[c-nsp] Are Nexus and per-interface or FEX MTU settings possible?

[John Gill]

That is correct, FCoE is the main driver for class-based MTU setting. 
It is actually a requirement in FCoE.

Regards,
John Gill
cisco

On 9/20/12 1:04 PM, Lars Christensen wrote:
> Hi Phil
>
> The rationale behind doing the L2 MTU on the whole platform is related to the whole QoS architecture and the Data Center Ethernet setup with FCoE and stuff.
>
> Remember, when you use FCoE, you are using a dedicated CoS class (or queue) for the FCoE traffic. As FCoE is requiring a MTU of roughly 2158 bytes with no-drop capabilities, you need to enable this on the whole switch.
>
> An example of a QoS policy on the switch could be:
>
> queue 1 used for CoS 0 (best effort), MTU 1500 bytes and drop
> queue 2 used for CoS 1, MTU 1500 bytes and drop
> queue 3 used for CoS 2, (iSCSI), MTU 9000 bytes and drop
> queue 4 used for CoS 3 (FCoE), MTU 2158 bytes and no-drop
> ...
>
> As you can see, we are using different MTU and drop-preference settings on the queues. As each port on the switch has 8 queues, the easiest way to implement this is by using a global MTU setting.
>
> On the Nexus 7k, we are using a per-port setting on each M-series linecard, but we are using global MTU on the F-series linecard, just as on the 5k series.
>
> There are probably several other rationales behind the design, but this gives you an example.
>
> Lars Christensen
> CCIE #20292
>
>
>
> Den 20/09/2012 kl. 11.17 skrev Phil Mayers <p.mayers at imperial.ac.uk>:
>
>> On 09/20/2012 07:20 AM, Reuben Farrelly wrote:
>>
>>> policy-map type network-qos enable-jumbo-frames
>>>    class type network-qos class-default
>>>      mtu 9216
>>>
>>> system qos
>>>    service-policy type network-qos enable-jumbo-frames
>>>
>>
>> Interesting.
>>
>> Does anyone know the rationale behind this way of setting the MTU on ths platform? It seems a bit peculiar, and as you note, a too-high L2 MTU is seldom harmful, so you would think per-interface (or even per-device) would suffice.
>>
>> FWIW N7k seems to work "normally" i.e. set physical interface & SVI MTU in the interface config.
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