Difference between Modular QoS and ETS

[xzevallos]

I am just starting to study Enhanced Transmission Standard.  It seems fairly simple in theory, but I cannot figure out the difference between configuring modular QoS on a switch port and ETS.  I can allocate guaranteed bandwidth and strict priority queues on different classes on both technologies.  What is the difference?  

[marce1000]

    - FYI : https://community.cisco.com/t5/switching/difference-between-policy-map-bandwidth-and-ets/m-p/4606380/highlight/true#M523435

  M.

[xzevallos]

Unfortunately that explanation is not very clear.  Can you elaborate a bit?  

[Joseph W. Doherty]

More-or-less, ETS supports some L3 QoS techniques upon L2 using .1Q priority tags.

Possibly the real distinction is hardware at the L2 level using complicated L3 QoS usually supported by software.

Possibly helpful might be to compare QoS capabilities of a software based router with a hardware based switch, especially a non-L3 switch and/or go back one or more switch generations.

[xzevallos]

So what would be the deciding factor to use regular modular QoS config versus ETS config on a Nexus switch for example?  

[Joseph W. Doherty]

So what would be the deciding factor to use regular modular QoS config versus ETS config on a Nexus switch for example? 

Capabilities of platform vs. what you want to accomplish.

If given an equal, or even somewhat equal, choice, if you can use L3, use it.

Why?

L3 ToS is end-to-end, while L2 is per L2 domain.

L3 ToS provides 6 bits vs. Ethernet CoS only providing 3 bits.

Ethernet CoS both adds to frame size and might not be recognized by all switches (latter unlikely using Enterprise or DC switches).

Basically, ETS is powered brakes on a horse cart.

[xzevallos]

So if ETS is limited to COS values and I cannot configure it based on DSCP, it seems kind of useless to me.  But there must be some advantage of using ETS that I am not aware of, which is what I am trying to understand.  

[Joseph W. Doherty · Accepted solution]

there must be some advantage of using ETS that I am not aware of, which is what I am trying to understand.

Non-IP.  A possible "poster child", FCoE and IP sharing links.  (Also a good example why you may find such features on a DC oriented switch, like a Nexus, but less likely on an Enterprise switch.)