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Cisco QoS FAQ: Traffic Policing and Shaping

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Cisco QoS FAQ: Traffic Policing and Shaping

Q1. Explain the points during the process of a single router receiving and forwarding traffic at which shaping and policing can be enabled on a router.

Answer: Shaping can be enabled for packets exiting an interface, subinterface, or individual VC. Policing can be performed both on packets entering an interface or exiting an interface.

Q2. Compare and contrast the actions that shaping and policing take when a packet exceeds a traffic contract.

Answer: Shaping queues packets when the shaping rate is exceeded. Policing either discards the packet, just transmits the packet, or it re-marks a QoS field before transmitting the packet.

Q3. Compare and contrast the effects that shaping and policing have on bandwidth, delay, jitter, and loss.

Answer: Shaping places packets into queues when the actual traffic rate exceeds the traffic contract, which causes more delay, and more jitter. Policing when making a simple decision to either discard or forward each packet causes more packet loss, but less delay and jitter for the packets that do make it through the network.

Q4. Describe the typical locations to enable shaping and policing in an internetwork.

Answer: Shaping is typically performed before sending packets into a network that is under some other administrative control. For instance, shaping is typically performed before sending packets from an enterprise into a service provider’s Frame Relay network. Policing, although supported as both an input and output function, is typically performed at ingress points, once again at the edge between two administrative domains.

Q5. Describe the reasons behind egress blocking in a Frame Relay network with a T1 access link at the main site, 128-kbps access links at each of 20 remote sites, with 64-kbps CIR VCs from the main site to each remote site.

Answer: Egress blocking can occur for frames leaving the Frame Relay network going to the main site, because the sum of the access rates of the 20 sites exceeds the access rate at the main site. Egress blocking occurs for packets leaving the Frame Relay network going to an individual remote site, because the access rate at the main site exceeds the access rate at each remote site.

Q6. If a router has a shaping tool configured, with a shaping rate of 256 kbps, and a Bc of 16,000 bits, what Tc value does the shaping tool use?

Answer: Because Tc = Bc/CIR, the answer is 16,000/256,000, or 62.5 ms.
If a router has a shaping tool configured, with a shaping rate of 512 kbps, and a Be of 16,000 bits, what Tc value does the shaping tool use?
Tc is not calculated based on Be, so you cannot know the answer with this limited amount of information. The formula to calculate Tc is as follows: Tc = Bc/CIR, where CIR is the shaping rate. If you know Bc and CIR, you can calculate the Tc value.

Q7. Define the terms Tc, Bc, Be, and CIR.

Answer: Tc: Time interval, measured in milliseconds, over which the committed burst (Bc) can be sent.
Bc: committed burst size, measured in bits. This is the amount of traffic that can be sent during every interval Tc. Typically also defined in the traffic contract.
Be: Excess burst size, in bits. This is the number of bits beyond Bc that can be sent in the first Tc after a period of inactivity.
CIR: committed information rate, in bits per second, defines the amount of bandwidth that the provider has agree to provide as defined in the traffic contract.

Q8. Explain the goal of the Da and Dc debt processes when using CAR.

Answer: CAR attempts to reduce congestion by getting the senders of the traffic to slow down, similar to WRED.

Q9. Describe the concept of traffic-shaping adaption, explaining the two triggers that cause shaping to adapt.

Answer: Adaption causes the shaper to reduce the shaping rate during congestion. Shaping can react to frames with the BECN bit set, or to Cisco Foresight congestion messages.

Q10. Describe the difference between interface output queues and shaping queues, and explain where the queues could exist on a router with 1 physical interface and 20 subinterfaces.

Answer: Output queues exist on the physical interface, and can be controlled with queuing tools such as CBWFQ, CAR, and WFQ. Shaping queues exist when traffic shaping is enabled; the shaping queue is associated with the particular instance of shaping. If shaping has been enabled on 20 subinterfaces on a single physical interface, for instance, 20 sets of shaping queues exist, all feeding into the single set of physical interface output queues. Traffic Shaping

Q11. What do the following intialisms stand for? FRTS, GTS, DTS, and CB shaping

Answer: Frame Relay traffic shaping, generic traffic shaping, distributed traffic shaping, and class-based shaping.

Q12. List the command, with the correct syntax, that sets a shaped rate of 128 kbps, a Bc of 8000, and a Be of 8000, when using GTS. Do not assume any defaults; explicitly set the values in the command.

