Wireless Access

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Contributor II
Posts: 37
Registered: ‎10-27-2011

Client MTU setting

Quick question regarding the MTU setting on client machines.

 

The AP creates a tunnel to the controller, encapsulates all client traffic and sends it to the controller.

 

If the client traffic has a maximum MTU size (1500) and the tunnel traffic is also a maximum size of 1500, won't a single frame of 1500 bytes from the client be fragmented from the AP to the controller since the ap has to add more information for the tunnel?

 

Would I be better served by setting client MTU's to 1400 so that the client data and the tunnel can both fit in under my networks 1500 mtu size and avoid fragmentation?

 

Thanks

 

 

MVP
Posts: 562
Registered: ‎11-28-2011

Re: Client MTU setting

Have you set a specific MTU on the APs? They do send jumbo frames I've noticed. If you need the APs or RAPs to send through a topology that doesn't allow giants/jumbos, that's when you set a lower AP/RAP MTU. If on a LAN, leave the MTUs alone I'd suggest. For instance, I set MTU on RAPs so they can get through Cisco ASAs etc.

 

If you've set an MTU on the AP for a good reason, then what the applications do next matters most.

 

I wouldn't start playing with client MTUs unless you're trying to resolve a linked issue. You'll make probelms for yourself long term.

 

If the MTU on the client is the same as the AP, the application (using TCP probably) will try to window up, but SHOULD back off when it hits around 1400-1480 ish. If it doesn't then you have an issue to act as your troubleshooting catalyst.

 

Cheers.

 

Kudos appreciated, but I'm not hunting! (ACMX 104)
Contributor II
Posts: 37
Registered: ‎10-27-2011

Re: Client MTU setting

Thanks for the explanation.

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