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CCIE R&S - Written - QoS

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What are the three steps of configuring the QOS MQC?   class-map, policy-map, service-policy  
What are the two functions of NBAR?   protocol analysis and traffic classification for QOS  
What is the name of NBAR definitions?   PDLMs  
What benefit does NBAR have over access-lists?   Deep packet inspection to the application layer  
Command to enable NBAR monitoring?   ip nbar protocol-discovery  
Layer 2 QoS marking? How many bits?   CoS, 3  
Layer 3 QoS markings? How many bits?   ToS - IP Precedence (First 3 bits of byte) and DSCP (First 6 bits of byte)  
MPLS QoS marking? How many bits?   Experimental Bits or Traffic Class, 3  
Frame-Relay QoS marking? How many bits?   DE, 1  
ATM QoS marking? How many bits?   CLP, 1  
What is the CoS 0 Application?   Best Effort (Routine)  
What is the CoS 01Application?   Medium Priority Data (Priority)  
What is the CoS 2 Application?   High Priority Data (Immediate)  
What is the CoS 3 Application?   Call Signaling (Flash)  
What is the CoS 4 Application?   Video (Flash-override)  
What is the CoS 5 Application?   Voice Bearer (Critical)  
What is the CoS 6 Application?   Reserved (Internet)  
What is the CoS 7 Application?   Reserved (Network)  
What are the last two bits of the IP ToS byte used for?   Flow Control  
What is the DSCP Class Selector PHB used for?   Pure backward compatibility with IP Precedence  
What is the DSCP Default PHB and bit value?   Best effort service (000 000)  
What is the DSCP Assured Forwarding PHB and initial bit values?   Guaranteed bandwidth services - 001, 010, 011, or 100  
Describe the DSCP AF drop probabilities.   010 - Low probability, 100 - Medium probability, 110 - High probability  
What is the DSCP EF PHB and bit value?   Voice, 101 110  
How do the drop probability values differ between AF and EF?   In AF, 110 means high probability of drop - In EF, 110 means don't drop  
In what three ways can QoS Policy Propagation through BGP classify packets?   Access lists, BGP Community lists, and BGP AS Paths  
What problem does QoS Priority Queuing introduce?   Starvation  
What problem does QoS Custom Queuing introduce?   Latency for voice  
What problem does Weighted Fair Queuing introduce?   Can't accommodate bandwidth reservations  
What is the default behavior for full queues in CBWFQ?   Tail Drop  
What is the default processing behavior in each CBWFQ queue?   FIFO  
What three types of weighting can be used in CBWFQ?   Bandwidth, Bandwidth Percent, Remaining Bandwidth Percent  
How many types of weighting can you use in one QoS service policy?   1  
What percentage of interface bandwidth should not be assigned by CBWFQ?   25%  
How is Low Latency Queuing different from CBWFQ?   It adds a priority queue  
How does Low Latency Queuing stop the priority queue from starving other queues?   the prioritized bandwidth is policed  
What is QoS Modified Deficit Round Robin?   Deficit Round Robin with a priority queue added  
What are the two modes of Modified Deficient Round Robin and how do they work?   Alternate - there will never be starvation, Strict priority - services the priority queue no matter what  
This congestion avoidance technique prevents global synchronization?   Random Early Detection (RED)  
What is the problem with Random Early Detection (RED)?   No way to differentiate what traffic to drop  
What is the default Weight Random Early Detection denominator?   10  
Explain the WRED min threshold.   Specifies at what level to start random drops  
Explain the WRED max threshold   Specifies at what level to start full drop  
Explain the WRED max drop probability.   Specifies what percentage of packets will be dropped (1/N)  
Command to enable WRED and in what MQC mode?   random-detect, policy configuration mode  
What is the default ToS type that WRED uses to determine a traffic profile?   IP Precedence  
Explain 3 ways that policing and shaping are different?   Shaping can only be outbound, Shaping buffers excess traffic, Policing drops or re-marks traffic  
What is the policing token formula?   CIR = Bc / Tc  
What is Tc?   timing interval  
What is Bc?   normal burst size  
What are the three states of a Dual-Token, Single Rate Bucket?   Conforming, Exceeding, Violating  
What two rates are enforced on a Dual-Rate Token Bucket?   CIR and PIR  
