QoS – Flashcard
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Give an example of an Integrated Services protocol
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RSVP
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What does RSVP do?
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it reserves bandwidth on all routers of the path, for a specific application to work
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What does Differentiated Services protocols do?
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they apply QoS mechanisms on a per-hop basis
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What are the characteristics of voice traffic in terms of bandwidth?
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deterministic, smooth, benign, sensitive to delay, jitter and packet loss
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What are the characteristics of video traffic in terms of bandwidth?
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deterministic, bursty, greedy
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true/false: QoS can help circumvent problems of severe lack of bandwidth
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false. QoS can be a solution for temporary congestion
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QoS can protect against which issues?
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packet loss, delay and jitter
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What are the four deployment methods of QoS?
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CLI, MQC (Modular QoS CLI), AutoQoS, QPM (QoS Policy Manager)
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Which QoS deployment method integrates witch CiscoWorks?
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QPM
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What's the difference between policing and shaping?
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if a limit is reached, policing drops or re-marks packets, while shaping queues packets
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What are the known QoS tools?
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classification, marking, policing, shaping, congestion avoidance, congestion management, Link efficiency,
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What does classification do?
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inspects one or more fields of a packet in order to group traffic in different classes
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true/false: RED is implemented on Cisco devices by default
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false.
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What's the disadvantage of RED?
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when buffers are filled, packets are randomly dropped
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true/false: RED is implemented on Cisco devices
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False. WRED is.
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With WRED, when all types of traffic have the same weight, what does a Cisco device do?
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it randomly drops packets, when the buffers get filled.
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What are the steps to configure MQC?
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1. configure traffic classes 2. configure Policy map 3. apply Policy map on the interface
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true/false: Only one Policy map is applicable per interface
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false. Only one Policy map is applicable per interface, per direction.
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What's the command to configure a class of traffic?
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(config)#class-map {match-all | match-any} {name}
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What are the possible criteria of matching traffic when creating class maps?
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"match-all" (the default), "match-any"
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What's the command that configures a Policy map?
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(config)#policy-map {name}
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true/false: we must always configure a default class
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false. There's a default class map called "class-default"
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To which boolean operators are "match-any" and "match-all" equivalent to?
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"match-any" = OR statement, "match-all" = AND statement
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Policy maps are not case sensitive
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false
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a policy map can manipulate one or more class maps
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true
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What's the syntax to configure actions for a class map, under a policy map?
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(config)#policy-map Policy-Map1 (config-pmap)#class Class1
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What's the command that applies a policy map to an interface?
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(config-if)#service-policy {input | output} {MyPolicy_Map}
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What's the command that displays the list of class maps?
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show class-map
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What's the command that displays the list of policy maps?
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show policy-map
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What does Marking do?
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writes to a packet according to the strategy that Classification follows
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Classification can be done at which layer?
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2 through 7
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Define the Trust Boundary
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the point in the network at which QoS markings are trusted
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Where should Marking be implemented?
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as close to the source as possible
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true/false: network devices inside a Trust Boundary should reconfigure QoS settings of the incoming packets
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false.
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Where can we summon the power of NBAR?
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inside a class map, with "match protocol..."
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in NBAR, what if there are new applications or new application versions, and you don't have them defined in IOS, under "match protocol" ?
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download NBAR PDLMs from Cisco.com
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How to activate NBAR protocol discovery?
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(config)#ip nbar protocol-discovery
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What are the steps to define new PDLMs in IOS?
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1. download PDLM from Cisco.com 2. put the downloaded PDLMs in router Flash 3. install the PDLM with (config)#ip nbar pdlm {path_of_PDLM_in_Flash}
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Which limitation prevents us sometimes from marking packets close to the source?
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some devices (ip phones and switches) do not support QoS marking
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At which layer can Marking occur?
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layers 2 and 3
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Give examples of Marking at layer 2?
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CoS, MPLS EXP bits, Frame Relay DE bit
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Which tools allow marking at layer 3?
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IP Precedence and DSCP
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What is the difference between class maps and policy maps?
