Re: [lvs-users] Apparent MTU problem using LVS-DR and Windows 2003 RealS

To: " users mailing list." <lvs-users@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [lvs-users] Apparent MTU problem using LVS-DR and Windows 2003 RealServers
From: Joseph Mack NA3T <jmack@xxxxxxxx>
Date: Tue, 15 Sep 2009 18:27:40 -0700 (PDT)
On Tue, 15 Sep 2009, Christopher Smith wrote:

> I have a somewhat weird (at least to me) problem with an 
> LVS-DR setup that has some Windows 2003 RealServers.
> Firstly, this whole setup is VMs running inside an ESXi 
> 4.0 host, in case that sets of anyone's alarm bells up 
> front.

Well, there is a standard networking problem with at least 
one VM but I would have hoped it would be fixed by now. Have 
a look in the HOWTO before bothering with what may be 
nonsense below.

> Then it just continues with the 'need to frag' messages indefinitely.

does the otherwise normal TCP service encapsulate packets?

> I had a bit of a look around on Google and the list 
> archives, but all the postings I could find were referring 
> to using LVS-TUN, not LVS-DR.

> Has anyone seen this problem before ?  I'm assuming it has 
> something to do with the larger data transfers of the 
> DICOM association needing packets to fragment, but the 
> smaller HTTP requests do not, but surely that shouldn't be 
> a problem with all hosts on the same vlan ?

The LVS-Tun MTU problem is solved, at least for LVS (but not 
for Linux networking in general). You could just try it 
blind - ie without any understanding of the problem - 
reducing the MTU using the same methods as for LVS-Tun.


Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at
Homepage It's GNU/Linux!

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