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Re: MTU and Postfix

On Mon, 05 Sep 2011 23:58:29 -0500, Stan Hoeppner wrote:

> On 9/4/2011 5:40 AM, Camaleón wrote:
>> On Wed, 31 Aug 2011 16:09:23 +0000, Camaleón wrote:
>> (...)
>>> I'm still monitoring this but if this is the "cure" to prevent such
>>> errors, are there any expected drawbacks for lowering MTU
>>> "system-wide"?
> Slightly lower overall performance when communicating with remote hosts.
>   Almost zero difference on the LAN.

The host is barely accesible from the LAN (just for admin tasks), it's  
mostly a 100% standalone and remote server which plays the role of a web 
server (very low traffic, it hosts a couple of dedicated sites) and e-
mail services.
> If you want optimal performance, you should enable jumbo frames on all
> LAN hosts (9000 bytes) since you're using GbE, and install an edge
> router that handles jumbo frames.  Then you need not worry about any of
> this.  Just make sure you find out from your service provider what size
> frames they use.  Then program the WAN interface on the router with that
> frame size (MTU).

I prefer to be able to deliver all of the e-mails, performance can go to 

Anyway, I dunno if that would be possible. The switch where the server is 
attached to does not allow jumbo frames so I would have to completely 
diconnect the server from the local LAN and leave it as a purely remote 
server and then yes, I can enable jumbo frames for the gigabit card. But 
now that I know where the problem is and how can mitigate it, I doubt if 
that would be worth of it.

> Problems such as yours are always caused by MTU mismatches.  In most
> cases the mismatch is between the customer's edge device and the service
> provider equipment.  Good routers will handle this just fine as long as
> the WAN port MTU is programmed to match the service provider equipment.

Yep, for what I've read, a badly configured appliance (firewall/IPS) can 
also give such results.
> Worth noting is that different network technologies use different frame
> sizes.  For instance, ethernet uses a 1514 octet frame.  Fiber channel
> uses 2112.  FDDI uses 4500.  SONET is 2430.
> You mentioned a "FTTH gigabit router" previously.  Is this SP equipment
> or your independent equipment?  

As of today, it's ISP's device. FTTH was installed this summer (June-
July) in the office and I'm still reluctant to make any change.

> The MTU mismatch most likely exists inside that box.  If it was
> provided to you, then someone probably didn't program it correctly.  It
> should have worked fine with different MTUs on both sides.

Yep, I also thought so but I couldn't find the manual of the provided 
device, just a very brief flyer with specs. It's a Comtrend WAP-5813n and 
dunno where the MTU setting is on this router nor how/if I have to tweak 



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