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class maps classify traffic. Policy maps tell what to do with them
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How many values are available for IP Precedence marking?
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8 (0 to 7)
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Define the values of marking. in a eight-class QoS policy?
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0: best-effort 1: medium-priority data 2: high-priority data 3: Call signaling 4: videoconferencing 5: voice bearer 6 and 7: reserved
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Define CoS?
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it's a 3-bit filed in the layer 2 header that allows to have up to 8 classes of traffic
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Define IP Precedence
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It uses the leftmost 3 bits of the ToS field of the layer 3 header, and allows to have up to 8 classes of traffic
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true/false: IP Precedence came after DSCP
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false
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How do you define actions under a policy map?
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(config-pmap-c)#set ...
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How does DSCP segment the ToS byte?
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leftmost 3 bits: the Major class middle 3 bits: Drop Preference 2 rightmost bits: ECN bits
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Why in Cisco routers we have the possibility to define 64 DSCP values or AF/EF values?
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That's for flexibility. It is recommended however, to adhere to the AF/EF naming standard
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List the AF Major classes
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AF1, AF2, AF3, AF4
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List the EF Major classes
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there's only EF.
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Map the DSCP values to the IP Precedence values
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0 -> 0 AF1 -> 1 AF2 -> 2 AF3 -> 3 AF4 -> 4 EF -> 5
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true/false: a DSCP value is composed of a Major class, a Drop Preference and a ECN bit
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false. It is composed of only the Major class and the Drop Preference
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true/false: FIFO gives delay and bandwidth guarantees
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false
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Which queuing method does Priority queuing use?
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Strict Priority
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How many queues are available with Priority Queuing?
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4: High, Normal, Medium and Low
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true/false: Priority Queue provides delay guarantees
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yes only for the high queue
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How many queues are available with Custom Queuing?
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16
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Which queuing method does Custom queuing use?
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Round Robin
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true/false: Custom Queue provides delay guarantees
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false
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What does WFQ do?
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the traffic that consumes the least bandwidth will be prioritized
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How many queues are available with Waighted Fair Queuing?
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one per flow
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How many queues are available with Class-based Waighted Fair Queuing?
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up to 256
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true/false: CBWFQ provides bandwidth guarantees
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yes. We can set percentages of interface bandwidth dedicated to a queue
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CBWFQ is a combination of which queuing mechanisms?
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CQ and WFQ
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Which queuing mechanism is suited for voice traffic?
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LLQ
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LLQ is a combination of which queuing mechanisms?
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PQ and CBWFQ
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What are the recommended LLQ bandwidth provisioning rule?
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- reserve 33% of the interface bandwidth to voice and video - ensure that no more than 75% of the interface bandwidth is allocated (whether it's voice, video or data)
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true/false: the higher the Drop Precedence value, the lower the probability the packet will get dropped (when buffers are full)
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false
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true/false: when we mark packets, we are giving priorities to them
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false. Marked packets are not prioritized. Priority is given to them through the use of queuing on downstream devices
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When queuing is configured under an interface, to which direction is it applied?
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outbound direction only
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What does the following command do? max-reserved-bandwidth
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There's a default bandwidth allocation threshold that we can not exceed when we want to make a bandwidth allocation with "bandwidth" command, under a policy-map. This default value can be changed with the "max-reserved-bandwidth" command
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What does "bandwidth remaining percent X" mean?
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allocate X percent of the remaining bandwidth
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why is it more useful to allocate bandwidth on a percent basis rather on a fixed-value basis?
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because with the fixed-value allocation, each time the interface physical bandwidth changes, we must change the value
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What is the max size of the TCP window?
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unlimited for most OSes
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What is TCP Synchronization?
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it's when network hosts ajust their TCP window size after packet drops
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true/false: What's the size of the UDP window?
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UDP does not have a concept of a window
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true/false: RED works for both TCP and UDP traffic
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false; It works only for TCP
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In Cisco implementation of WRED, how many RED profiles are available?
