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As I understood, the main difference between H-QoS and QoS is the level of category granularity, in which we could apply different QoS tools and policies. For example in simple QoS, just differentiation between services is possible(like different priorities for voice, video, date and so on), but when H-QoS comes in, it will be possible to apply QoS policies to different users, VLANs, maybe logical interfaces and so on.

Is that all about H-QoS? or maybe I'm completely wrong?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 10:03
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H-QoS allows you to use nested service-policies. For example, in a three-level hierarchical policy, use the service-policy command to apply:

•Bottom-level policy to a middle-level policy

•Middle-level policy to a top-level policy

•Top-level policy to an interface, subinterface, VC, or VLAN

Hierarchical QoS allows you to specify QoS behavior at multiple policy levels, which provides a high degree of granularity in traffic management. A hierarchical policy is a QoS model that enables you to specify QoS behavior at multiple levels of hierarchy. You can use hierarchical policies to:

•Allow a parent class to shape multiple queues in a child policy

•Apply specific policy map actions on the aggregate traffic

•Apply class-specific policy map actions

•Restrict the maximum bandwidth of a virtual circuit (VC) while allowing policing and marking of traffic classes within the VC.

Cisco doc for ASR

Another presentation about HQoS

  • What do you mean by "level"? Could you please give me an example which shows these 3 level of policies? Why don't we separate different services initially and apply simple QoS policies to each of them? ( instead of nested policies) Actually, I can't understand the benefits of H-QoS over QoS! – A.A Nov 21 '18 at 9:55
  • @A.A check links in a bottom of my answers. They contains simple and complex examples. – Andrey Prokhorov Nov 21 '18 at 9:57
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The main difference is that with HQoS, policies are applied bottom-up in hierarchical order, rather than being applied independently at different levels.

Let's take a simplified two-level example with subscriber-level and VLAN-level, where there are multiple subscribers per VLAN, with data rate limits for each. Suppose all subscriber traffic exceeds the VLAN limit by 2%, but two of the subscribers exceed their subscriber limits by 10%.

If we apply the VLAN limit to all traffic without considering the per-customer limit, we'd drop 2% of all traffic on the VLAN. Yet if we had applied the per-subscriber limit first and only consider non-dropped traffic for the per-VLAN limit, we might end up being below the per-VLAN limit and not have to drop any other customer traffic at all. So, well-behaved subscribers could be punished for the behavior of the oversubscribers, which is not what we want.

With non-hierarchical QoS, you get no guarantees about the order, and in many implementations there would be no order at all: both levels would be applied independently and concurrently, so not only would you end up with lower throughput than the VLAN limit, but the two oversubscribing customers would get less than their traffic limit.

With HQoS, each oversubscribed customer gets 10% dropped, and every customer (including these two) get an equal percentage dropped as necessary to bring the rest of the traffic under the per-VLAN limit.

With an implementation that applies QoS at different levels concurrently, all customers would have 2% of traffic dropped. In addition, the oversubscribed customers would have 10% of their traffic dropped. Some packets would be marked for dropping by both limiters, so the total drops for these customers would be just under 12% (actually 10% + 2 % - (10% * 2%)).

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