QoS Flashcards
The measure of transmission quality and service availability of a network
QoS
Not real-time traffic. Bursty and widely varying packet arrival times.
Data Traffic
Real-time traffic. Constant and predictable bandwidth and packet arrival times. Packets are small but are delay sensitive and should be less than 150 ms.
Voice Traffic
Varied bandwidth requirements, can be real time but not always
Video Traffic
What are the quality issues in converged networks?
Lack of bandwidth End-to end delay Variation of delay (jitter) Packet loss
What QoS mechanism identifies and categorizes traffic into classes?
Classification and marking
QoS mechanism that controls the maximum rate of traffic sent or received on an interface.
Policing
What is the difference between policing and shaping?
Policing drops exceeding traffic
Shaping delays exceeding traffic
What QoS mechanism uses class-based weighted fair queue, which guarantees specific amount of bandwidth to different classes of traffic?
CBWFQ
Often configured on interfaces at the edge of a network to limit traffic and is typically used by service providers to offer customers sublate access
Policing
Bandwidth assigned to a class is the _____ bandwidth allocated to the class during congestion.
minimum
What congestion management or scheduling feature prioritizes voice traffic so it gets to the final destination first
Low-latency queue
QoS feature that allows certain types of connections, such as WAN links, to be provisioned with special traffic handling tools such as fragmentation.
Link-specific tools
A point in the network where packet markings are not necessarily trusted
Trust boundary
The network locations where packet markings are accepted and acted upon
Borders of a trust domain
Devices with user access such as PCs and printers are classified under the _____ domain
untrusted
Part of the network with only network administrator access such as routers and switches.
trusted domains
Tool that inspects packets within a field to identify the type of traffic that the packet is carrying
Classifier
What are the four policy-enforcement mechanisms?
Marking
Queing
Policing
Shaping
Tool that writes a value in the header of a packet, frame, cell, tag, or label
Marker
Layer 2 Ethernet marking that contains three bits to mark the Ethernet frames
Class of Service (CoS)
Describes a 3-bit field in the QoS control field of wireless frames
Traffic Identifier (TID)
Generally used to indicate the Layer 3 packet field. Comprised of 8 bits, 3 of which are designated as the IP precedence field
ToS (Type of Service)
Set of 6-bit values that is used to describe the meaning of ToS. It is backwards compatible with IP precendence.
DSCP (Differentiated services code point)
Where should classification and marking be performed?
Close to end devices
What are the three most common ways to classify traffic?
Markings
Addressing
Application signatures
Router needs to inspect traffic beyond layer 4
Deep packet inspection
Tool that recognizes well known applications
Network Based Application Recognition (NBAR)
How does NBAR recognize packets?
By examine the data payload
Smoothes out traffic and if it exceeds the offered bandwidth, it buffers or delays it.
Shapers
Where should policers be placed to drop traffic as soon as possible so it doesn’t waste resources?
ingress
When traffic exceeds the allotted bandwidth, what can policers do with the traffic?
Drop or re-mark
When congestion occurs, packets get ordered in output buffers
Queuing
Decides which packet should be sent out next and occurs regardless of whether there is congestion
Scheduling
What are the three scheduling mechanisms?
Strict priority
Round-robin
Weighted fair
Queues with lower priority are only served when the higher priority queues are empty. There is a risk that lower priority traffic will never be processed (traffic starvation)
Strict priority
Packets in queues are served in a set sequence
Round-robin
Queues are weighted so that some are served more frequently than others. Solves starvation and also gives priority to real time traffic
Weighted fair
What queueing mechanism guarantees fair bandwidth for traffic classes. No latency guarantees and is only suitable for data networks
Class-based weighted fair queuing
Takes the class-based weighted fair and adds a queue with strict priority for real time traffic
Low-latency queueing
Default queuing response to congestion and drops packets as they arrive
Tail drop
Congestion avoidance mechanism that randomly drops packets before the queue fills up. Randomness is skewed by traffic weights.
