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Occasional Contributor II

MTU modification on 7210 Controller

Hello,

 

I have a 7210 Mobility Controller with an AP-115 directly connected on it. I created 2 SSIDs. I modified the MTU (SAP MTU) in the AP-System-Profile to 1200. But once connected to a SSID, I'm not able to ping the controller from my Device with a packet size from 1200 Bytes and the "don't fragment" bit active.

Only a packet size from 932 Bytes allows me to ping the controller successfully without fragmenting the packet.

 

Do I have to modify another value? How could such a difference be explained?

 

Thank you for responding,

Kind regards,

Nicolas.

MVP

Re: MTU modification on 7210 Controller

did you check / are you sure the low mtu isn't caused by another device in the path?

Koen (ACMX #351 | ACDX #547 | ACCP)

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Re: MTU modification on 7210 Controller

Only 11ac APs support jumbo frames:

 

http://www.arubanetworks.com/techdocs/ArubaOS_64x_WebHelp/Content/ArubaFrameStyles/Network_Parameters/Jumbo_Frame_Support.htm

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Occasional Contributor II

Re: MTU modification on 7210 Controller

Hi Koen,

 

No the AP is directly connected to the controller via a standard switch with default MTU 1500 Bytes.

Is the MTU set in AP-System-Profile set for all SSIDs? The maximal packet-size without fragmentation should be (MTU - GRE Header) right?

 

Re: MTU modification on 7210 Controller

Sorry, I completely misread the original post ..

Is the SAP MTU blank? It should be by default. If so, expect an MTU of at
least 1500 bytes.

You can confirm MTU with the following command: show AP debug system-status
ap-name XXXX. Copy/paste into notepad and search for MTU.
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Occasional Contributor II

Re: MTU modification on 7210 Controller

Hello,

 

Thanks for this command, I will test it tomorrow and send the feedback.

 

Regards,

Nicolas.

Occasional Contributor II

Re: MTU modification on 7210 Controller

Hi,

 

I made more tests on my Lab. SAP MTU ist set to 1200 in the AP-System-Profile.

The output from "show ap debug system-status ap-name AP01 shows me that the Switch MTU is 1200, and the AP interfaces bond0 and br0 MTU is 1500.

As a client connected to one SSID, I can ping with a maximal packet-size from 1472 without fragmentation.

On my Controller, I can ping the AP IP-Address with a maximal packet size from 932 without fragmentation (CPsec enabled).

 

I'm a little bit confused about how the system should work. I think that I should not be able to ping with a packet-size from 1472 using my laptop is that correct?

Re: MTU modification on 7210 Controller

Well, unfortunately I can't reproduce your exact problem.  Setting the SAP MTU on the AP is not affecting wifi client pings when DF bit set.  At 1200 byte SAP MTU, I'm able to ping up to 1472 bytes.  Same for pings from the controller to the AP.

 

Do you have a need for setting the MTU to 1200?  Are your APs traversing an IPSec tunnel?  If not, I'd remove the SAP MTU.

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