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8 RED profiles in IP Precedence, 64 profiles in DSCP
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Define the MPD in WRED
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Mark Probability Denominator it's a value that defines how many packets are dropped. e.g 1/100 of packets are dropped
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Define WRED thresholds
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The Minimum threshold: the threshold at which packets become to get dropped. The Maximum threshold: the threshold over which 100% of packets are dropped
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Which command shows the number of marked packets?
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show policy-map interface
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Acronym of ECN?
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Explicit Congestion Notification
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What does ECN add to WRED?
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ECN makes WRED proactive: allows a router to detect a congestion and notify the upstream device. This one notifies the network software to slow down the packet stream
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Which bits of the TOS byte are for ECN?
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rightmost 2 bits
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What are the possible binary values of ECN bits?
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00: device does not support ECN, 01 and 10: device is ECN-compatible, 11: there is a congestion
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What's the purpose of ECN-Echo packet?
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When a downstream router inspects a packet and finds that ECN=11, then it notifies the upstream router with a ECH-Echo packet that there's a congestion
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true/false: the end hosts must have network software that supports ECN
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true
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true/false:ECN is useful between two routers to detect a link congestion?
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false.
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How to activate WRED under a policy map?
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(config-pmap-c)#random-detect
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How to activate ECN?
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(config-pmap-c)#random-detect ecn
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What does the "load-interval 60" do?
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sets NBAR IP protocol discovery interval to 60 sec
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What are the two known tools of Link Efficiency?
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Header compression, LFI
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Which parts of a packet can be compressed?
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payload, header
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What is RTP header compression known as?
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cRTP
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which command activates cRTP?
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(config-pmap-c)#compression header ip rtp
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Which command displays information about cRTP?
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show ip rtp header-compression
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On which L2 WAN technology can LFI be configured?
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on PPP and Frame Relay links only
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what's the serialization delay?
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the time it takes to put a frame into the wire. It depends on the size of the frame and the link speed (configured with "bandwidth" command)
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true/false: LFI is configurable with MQC
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false
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Cisco recommends not to configure LFI for links whose bandwidth is greater than 768kbps
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true
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How to configure LFI for PPP links?
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enable PPP multilink PPP multilink, create a PPP multilink group, create a PPP multilink interface (this is a logical interface), associate both the physical and the PPP multilink interfaces to the same multilink group
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true/false: QoS on routers is done in software. QoS on Catalyst switches is done in hardware
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true. On routers, QoS configuration is processed by the CPU. Watch out for CPU utilization!
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True/false: Policing should not be done as close to the source as possible
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false
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QoS switch capabilities are defined in terms of...?
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transmit queues and thresholds. E.g: 2Q2T, 1P2Q2T
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What types of queues can be configured on Catalyst switches?
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Priority Queue, Weighted Round Robin (WRR), WRR with Priority Queue
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What's the default marking on a Catlayst switch?
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mark frames with CoS=0
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true/false:shaping is good for temporary bursts
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true
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What are the three possible actions with policing?
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conform-action, exceed-action, violate-action
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What does the following command do? police 56000 conform-action transmit exceed-action drop
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- allocate a CIR of 56kbps - if CIR is reached then transmit packets, - if exceeded then drop the packets
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Define Be:
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Excess Burst: the amount of bandwidth we are allowed to have above the Bc
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Define Bc
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Bc: Committed Burst
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What's the difference between legacy shaping and MQC shaping?
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Legacy shaping is done on frame-relay networks on a per-interface basis. MQC shaping is on a per-class basis
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what is MQC shaping also called?
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class-based shaping
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Define LAR
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Local Access Rate LAR: the bandwidth that WAN physical interface can support, ie T1, E1
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Define CIR
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Committed Information Rate CIR: the bandwidth guaranteed
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True/false: LAR <= CIR
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false
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Define Tc
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Tc: Committed Time: it's a slice of a second
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What's the formula that ties CIR to Tc?
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CIR=Tc*Bc
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What are the strategies of traffic shaping?