What interfaces are supported for AutoQoS?   Physical Serial (HDLC and PPP), Frame-Relay P2P subinterfaces, and ATM P2P subinterfaces  
What is the interface speed limit for ATM subinterfaces?   768k  
What is a requirement of AutoQoS for slow speed links?   It must be configured on both sides  
What is a requirement of AutoQoS for an ATM PVC?   a bit rate has to be set  
What 5 signaling protocols is AutoQoS built to identify?   H.323, H.225 (Unicast Only), SIP, 'Skinny', and MGCP  
"Where does the command ""auto qos voip"" go in a frame-relay environment?"   in dlci config mode  
What are the two phases of AutoQoS for Enterprise?   Discovery Phase, Installation Phase  
How is AutoQoS similar to AutoQoS for Enterprise?   Same set of restrictions  
Command to enable Phase 1 of AutoQoS for Enterprise?   auto discovery qos  
Command to enable Phase 2 of AutoQoS for Enterprise   auto qos  
What is the limit of CoS, IP Precedence, and DSCP values that can be listed on a single match command inside the MQC?   CoS: 4, IP Precedence: 4, DSCP: 8  
In what two ways can the bandwidth be set for a priority queue in the MQC?   Bandwidth, Bandwidth Percent  
What does the priority command's option burst value default to?   20% of configured priority bandwidth  
What is the requirement in a QoS service policy to classify or mark based on CoS?   The interface must be a 802.1q trunk  
If a QoS policy map defines more than 100% of the link's bandwidth, what action does IOS take when the command is typed in?   It rejects the command  
What is required to perform QoS marking?   CEF enabled  
Where should QoS marking be done?   As close to the ingress network edge as possible by a trusted device  
What happens to packets that don't match a defined QoS class?   They are put in the class-default class  
What is the limit of number of services classes that Cisco Recommends?   4 or 5  
What action is taken on packets that are in an MQC policy with no 'set' command?   They are not marked  
What is the restriction of classification and marking for non-IP headers?   They can only be classified inbound and marked outbound  
What is the requirement for Layer 2 Ethernet marking and classification?   802.1q or ISL trunks  
What field are the QoS values stored in on a 802.1 tag? Size of this field?   Tag Control Field, 2 bytes  
What are the 3 QoS bits of the Tag Control field called? Which bits are they?   user-priority bits, 3 most significant bits  
What field are the QoS values stored in on a ISL header? Size of this field?   User Field, 1 byte  
What are the 3 QoS bits of the User field called? Which bits are they?   CoS (Class of Service), 3 least significant bits  
What command is used to change interface statistics more frequently for better analysis? What is the default value? Lowest value?   load-interval, default is 50 minutes, lowest value is 30 seconds  
What is QoS pre-classification?   Classification done on VPN endpoint routers to make egress decisions based on original traffic before encryption  
How does QoS preclassification work?   Router stores unencrypted traffic in memory until egress QoS actions are taken  
What three places can the 'qos pre-classify' command be applied?   tunnel interface, virtual-template, or crypto map config modes  
How can traffic be marked in PBR?   Using the set command to mark IP Precedence or entire ToS byte  
What are two other names for the hardware queue?   Transmit ring or transmit queue  
What does IOS do to better utilize the queuing tool present?   Shrink the length of the hardware queue  
Each class in the policy map defines a _____________.   single queue  
How many queues does CBWFQ and LLQ support?   64  
What types of queuing can be used inside of the class-default queue?   FIFO or WFQ  
In CBWFQ, what happens if some queues are empty?   Unused bandwidth is shared proportionally across all other classes  
How is CBWFQ bandwidth remaining percent calculated?   max reserved bandwidth (percentage) times interface bandwidth  
What is the 'max-reserved-bandwidth' default value?   75  
In QoS, what constitutes a 'time of congestion'?   Hardware queue is full  
How does LLQ treat multiple priority queues?   Grouped into a single LLQ with packets serviced according time of arrival  
What is different with bandwidth command in LLQ vs. CBWFQ?   LLQ can use both bandwidth and bandwidth percent in the same policy map. CBWFQ can only use one of the three available.  
