[c-nsp] newbie questions about jumbo packets

[Tim Stevenson]

Hi Scott, please see inline below:

At 07:09 AM 12/18/2014  Thursday, Scott Voll announced:
>I'm working on my Nexus 5k's. I need to adjust QoS for the first time.  I'm
>following the white papers about input QoS, Network QoS, and Queuing QoS.
>I currently have a very simple network QoS with MTU of 9216 to allow jumbo
>frames on the pair of Nexus (storage).
>
>But now that I'm adjusting QoS I will be classifying my traffic, rather
>than just using the default.

On n5k you use the network-qos (NQ) policy to adjust the MTU for all 
ports in the box. This does not change any other QoS behaviors, 
either internal or external to the box. For example, it does not 
cause the switch to mark/remark packets or classify/queue any 
differently than before.

Note that on nexus in general, there is some level of 'baseline' 
classification for queuing purposes, typically at least a low/high 
priority queue to ensure network control traffic is prioritized. More 
granular configs are possible but out of the box it's pretty basic.


>   What happens if I add jumbo frames to all my
>network classifications and it's not enabled everywhere (other switches
>connected(Ethernet))?


If you enable jumbos on some switches but not all, obviously if a 
jumbo passes to a switch with it disabled, it will be dropped.


>   I Believe on networks where the MTU is smaller it
>will just down size to the standard 1500.  Is that correct?


Most network gear does not fragment, or only fragments in software, 
so this is something to avoid.


Hope that helps,
Tim




>TIA
>
>Scott
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Tim Stevenson, tstevens at cisco.com
Routing & Switching CCIE #5561
Distinguished Engineer, Technical Marketing
Data Center Switching
Cisco - http://www.cisco.com
+1(408)526-6759