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List:       linux-vlan
Subject:    Re: [VLAN] Experts needed (Linux 2.4.16, eepro, mtu)
From:       Stephane LE COZ <caramb () wanadoo ! fr>
Date:       2001-12-12 19:40:42
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Hi,

Thank you Juan Pablo, thank you Holger.

Because I was in a hurry (I need this to work for tomorrow morning), I 
made some new tests and I've finally patch the intel driver this morning 
(I'm a bad guy, shame on me, I made some experiments on a production box :((

First I confirmed the issue (let's do some ascii art ;) :
+---------------+        +------------+                     
+--------------+          +----------+
| Web server |--Ethernet untagged--| Cisco 3548XL |--802.1q--| Firewall |
+------------+                     +--------------+          +----------+

1) Tried to browse the site on the server : the connection stalled
2) Changed the Web server interface MTU to 1400 : browsing is ok
3) Changed the Web server interface MTU to 1496 : browsing is ok (802.1q 
header is 4 bytes long)
4) Changed the Web server interface MTU back to 1500 : browsing stalls

Then manually patched the intel drivers, rebooted the firewall (sic!)
And, houra!, everything is now ok ; no more MTU issue.

Stéphane.

Stephane LE COZ wrote:

> Hi there (from France ;),
>
> I took a look to the 2001 archives of this list but I didn't really 
> find answers to the following questions :
>
> - First, I use a 2.4.16 linux kernel with the "buit-in" vlan support 
> on a 3 Intel eepro equiped Linux firewall. I use the lastest (1.5) 
> vconfig software. This box is connected to a Cisco Catalyst 3548XL-EN 
> switch.
> - I successfully set up 802.1q trunking between the firewall and the 
> switch and I also successfully pinged a web server assigned to a vlan.
>
> - The problem I'm façing is the following : using a telnet command 
> (telnet a.b.c.d 80) to perform a manual GET / request on the web 
> server is ok but when I use a browser (Netscape or IE), the connection 
> stalls... Sounds like some MTU problems. (I checked everything (even 
> sniffed the network), and everything seems to be ok)
>
> - About this definitely famous issue I found no real explaination.
> - Is changing the MTU of my trunk interface on the linux box to 1504 
> with ifconfig a solution to this issue? Any performance drawbacks ?
> - Of course I saw, there is a small patch to the Intel Pro drivers but 
> I wonder about the effect of this patch : Is this patch only corrects 
> some buffer size issue or is it changing the MTU ? Because, as I told, 
> I have 3 Intel interfaces on the firewall and the two others are 
> connected to "untagged" ethernet networks. If I patch the driver, will 
> it be ok on those two normal interfaces ? (I ask instread of testing 
> because the box is production)
>
> Do you confirm this is probably a MTU related issue ?
> What is the best solution ; changing MTU using ifconfig or patching 
> the driver ?
>
> Regards.
> Stephane.
>
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>


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