What is a restriction of the class-default queue?   It cannot be a priority queue  
What is the queue structure of Modified Deficit Round Robin (MDRR)?   seven round-robin queues (0-6) plus a priority queue (7)  
What is the downside of using the alternate mode in MDRR?   jitter and latency for priority queue  
How does alternate mode MDRR service all queues when they are full?   Services priority queue in between every other queue (0, P, 1, P, 2, P…)  
What is the MDRR Quantum Value (QV)?   Number that specifies how many bytes can be sent per queue, per round  
What happens if more bytes are sent than allowed in MDRR?   That queue has a deficit that will be added to the QV for the next round  
How many switch ingress queues are there?   2  
How many switch ingress queues can be configured as priority queues?   1  
What does switch ingress queuing using to set discarded thresholds for each queue?   Weight Tail Drop (WTD)  
How many thresholds are created on switch ingress queuing?   3  
Which switch ingress queues are configurable?   1 and 2  
What is the behavior of switch ingress queue Threshold 3?   It will drop traffic when the queue gets to 100%  
What method is used to control the packet send rate from ingress queue to internal switch fabric?   Shared Round-Robin (SRR)  
What do the relative weights represent in SRR?   Ratio of bandwidth sharing  
In what mode are the switch ingress queuing commands applied?   Global configuration mode  
How can traffic be mapped for QoS in switch ingress queuing?   CoS or DSCP value  
Which queue is CoS 5 traffic placed into by default in switch ingress queuing?   Queue 2  
Which queue is non CoS 5 traffic placed into by default in switch ingress queuing?   Queue 1  
What does the command 'mls qos srr-queue input priority-queue (queue-id) bandwidth (weight)' accomplish? What does the 'weight' variable specify?   enables priority queuing, weight specifies link bandwidth percentage to use  
What does the command 'mls qos srr-queue input buffers (percentage1) (percentage2)' accomplish? What are the variables' default values?   Specifies how to divide ingress buffer to the two queues, default is 90% to queue 1 and 10% to queue 2  
What does the command 'mls qos srr-queue input bandwidth (weight1) (weight2)' accomplish? What are the variables' default values?   Specifies frequency of packets being taken from two buffers, Default values are 4 and 4  
How many switch egress queues are there?   4  
What is the restriction on a priority egress queue?   Must be queue 1  
When a priority egress queue is configured, how are bandwidth commands pertaining to that queue in the share or shape commands treated?   Ignored  
How does switch egress queuing determine which four queues to place packets in?   Frames DSCP value compared to DSCP-to-CoS map, then that is compared to CoS-to-queue map  
Is the DSCP-to-CoS map on a global or interface level?   global  
Is the CoS-to-queue map on a global or interface level?   interface  
What are the two options for switch egress scheduler?   Shared, Shaped  
What is the default weights of the four switch egress queues?   25 percent each  
What is the difference between shared and shaped mode for egress queuing?   Shared mode only polices during congestion, shaped polices at all times  
Can a 3560 have shared and shaped mode on the same interface?   Yes  
What can be configured for each egress queue-set?   buffer and WTD thresholds  
How many queue-sets are there for switch egress queuing?   2  
How are reservations made in RSVP?   On a flow-by-flow basis  
How is an RSVP flow defined?   Destination IP address, protocol ID, and destination port  
How many reservations are made for one host communication with another?   2  
What type of RSVP message is sent from the source Router towards the destination?   PATH  
What does a RSVP PATH message contain?   Previous Hop information, bandwidth, and QoS needs  
What does a non-source, non-destination, RSVP-aware router do when it receives a PATH message?   Change PHOP and forward  
What does the last router do when it receives the RSVP PATH message?   Replies with a RESV message to source router and initiates PATH message to source router  
What does a non-source, non-destination, RSVP-aware router do when it receives a RSVP RESV message and cannot fulfill the requirements?   Send error message to originator and drops RESV packet  
