Nexus 5k MTU size question

Answered Question
May 27th, 2010

I have 2 Nexus 5k's running NX-OS 4.2(1)N1.   The 2 5k's are not directly connected to each other.  These are connected to Nexus 2k's which are in the top of our server racks.   Initially, my higher ups asked to have jumbo frames enabled, so i performed the following:


switch(config)# policy-map type network-qos jumbo<br/>switch(config-pmap-nq)# class type network-qos class-default<br/>switch(config-pmap-c-nq)# mtu 9216<br/>switch(config-pmap-c-nq)# exit<br/>switch(config-pmap-nq)# exit<br/>switch(config)# system qos<br/>switch(config-sys-qos)# service-policy type network-qos jumbo</code></p><p><code><span style="font-family: arial,helvetica,sans-serif;">I guess this enables jumbo frames to the switch as a whole.   They are concerned this will degrade performance being that there will be numerous servers and whatnot that require the normal MTU of 1500.   However, we are wanting at some point to integrate the SAN to the 5k / 2k scenario which requires jumbo frames.   So my question to you is.....Is by applying the jumbo MTU size in the manner above going to hurt the non-san server performance, or should I look at applying jumbo frames on a per interface level.   And if so...how does one go about doing it on a per interface level.   Any info you can give me on this subject, would be fantastic....</span></code></p><p><code><span style="font-family: Arial;">Thanks again in advance. </span>
Correct Answer by Lucien Avramov about 6 years 9 months ago

Correct, so even without enabling Jumbo globally the 5K will process jumbo mtu when those are received.

You will still see it in the show interface ethernet x/y output.

Correct Answer by ryan.lambert about 6 years 9 months ago

I think what he was trying to get at is, there is some concern that things still running at 1500 MTU on that switch would be negatively impacted if this were a global change.


Theoretically I am guessing the answer to that is 'no, it will not negatively impact servers at 1500 MTU' ... unless they had a MTU mismatch with a device they were talking to. I can't 100% confirm that, but the policy as it is applied seems to me to be more of 'support (not require) jumbos globally, business as usual for everything else'. Lucien, perhaps you can clarify?

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Lucien Avramov Thu, 05/27/2010 - 11:52

On the interface level there is not much to do.

N5K will process jumbo frames and not block them.


Now onto your next question coming : how do I disable jumbo since I enabled it?


Here it is:


go to system qos and then:

service-policy type network-qos default-nq-policy

Correct Answer
ryan.lambert Thu, 05/27/2010 - 12:38

I think what he was trying to get at is, there is some concern that things still running at 1500 MTU on that switch would be negatively impacted if this were a global change.


Theoretically I am guessing the answer to that is 'no, it will not negatively impact servers at 1500 MTU' ... unless they had a MTU mismatch with a device they were talking to. I can't 100% confirm that, but the policy as it is applied seems to me to be more of 'support (not require) jumbos globally, business as usual for everything else'. Lucien, perhaps you can clarify?

Correct Answer
Lucien Avramov Thu, 05/27/2010 - 12:41

Correct, so even without enabling Jumbo globally the 5K will process jumbo mtu when those are received.

You will still see it in the show interface ethernet x/y output.

jonesl1 Thu, 05/27/2010 - 13:22

Thank you....yes that is what I was getting at.   I just wanted to make sure I wasn't hindering anything by making

a global change.   Sometimes, its tough for me to word it correctly being that alot of this is over my head.


Thanks Ryan / Lucien

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