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average, peak
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What's the command to configure shaping, at CIR = 512kbps?
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(config-pmap-c)#shaping {peak|average} 512
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true/false: with shaping, Cisco recommends to configure Bc and Be
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false. Cisco IOS integrates an IOS that automatically determines and sets Bc and Be
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true/false: it's possible to nest a class-map inside another
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true
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Acronym of SAA
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Service Assurance Agent
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What does IP SLA (previously known as Service Assurance Agent SAA) allow to do?
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active monitoring of network parameters (delay, jitter, packet loss...) and simulates popular network traffic to test QoS settings
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What is SAA also called?
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Router Time Responder
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What are the steps to configure SAA?
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1. configure SAA identifier (config)#rtr 100 2. configure SAA type (config-rtr)#type ... 3. schedule the SAA (config-rtr)"rtr 100 schedule... 4. on the other router, configure SAA reponder (config)#rtr responder
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true/false: we can configure a SAA probe to respond to certain conditions
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true
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what's the command to verify SAA statistics and operations?
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show rtr {...}
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Acronym for AVBO?
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Advanced Voice Busy Out
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What does AVBO do?
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allows proactive monitoring of certain voice parameters, using a SAA probe. If these parameters fall below certain thresholds, AVBO marks the link as "busy out"
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What's the advantage of using AVBO?
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to reroute a voice call on a "non-busy" link and avoid the user to wait 10-15 sec before hearing the ringtone
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true/false: AVBO is configured on the global configuration mode
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false. It's configured under voice ports
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true/false: it's recommended to leave 10% of bandwidth to best effort traffic
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false. It's recommended to leave 25% of bandwidth to best-effort traffic.
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true/false: Serialization delay is a concern for links with bandwidth <= 768kbps
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true
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true/false: cRTP is not efficient for links with bandwidth <=768kbps
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false. It's needed for links with bandwidth <=768kbps
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Define PAK Priority
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it's an internal prioritization mechanism used for routing packets. The router marks routing packets with IP Precedence/CoS value 6
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On which IOS version do we have to tune the TX_Ring value?
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on IOS prior to 12.1
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What's the range of values of TX_Ring?
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3-64 packets
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What's the recommended value of TX_Ring on links <=768kbps?
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3 packets
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Which queuing strategy is applied to TX_Ring?
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FIFO, and only FIFO
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What is TX_Ring?
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it's the hardware queue that receives packets right before transmitting them to the wire
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true/false: in newer IOS releases, TX_Ring value is automatically adjusted
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true
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In which scenarios is auto-qos suitable?
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- small and fast deployments of QoS - large scale deployments where consistency is needed - service providers deploying managed VOIP
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What are the requirements to deploy Auto QoS?
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- the device is a mainstream hardware (eg 2960, 3550,1800,...) - IOS that supports Auto QoS - CEF is enabled - "bandwidth" command is set - interface is not admin down
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Which command activates auto QoS?
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(config-if)#auto qos voip
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What are the commands needed to disable auto QoS?
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(config-if)#no auto qos voip
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Why we classify traffic ?
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to prepare it for special treatment downstream the path
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Why do we mark traffic ?
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to avoid doing classification on traffic a second time, on the next hop
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voice and video applications are sensitive to what?
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voice and video applications are sensitive to delay, jitter and packet loss.
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How can jitter happen?
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The sender is streaming packets at a steady rate, but the processing delay and queueing delay are variable. This creates jitter.
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What are the fixed delays and the variable delays?
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fixed delays: propagation delay and serialization delay; and variable delay: processing delay and queuing delay
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true/false: data traffic is not bursty
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false. It can be steady sometimes and bursty other times.
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Define end-to-end delay.
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End-to-end delay, is the time it takes for a packet to go from a sender to a receiver. It includes all delays across the network path.
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When can packet loss occur?
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Packet loss occurs when one of the following conditions happens: - tail drop, - Input queue is full, - frame errors such as CRC, runts and giants - overrun
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Define the maximum bandwidth on a network path.
