QoS

Question 1

QoS in a given router always occurs based on the ___ label

Answer 1

Top Label

Question 2

Consider an MPLS imposition on a packet with IP prec 5, what should be the value of the exp bits?

Answer 2

5

Question 3

Consider an MPLS imposition on an MPLS frame with exp bits 3, what should be the value of the exp bits of the new label?

Answer 3

3

Question 4

Consider an MPLS swap on an MPLS frame with exp bits 3, what should be the value of the exp bits of the new label?

Answer 4

3

Question 5

Consider an MPLS pop on an IP packet with 2 labels, the outer label with exp bits 3, the inner label with exp bits 4. Will the bits from the outer label be copied to the inner label?

Answer 5

No, there is no reflection in disposition/pop

Question 6

Consider an MPLS pop on an IP packet with 1 label, the transport label with exp bits 3, IP prec of the IP packet 5. Will the bits from the transport label be copied to the TOS field?

Answer 6

No, there is no reflection in disposition/pop

Question 7

What are the 3 models of diffserv tunnels?

Answer 7

Pipe, Short Pipe, Long Pipe

Question 8

diffserv tunnels - Pipe: At the ingress LSR, marking can be done by ToS reflection or manually executed by policy. At the egress LSR, treatment (scheduling and discarding) is based on Exp bits (LSP DiffServ tunnel) and QoS marking is not copied from Label to IP Prec of the IP packet

Answer 8

Pipe

Question 9

diffserv tunnels - Short Pipe: At the ingress LSR, marking can be done by ToS reflection or manually executed by policy. At the egress LSR, treatment (scheduling and discarding) is based on DSCP bits (DiffServ tunneled information) and QoS marking is not copied from Label to IP Prec of the IP packet

Answer 9

Short Pipe

Question 10

diffserv tunnels - Uniform: At the ingress LSR, marking must be done by ToS reflection. At the egress LSR, marking must be copied from Exp bits to DSCP bits. If Exp bits are altered in transit, this change will be reflected to the IP packet

Answer 10

Uniform

Question 11

What are the two supported implementations for DS-TE?

Answer 11

IOS and IOS-XR support both DS-TE Standard and Pre-standard implementations

Question 12

Are the two DS-TE implementations interoperable?

Answer 12

False, they are not interoperable as they use different OSPF extensions

Question 13

Do both DS-TE implementations support MAM and RDM?

Answer 13

False, the standard supports both while the pre-standard only supports RDM

Question 14

CT0 Global Pool and CT1 global subpool

Answer 14

True

Question 15

What is the decimal value of EF?

Answer 15

46

Question 16

Queueing Models: FIFO - First packet in, first packet out. Fair - Resources (bandwidth) are shared equally among the flows. Weighted Fair Queueing (WFQ) - Bandwidth is allocated per flow proportionally to weight (marks).

Answer 16

FIFO, Fair, Weighted Fair Queueing (WFQ)

Question 17

________ - Successor of WFQ, with the ability to create custom Traffic Classification Classes. Based on Weights (Bandwidth). By default, only ___ of interface traffic can be assigned by this queue, the remaining ___ is for routing and control plane protocols. The default packet dropping method is _______, but it can be changed to ______ as well.

Answer 17

Class Based Weighted Fair Queueing (CBWFQ), 75%, 25%, Tail drop, WRED

Question 18

What are the 3 QoS models?

Answer 18

Best Effort, IntServ, DiffServ

Question 19

What are the two least significant bits of the TOS field used for?

Answer 19

Explicit Congestion Notification - ECN

Question 20

IPv6 has a Flow Label field that allows per-flow processing at the IP layer. The field is set by the source host and can be modified by intermediate nodes.

Answer 20

False, it cannot be modified by intermediate nodes

Question 21

_________: Used for best-effort service (bits 5 to 7 of DSCP = 000). ___________: Used for low-delay service (bits 2 to 7 of DSCP = 101110). ___________: Used for guaranteed bandwidth service (bits 5 to 7 of DSCP = 001, 010, 011, or 100). Class-Selector PHB: Used for backward compatibility with non-DiffServ-compliant devices (RFC 1812-compliant devices [bits 2 to 4 of DSCP = 000]).

Answer 21

Default PHB, Expedited Forwarding Per-Hop Behavior (EF PHB), Assured Forwarding Per-Hop Behavior (AF PHB)

Question 22

Cisco IOS XR DS-TE Implementation: Two types of tunnels can be defined - ____ tunnels (constrained via global pool), ____ Tunnels (constrained via subpool bandwidth). The entire global pool is available when a subpool is not used.

Answer 22

Data Tunnels, Voice Tunnels

Question 23

What are the 3 possible operations on a packet in the network?

Answer 23

Expedited, Delayed, Dropped

Question 24

In congestion management, queues are used to prioritize packets of defined classes so they can be forwarded even under congestion conditions. What queues can be used?

Answer 24

LLQ, WFQ

Question 25

To avoid congestion, algorithms that drop packets in the buffer are used. Name algorithms for congestion avoidance

Answer 25

Tail drop, RED, WRED

Question 26

Does tail drop cause the problem of global synchronization?

Answer 26

True

Question 27

___: Market standard, randomly drops packets, benefits only TCP as UDP does not use windowing.

Answer 27

RED

Question 28

___: Cisco’s implementation of RED, same basic operation as RED but with class support.

Answer 28

WRED

Question 29

In WRED, do packets start to be dropped at the maximum threshold?

Answer 29

False, at the minimum threshold

Question 30

In WRED: Below the min threshold there are no drops, between the min and max thresholds drops occur gradually (this is the _____________), after the max threshold drops are total.

Answer 30

MPD (Mark Probability Denominator)

Question 31

Does shaping imply a queue and enough memory to buffer packets, while policy does not?

Answer 31

True

Question 32

What are the methods of implementing QoS?

Answer 32

CLI, Cisco Auto QoS, MQC, Cisco Works QPM - QoS Policy Manager

Question 33

How many bits are there for COS?

Answer 33

3

Question 34

What type of queue distributes packets equally?

Answer 34

Round robin

Question 35

What is the difference between round robin and weighted round robin?

Answer 35

In weighted, prioritization is possible, prioritized class traffic will be transmitted more

Question 36

What is the disadvantage of round robin?

Answer 36

It does not consider packet size, which leads to irregularity

Question 37

How does deficit RR solve the problem of RR?

Answer 37

Keeps track of extra bytes sent by a sender, a kind of credit and debit of byte slots for fairer balancing

Question 38

The difference between RR and Modified Deficit RR is that MDRR supports classes in 2 models: Strict: Empties the “LLQ” first while it has data, subject to starvation. Alternate: Serves the LLQ and other queues with data in round robin but with weights. Operates by keeping track of extra bytes served to a queue to decrement in the next slot. If packets arrive at the LLQ while serving another queue, the scheduler will finish serving the current queue before serving the LLQ, causing delay and jitter.

Answer 38

False, this is the difference between MRR and MDRR

Question 39

________ is the default dropping scheme for CBWFQ, ______ can be used to prevent class congestion.

Answer 39

Tail drop, WRED

Question 40

Which queue is configured with the command ‘bandwidth percent 30’

Answer 40

CBWFQ

Question 41

In Cisco IOS, there are two Priority Queues Q1 and Q2, Q1 is more prioritized.

Answer 41

False, in IOS XR

Question 42

Which queue is configured with the ‘priority’ command?

Answer 42

LLQ