How does the source router know that reservations have been made in both directions?   It receives a RESV CONF message  
What does RSVP use to be able to dynamically adjust to network changes?   periodic refresh messages  
What values are uses if these variables are not set? 'ip rsvp bandwidth (total-kbps) (single-flow-kbps)'   total-kbps = 75% of interface bandwidth, single-flow-kbps = any size flow  
What QoS method does RSVP use by default? How to change this behavior?   WFQ, ip rsvp resource-provider none  
How does the command 'ip rsvp data-packet classification none' change RSVP behavior   It prevents RSVP from trying to classify every packet  
What considerations need to be taken when setting the RSVP bandwidth using LLQ?   priority queue minus Layer 2 overhead  
What is the average queue depth in WRED?   The measurement compared with minimum and maximum thresholds to determine when packets should be dropped  
When the average queue depth is above the max threshold, what happens?   Full Drop  
How is full drop different from tail drop?   full drop can discard packets even when the queue is not full  
What three places can WRED be configured?   physical interface with FIFO, non-LLQ class in CBWFQ policy map, and ATM Virtual Circuit  
This WRED value is the calculation of the rolling average queue depth.   exponential weighting constant  
What does a low Exponential Weighting Constant mean?   Old average is a small part of the average calculation, this makes the average change more quickly  
When does AutoQoS trust the values it receives?   When the link is a uplink port, trunk port, or Cisco phone  
What does a switch do if the access port is not connected to a Cisco phone?   Mark all traffic down to DSCP 0  
If the switch trusts QoS markings, which 5 traffic types are placed into the priority queue?   Voice and Video control, Video, Voice, routing protocol, and STP traffic  
When a switch trusts the QoS markings which traffic goes into the normal input and output queues?   Input: non Voice and Video control, non Video, no Voice, non routing protocol, and non STP traffic; Output: non Voice traffic  
What is enabled on links with speeds 768k or less for AutoQoS?   Compression and Fragmentation  
In AutoQoS, what will happen to non-frame relay serial interfaces that are 768k and less? Why?   Their encapsulation will be changes to PPP, to enable link fragmentation and interleaving  
If the 'trust' keyword is used, what action does the router take on that AutoQoS interface?   Class maps are created based on DSCP values  
If the 'trust' keyword is omitted, what action does the router take on that AutoQoS interface?   Access lists are created to match voice, video, and call control ports  
What technology does CB Policing replace?   CAR  
What 7 actions can be taken in CB policing?   transmit, set dscp and transmit, set precedence and transmit, set qos and transmit, set clp and transmit, set frde and transmit, drop  
What does 1 token represent in policing?   1 byte  
How often does a policer refill a bucket?   Whenever a packet arrives  
How does a policer refill the bucket?   Prorated number of tokens [(CurrPktArvTime - PrevPktArvTime) * PolRate] / 8  
How are dual-rate tokens bucket filled and drained?   Bc filled based on CIR, Be filled based on PIR, tokens drained from both bucks for packets conforming to CIR  
What configuration constitutes three colors in CB Policing?   violate action or Be higher than 0  
What configuration constitutes dual rate?   CIR and PIR configured  
What command is used to get into multi-action policing (sub configuration) mode?   omit action parameters from the 'police' command  
In what two ways can a policing rate be set?   bps or bandwidth percentage  
What is the only flavor of policing that CAR supports?   Singe rate, two color  
What are the 4 main differences that CAR has in comparison to CB Policing?   non-MQC 'rate-limit' command, can used nested (cascading) rate-limit commands, supports Be but not violate action, different internal logic to determine if a packet conforms or exceeds  
What is the purpose of using nested (or cascading) rate-limit commands in CAR?   Allows multiple commands to process the same packet  
What are the two ways to classify traffic in CAR?   Normal IP ACLs or Rate-Limit ACLs  
What can a CAR Rate-Limit ACL match?   MPLS Exp Bits, IPP, or MAC address  