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On a network path, the maximum bandwidth is the bandwidth of the slowest link.
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Define the available bandwidth on a network path.
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On a network path, the available bandwidth is calculated with the following formula: maximum bandwidth/number of flows
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What are possible solutions to reduce packet loss?
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increase available bandwidth, use queueing techniques, use shaping techniques
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What are voice traffic and video traffic QoS requirements?
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less than 150ms of one-way delay, less than 30ms of one-way jitter, less than 1% of packet loss
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What are Telepresence traffic QoS requirements?
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less than 150ms of one-way delay, less than 30ms of one-way jitter, less than 0.05% of packet loss
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When planning bandwidth requirements for video traffic, how much bandwidth should we provision for traffic burst?
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provision at least 20% for burst traffic
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What needs to be done before identifying traffic classes?
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- determine which applications run on the network (network audit), with tools such as NBAR, Netflow and Wireshark, - audit the company departments to know which applications they use and consider as most important(business audit) - determine the requirements of each traffic type in terms of bandwidth, delay, jitter and packet loss (the service levels)
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What can be done to ease the task of affecting priorities to certain traffics over others in an organization?
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get the sponsorship of executive stuff.
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Define Qos Policy
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a coporate-wide document that defines classes of traffic, their priorities and their assigned bandwidth amounts.
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true/false: data traffic is more tolerant to delay and packet loss than voice or video traffic.
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true. Packet drops for example are usually not an issue with the TCP retransmission mechanism
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Which tools can be used to monitor QoS?
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IP SLA, Performance Monitor, mediatrace, SNMP
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true/false: with QoS, network engineers have the ability to choose which packets, where and when they can be dropped
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true.
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What are the steps to implement a QoS policy?
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1- determine the traffic types and their requirements 2- put traffic into classes, 3- apply a policy that meets the traffic requirements defined in step 1.
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Classification can be done at which level?
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at the input interface of each qos-aware device of the network path
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Where should marking be done?
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as close to the network edge as possible
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Where are congestion management techniques configured?
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at the output side of an interface
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Where are Policing and Shaping configured?
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- Policing: at both the input and output direction of an interface - Shaping: only at the output direction of an interface
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true/false: RSVP is the only RFC protocol that provides end-to-end bandwidth guarantees.
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true
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true/false: RSVP applies software QoS to the network
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false. RSVP applies hardware QoS to the network.
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What are the two RSVP signaling messages?
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RSVP signaling messages: PATH and RESV
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Which RSVP traffic occurs at the control plane and which occurs at the data plane?
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RSVP signaling and CAC occur on the control plane. All other traffic is treated on the data plane.
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How is data treated in RSVP?
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RSVP flows are put in a priority queue. All other traffic is managed by WFQ if the chosen service type is Guaranteed rate, and WRED if the chosen service type is Controlled load.
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true/false: the bandwidth available for RSVP is equal to the total interface bandwidth.
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false. The bandwidth available for RSVP is not necessarily equal to the total interface bandwidth. It is, by default, equal to 75% of the total interface bandwidth. This can be changed with the command max-reserved-bandwidth
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What are the three service levels provided by RSVP?
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best effort, guaranteed rate and controlled load.
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What is the queuing mechanism for best effort model?
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FIFO
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true/false: RSVP is less scalable than the Best Effort model.
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true.
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True/false: only routers need to be RSVP-capable
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false. All nodes and applications that need to benefit from RSVP must be RSVP-capable.
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true/false: DiffServ does not provide an end-to-end guarantee of QoS
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true.
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What are the four PHBs defined by IETF?
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default, EF (bandwidth guarantee and low delay, excess bandwidth is policed), AF (bandwidth guarantee), class selector.
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In which format can DSCP values be written in binary?
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If we want to write DSCP encodings in binary, they are in the following format: aaadd0, where aaa is the class and dd is the drop probability. Notice that the least significant bit is always 0.
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What are the possible values of aaa in a DSCP value?
