MTU discovery with Windows > Linux

Soldato
Joined
18 Oct 2002
Posts
5,299
I'm having an odd issue I'm hoping someone can shed some light on.

Site A is a windows box.

Site B is linux.

Connecting both sites are two adsl lines with two vpn routers. When going over the first range 10.10.1.0, to the linux box, all is well and the samba shares are viewable etc.
When going over the second IP range (10.10.2.0, same boxes, same shares) any shares one level below the root are not visible unless the folders are empty. Samba config is about as basic as it gets with no auth.

The only thing that differentiates the two is that the second vpn is using PPPOE passthrough to an RV042.

Pinging site B from A over the first range reveals MTU DF needed @ 1473 bytes.

Ping site B from A over the second range reveals MTU DF needed @ 1413~ bytes which presumably means the MTU is capped @ 1440 along the way?

I've managed to get onto the PPPOE device (Vigor 120) and can see MTU was 1442. I've tried increasing this to 1458, 1460, 1492 and 1500 (bearing in mind max MTU for PPPOE is supposedly 1492) but regardless of what I set, DF is returned on the ping request at the exact same value every time, so it looks like the Vigor is not applying these settings. Thinking about it, the whole point of passthrough is that it synch and passes authentication back, isn't it? MTU is currently 'auto' on the RV042; I've tried forcing 1500 to no avail.

I didn't set this up initially so I'm about to throw it out in favour of a native PPPOA unit but just wondering if anyone had any thoughts... am I barking up the wrong tree here?

Shouldn't PMTU sort this out itself?
 
Update: switched to a PPPOA unit and problem has vanished.

Annoyingly, ping -l -f still shows fragmentation in the exact same place so smugness has been replaced with confusion.
 
MTU stands for "Maximum Transmission Unit" and it denotes the maximum size the Ethernet frame can be including headers, addressing data and footers.

if SiteB is asking for an MTU of ~1413B, then set the MTU to something lower then 1413. 1400 is a good place to start and then work up.
 
I'll add: ensure end-to-end ICMP availability first and foremost would be my recommendation as PMTU uses ICMP to pass messages back and forth.

Best of luck.
 
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