What are the three typical causes of QoS problems?   No proper planning, outdated configuration, no use of documentation  
What is egress blocking?   SP frame-relay or ATM switch has to queue data before it can send it out the other end of the VC  
What is the difference between the terms CIR and shaping rate?   CIR is the contractual rate, shaping rate is the configured rate  
What does 1 token represent in shaping?   1 bit  
When is the token bucket filled in traffic shaping?   At the beginning of every Tc interval  
In the single token bucket model, what happens to extra tokens that do not fit in the bucket?   They are wasted  
In the single token bucket model, how is Be implemented?   The bucket capacity is enlarged  
When congestion occurs, the shaper can lower the shaping rate until __________________ or _______________.   It reaches the minimum information rate or congestion stops  
How is adaptation triggered in traffic shaping?   Router receives a frame with the BECN bit set or a Cisco ForeSight congestion message  
Each time the router receives a congestion notification, how does the shaping rate change?   Bc and Be decreased by 25% of maximum  
When does the shaping rate grow after being slowed down due to congestion messages?   Grows after 16 consecutive Tc values without a congestion message by 1/16 until maximum rate reached  
In Generic Traffic Shaping (GTS) what is the only value required?   shaped-rate  
In Generic Traffic Shaping (GTS), if Bc or Be is not required, what are their values?   Both set to 1/4 of the shape-rate  
What command is added in GTS to configured adaptive shaping on a Frame Relay link?   traffic-shape adaptive bit-rate  
What method of shaping allows the creation of a settings group to be applied on multiple interfaces?   Class-Based Shaping  
What two ways can a shaping rate be specified?   bps or bandwidth percentage  
What is the default shaping rate for a subinterface with no bandwidth command?   1544  
In Class-Based Shaping, how is Bc and Be measured if configured?   Number of milliseconds  
If shape rate <= 320 kbps in MQC, what are the values for Bc, Be, and Tc?   Bc = Be = 8000 bits, Tc = Bc / shape rate  
If shape rate > 320 kbps in MQC, what are the values for Bc, Be, and Tc?   Tc = 25 ms, Bc = Be = shaping rate * Tc  
What type of queuing does software queues default to?   WFQ  
What type of queuing does shaping queuing default to?   FIFO  
What is the benefit of small Tc values?   Low latency  
What does shaping become active? Inactive?   When a single packet exceeds the policy, Inactive when all shaping queues are empty  
What can FRTS learn?   CIR, Tc, Bc, Be from frame-relay switch  
What restriction does FRTS have on a physical interface?   Can't apply special queuing tools with FRTS  
What is structure is used to assign a shaping policy in FRTS?   Map Class  
What is the preference of assigning a map class to an interface in FRTS?   First looks under the interface DLCI. If not there, looks under the sub-interface. If not there, it looks under the physical interface. If not there, it applies the defaults  
If a map class is not found on an interface, what default values are applied?   Shaping rate = 56 kbps, Bc = 7000 bits, Tc = 125 ms  
How are frame-relay values explicitly set in FRTS? Why is this used?   under the 'map-class frame-relay NAME' command, used to achieve small Tc  
In a non map-class configuration with FRTS, what is the value of Tc?   125 ms  
In a non map-class configuration with FRTS, what is needed to use a Be?   peak rate must be configured  
What must be used in FRTS to enable adaptive sharing?   map-class with commands frame-relay adaptive shaping {becn | foresight}  
What is the default mincir rate for adaptive sharing in FRTS?   50% of shaping rate  
What must be created to use the MQC with FRTS?   default-class  
If FRTS and fragmentation are both applied to a PVC with the MQC, what type of queue will be used?   dual FIFO queue, 1 for voice and control traffic, the other for everything else  
When using nested policy maps in FRTS, what is the total rate available to the child?   Parent shaping rate  
If both the shape average and shape adaptive command is configured in the MQC for FRTS, which bandwidth is used?   shape adaptive bandwidth  
What replaces the 'frame-relay ip rtp priority' command for FRTS in the MQC?   LLQ  


   





 
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