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The possible values of aaa are: 000, 001, 010, 011, 100 and 101. If aaa = 000 then we have PHB default. If dd=00 then we have class selector. If aaa= 101, then it is EF PHB. This is the highest user-defined PHB, because aaa= 110 and aaa = 111 correspond to IP Precedence values 6 and 7, which are reserved. Else, we are left with aaa= 001, 010, 011 and 100 in binary, which all correspond to different classes of AF PHB.
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What are the possible values of ddd in a DSCP value?
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The possible values of dd are 00, 01, 10 and 11 in binary, which correspond to decimal values 0, 1, 2 and 3. The higher the value of dd, the higher the drop probability, which means the packet is more likely to be dropped.
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Which type of traffic needs EF PHB?
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Traffic that needs EF PHB is a traffic that requires guaranteed bandwidth and low delays. Packets that belong to this traffic should be marked with DSCP 46 (1011110 in binary ). EF PHB traffic is usually voip traffic.
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True/false: Each PHB is not always associated with a DSCP value or values.
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false. It is. For example, EF PHB is associated with DSCP 46.
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How can we determine the DSCP value that corresponds to a PHB?
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To determine the corresponding DSCP value of a PHB in the form of EF or AFxy, either convert the PHB to binary then convert the binary value to decimal, or do this formula: DSCP value = x*8 + y*2. For example, the DSCP value of AF32 is equal to 3*8 + 2*2 = 28. How to verify it? well, AF32 is in binary 011 10 0 (remember the aaadd0 format), which in decimal equals to 28.
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What are the classes of AF PHB?
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AF PHB is segmented in four AF classes: AF1, AF2, AF3 and AF4
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Define BA
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Behavioural Aggregate: a collection of packets that have the same DSCP value, and traverse a link in the same direction. BA is associated to a PHB. BA is configured in IOS by the use of class maps.
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Define PHB
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Per Hop Behaviour: the QoS treatment that is applied to a BA such as policing, shaping, queueing,... PHB is configured in IOS by the use of policy maps
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On how many bits is IP Precedence written?
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IP Precedence spans the most significant 3 bits
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On how many bits is DSCP written?
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DSCP spans the most significant 6 bits. The remaining 2 bits of the ToS byte are reserved for ECN
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Compare the DS field with the ToS byte
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The DS field defined by RFC occupies ToS byte in the IPv4 packet format. So its length is one byte.
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When do we have a Class Selector PHB?
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when the least significant 3 bits of the DSCP field are set to 0, we have a class selector.
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Define Class Selector Codepoints
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They are the possible values of Class Selector PHB
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true/false: Class Selector codepoints get similar treatment regardless of their orders.
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false. The higher the order of the Class Selector codepoint, the better the timely forwarding is.
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What is the purpose of Class Selector PHB?
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backward compatibility with IP Precedence-based devices.
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What is the format of Class Selector codepoints?
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Class Selector PHB is written as xxx000 in binary
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true/false: for better packet treatment, it is recommended to use Class Selector codepoints of DSCP 48 and DSCP 56.
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False. These codepoints (48 /8 = CS6 and 56 /8 = CS7) are reserved for routing traffic and must not be used for user-defined traffic.
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How de we find the DSCP value of a Class Selector codepoint?
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To convert a Class Selector codepoint to its DSCP value, multiply by 8. For example, the DSCP value of CS2 is 2*8 = 16.
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Why, in networking, do we have Class Selector codepoints?
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The purpose of creating codepoints for Class Selector PHB is to give preferential timely forwarding of packets, according to the order of the codepoint.
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true/false: DiffServ model is highly scalable
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true
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Define Congestion Avoidance
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a mechanism that drops packets based on QoS markings before a congestion occurs
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Define Policing
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a mechanism that drops misbehaving packets to maintain network integrity
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Define Shaping
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a mechanism to control traffic by delaying bursts
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Define Congestion Management
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a mechanism that prioritizes the transmission of packets based on QoS markings.
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What are the three parameters that must be defined for a traffic class, in a QoS policy?
answer
- bandwidth (minimum and maximum) - priority - queuing strategy
