic is grouped into classes

• Classification process defined at network edge

• Classification can be encoded inside packet itself

– Known as packet’s marking

– QoS behavior is defined by traffic’s class

• Called Per-Hop Behavior (PHB)

QoS Markings

• Defines classification of the packet

• Different types of markings

– IPv4 & IPv6 ToS Byte

• DSCP (6 bits)

• IP Precedence (3 bits)

– Layer 2 Markings

• Frame-Relay DE bit (1 bit)

• MPLS EXP bits (3 bits)

• 802.1q/ISL CoS bits (3 bits)

DSCP Default & EF PHBs

• Default

– Best Effort (Worst)

– DSCP value 0 (000000)

• Expedited Forwarding (EF) (Best)

– Priority Traffic

– DSCP value 46 (101110)

DSCP AF PHB

• Assured Forwarding (AF)

– Bandwidth Guaranteed

– Four classes

• AFxy where x = 1 – 4

• Higher is more preferred

– Three drop precedences

• AFxy where y = 1 – 3

• Higher means higher drop precedence

– DSCP value (xxxyy0)

DSCP CS PHB

• Class Selector (CS)

– Backwards compatible with IP Precedence

– Seven classes

• CSx where x = 1 – 7

• Higher is more preferred

Queueing & Congestion Management

• Queueing occurs when packets are delayed by router

• Congestion Management techniques define what happens when queueing occurs

• Techniques include…

– FIFO Queueing

– Fair Queueing & WFQ

– Priority & Low Latency Queueing

– Random Early Detection & WRED

• Technically “congestion avoidance”

– Shaped & Shared Round Robin

Queueing Techniques (Outbound)

• FIFO Queueing

– First packet to enter is first to leave

• Fair Queueing

– Shares resource equally amongst flows

• Weighted Fair Queueing (WFQ)

– Allocates bandwidth per flow proportional to weight

Queueing Techniques (Outbound)

• Class Based Weighted Fair Queueing (CBWFQ)

– WFQ class and weighting are manually defined

– Used to provide bandwidth guarantee to traffic

• Priority Queueing (LLQ)

– Packets marked as priority leave the queue first

– Used to provide low delay to traffic

• Weighted Random Early Detection (WRED)

– Drops packets randomly before queue is full

– Higher a packet’s weighting, less likely it is to be dropped

– TCP Windowing can be modified to slow down conversation

Shaping & Policing

• Used to force congestion to occur

• Traffic Shaping

– Goal is to normalize traffic flow to specified rate

– Delay and Queue exceeding traffic

– Can only apply outbound

• Traffic Policing

– Marks packets that exceed metered rate

• Drop is also a mark action

– Does not queue excess traffic

– Can be applied inbound or outbound

MQC

• Modular Quality of Service Command Line Interface

– Syntax implementation of QoS

– Allows multiple QoS methods per interface per direction and for hierarchy

– “Legacy” QoS did not

• Previously called CBWFQ

– Class Based Weighted Fair Queueing

– Now called Hierarchical Queueing Framework (HQF) as of IOS 12.4(20)T

MQC Configuration

• Define traffic classes

– class-map

– Define traffic match criteria

• Define traffic policy

– policy-map

– Define actions

• Apply policy

– service-policy [in/out] on interface

MQC Classification Options

• Match-Any vs. Match-All

• Access-Lists

• DSCP/IP Precedence

• NBAR

• Source Interface

• Source/Destination MAC address

• Can combine multiple matches in one class

MQC QoS Types

• Admission Control

– Policing

• Classification and Marking/Re-Marking

– Shaping

• Congestion Management & Avoidance

– Queueing Disciplines

• Link Optimization

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