WRED
Monitors the buffer depth and performs early discards on random packets.
Random Early Detection
What three features are properties and one-way requirements for voice traffic?
Smooth
Latency below 150ms
30-128 kbps
Jitter <30ms
Where does Cisco recommend that traffic be classified and marked?
As close as possible to the network edge
The trust boundary lies between the _____ interface of the device that will trust the QoS markings and the _____ interface of the device marking the traffic.
ingress
egress
Smooths out temporary bursts of traffic and enables a network interface to more efficiently handle traffic flows whose combined rates approach the maximum bandwidth available on the interface.
shaping
You have enabled QoS on a Cisco router. The router receives marked traffic on an interface that uses the default settings. What will occur?
Traffic will be classified and re-marked without regard to QoS markings
Where does the trust boundary lie?
Between ingress interface of device that will trust the QoS markings and the egress interface of the device marking the traffic
By default, how are ports marked when QoS is first enabled?
untrusted
What do OSI Layer 2 QoS markings such as Ethernet Class of Service (CoS) values do?
Enable devices incapable of accessing Layer 3 header information to make informed QoS decisions
A queuing system has three queues serviced with round robin scheduling and one low latency queue that holds all voice traffic. Round robin queue 1 holds predominantly UDP traffic, while round robin queues 2 and 3 hold predominantly TCP traffic. The packets in each queue happen to have a variety of DSCP markings per the QoS design. In which queues would it make sense to use a congestion avoidance (drop management) tool?
Queues 2 and 3
Drop management relies on the behavior of TCP, in that TCP connections slow down sending packets due to the TCP congestion window calculation. Voice traffic uses UDP, and the question states that queue 1 uses UDP. So, queues 2 and 3 are reasonable candidates for using a congestion management tool.
On networks that utilize Low Latency Queuing (LLQ), _____ is a common problem that can be seen under times of high network load. In these circumstances the routers will always give preference to the voice or video traffic classified into the LLQ, leaving all other traffic to sit idle in a queue.
queue starvation
You want to guarantee that more important traffic will always be sent before less important traffic. What are you most likely to configure?
priority queuing
What are the four basic queuing mechanisms available on most Cisco hardware platforms?
FIFO (First in First out), PQ (Priority Queuing), Custom queuing (CQ), weighted fair
What type of queuing mechanism does the packet exit a queue in the same order that they entered?
FIFO
What is the downside of using FIFO?
Can introduce considerable amounts of delay and jitter because of the inherent lack of prioritization.
What queuing is generally used by default on Cisco devices?
FIFO
What protocol is used to mitigate VoIP quality problems and can reserve bandwidth along an entire path?
RSVP (Resource Reservation Protocol)
Where is traffic policing typically implemented?
ingress traffic to limit the amount of processing for traffic that will inevitably be discarded
Implemented on egress traffic to maintain service level agreements (SLAs) and optimize available bandwidth without exceeding a configured committed access rate (CAR).
Traffic shaping
Which statement about QoS default behavior is true? - Ports are untrusted by default
- VoIP traffic is passed without being tagged
- Video trafic is passed with well-known DSCP value of 46
- Packets are classified internally with an environment
- Packets that arrive with a tag are untagged at the edge of administrative domain
Packets that arrive with a tag are untagged at the edge of administrative domain
Which option describes the purpose of traffic policing? It prioritizes routing protocol traffic It remarks traffic that is below the CIR It drops traffic that exceeds the CIR It queues and then transmits traffic that exceeds the CIR
It drops traffic that exceeds the CIR
Which statement about QoS default behavior is true?
A. Ports are untrusted by default.
B. VoIP traffic is passed without being tagged.
C. Video traffic is passed with a well-known DSCP value of 46.
D. Packets are classified internally with an environment.
E. Packets that arrive with a tag are untagged at the edge of an administrative domain.
E. Packets that arrive with a tag are untagged at the edge of an administrative domain.
