Exam 300-420 topic 1 question 102 discussion

to correctly understand this question, firstly eliminate answer which are definitely no correct, A & D; then look at answer B &C, hierarchical QoS, which is a method to put 2 policy-map nested together, the parent policy-map control the overall channel bandwidth granted, the child policy-map control actual bandwidth allocate to each type of service. in the question, the service provicer grant CIR of 50Mbps which is allocated to parent policy-map and the voice traffic is allocated in child policy-map. As 50Mbps CIR is granted by ISP so (B) policing on parent policy is correct. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_plcshp/configuration/xe-16/qos-plcshp-xe-16-book/qos-plcshp-hier-clr-plc.html https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k_r4-2/qos/configuration/guide/qc42hqos.html

The answer is C. Always SHAPE on egress towards a service provider in the parent policy, then use child policies to set priorities as you see fit (in this case VoiP). Generally, shaping = egress (i.e. towards ISP), policing = ingress (this is how ISPs drop traffic that exceed the CIR you are paying them for). Sources: https://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-policing/19645-policevsshape.html#anc17

It's a sub line rate circuit. You need to shape that or it'll be messy. Shaping is a hard requirement, not policing. You'll attach a child policy with a strict priority queue. That queue technically is policed, but you need a shaper as the parent at the very minimum.

For this specific scenario, the only way to guarantee the ISP will not drop the voice traffic is to make sure not to exceed the CIR from the customer side. This can only be guaranteed by policing out. All comments about shaping is correct but for this particular question.....policing.

because it's voice.

Egress = Shaping , ingress Policing is the basic principle. To add, following this discussion, https://community.cisco.com/t5/routing/hierarchical-qos/td-p/456350 , I'm even more convinced the answer is C

we will use shaping not policing

I think that C is the correct answer. The engineer can configure Shaping on Egress interface towards ISP. ISP is Policing on Ingress interface, the engineer has no access nor can configure ISP devices. Anyone agrees?

Pull up any hierarchal QOS example from Cisco on an ethernet interface an you will see it is the shape command for the parent policy.

Oh men after analyzing this question i just realized something, they want us to set this QoS on our Router, not the ISP router, and then it clicked, Shaping is using on Egress Interfaces and Policing is configured on Ingress Interfaces, in this case Shaping will be setup on our Router, but we can't do policing because we are not managing traffic into our network, we are managing packets out of our router (Egress) for the voice going out. This is Shaping. Makes sense

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_plcshp/configuration/xe-16/qos-plcshp-xe-16-book/qos-plcshp-oview.html#:~:text=in%20the%20bucket.-,Traffic%20Policing,the%20traffic%20to%20the%20speed%20of%20the%20interface%20receiving%20the%20packet.,-Was%20this%20Document

B is correct. Whats the difference between Policing and Shaping? -If there is an exceeding burst of traffic Policing will cut off the exceeding traffic -If there is an exceeding burst of traffic Shaping will put the traffic into a que and try to send out the packets once the congestion is gone. It is silly to start Shaping (aka.: "Queuing") Voice traffic since it would be all over the place Also for QoS we are usually doing "LLQ" to prioritize Voice traffic, but that would mean voice traffic could totally overtake the whole bandwitdh > Thatswhy we put a "hard policing" at a certain bandwith on policing (not to take over the whole bandwitdth) Configuration snippet for example: https://networklessons.com/quality-of-service/policing-configuration-example

C is correct. Never use Policing on Voice