Answer: traffic-shape rate 128000 8000 8000

Q13. Along with the class-map, policy-map, and service-policy commands, CB shaping requires one specific command that actually sets values used for the shaping function. List the command, with the correct syntax, that sets a shaped rate of 128 kbps, a Bc of 8000, and a Be of 8000, when using CB shaping. Do not assume any defaults; explicitly set the values in the command.

Answer: shape 128000 8000 8000

Q14. Along with the class-map, policy-map, and service-policy commands, DTS requires one specific command that actually sets values used for the shaping function. List the command, with the correct syntax, that sets a shaped rate of 128 kbps, a Bc of 8000, and a Be of 8000, when using DTS. Do not assume any defaults; explicitly set the values in the command.

Answer: shape 128000 8000 8000

Q15. Many commands are needed to configure FRTS, but the actual shaping parameters, such as CIR and Bc, are set under the map-class frame-relay command. List the map-class subcommands, with the correct syntax, that sets a shaped rate of 128 kbps, a Bc of 8000, and a Be of 8000, when using FRTS, and using multiple commands. Do not assume any defaults; explicitly set the values in the commands.

Answer:frame-relay bc 8000frame-relay be 8000frame-relay cir 128000

16 Compare and contrast the use of the class-map and map-class commands in terms of how each is used by FRTS and CB shaping.

Answer: FRTS uses map-class to create a common set of shaping parameters, which are then applied to an interface, subinterface, or VC. FRTS does not use the class-map command for shaping. Likewise, CB shaping does not use the map-class command. It does, however, use the class-map command for defining classification options. Interestingly, FRTS can use the class-map command indirectly, when using CBWFQ or LLQ. (Example 5-9 shows a sample configuration.)

Q17. Describe the context inside the configuration mode under which the service-policy command can be used to enable LLQ on an FRTS shaping queue. (“Context” means what part of configuration mode—for instance, global-configuration mode, interface configuration mode, and so on.)

Answer: You can use the service-policy command inside map-class configuration mode.

Q18. Explain the context inside the configuration mode under which the service-policy command can be used to enable LLQ on a CB shaping queue. (“Context” means what part of configuration mode—for instance, global-configuration mode, interface configuration mode, and so on.)

Answer: CB shaping requires a policy map, with class commands inside the policy map. Inside class configuration mode inside the CB shaping policy map, the service-policy command can refer to another policy map, which could enable LLQ for the class.

Q19. Explain the context inside the configuration mode under which the service-policy command can be used to enable LLQ on a GTS shaping queue. (“Context” means what part of configuration mode—for instance, global-configuration mode, interface configuration mode, and so on.)

Figure: Sample Network Used for GTS Configuration Examples

cisco-qos-faq-traffic-policing-shaping

Answer: GTS does not support LLQ as one of its shaping queues.

Q20. GTS has been configured under subinterface s0/0.1. What show command lists statistics for GTS behavior just for that subinterface?

Answer: show traffic-shape statistics interface s0/0.1

21. DTS has been configured under subinterface s0/0.1. What show command lists statistics for DTS behavior just for that subinterface?

Answer: show policy-map interface s0/0.1

Q22. CB shaping has been configured under subinterface s0/0.1. What show command lists statistics for CB shaping behavior just for that subinterface?

Answer: show policy-map interface s0/0.1

Q23. FRTS has been configured under subinterface s0/0.1. What show command lists statistics for FRTS behavior just for that subinterface?

Answer: show traffic-shape statistics interface s0/0.1

Q24. Which of the traffic-shaping tools can be enabled on PPP interfaces?

Answer: GTS, DTS, and CB shaping.

Q25. Which of the traffic-shaping tools can be enabled on each VC on a Frame Relay multipoint subinterface?

Answer: FRTS.

Q26. Which of the traffic-shaping tools support adaptive shaping?

Answer: FRTS, GTS, DTS, and CB shaping.

Q27. For which of the traffic-shaping tools does IOS perform shaping processing on 7500 VIP cards?

Answer: DTS.

28 Which of the traffic-shaping tools can classify traffic to shape subsets of the traffic on a subinterface?

Answer: GTS, DTS, and CB shaping.

Q29. Which shaping tools do not support WFQ and PQ to be used for the shaping queues?

Answer: GTS, CB shaping, DTS.

Q30 Which shaping tools do not allow WFQ or LLQ to be used for the interface output queuing at the same time as the shaping tool is enabled on the same interface?

Answer: FRTS

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