[j-nsp] MTU problems over VPLS

[Chris Kawchuk]

How does one send back an ICMP please-fragment-this Message when you're emulating a blue wire?

No router in the middle to send back to the customer. it's an L2 service. You're transparent to them IP-wise. No IP interface anywhere inside their bridge to source a packet from.

- Ck.


On 2013-02-12, at 5:57 PM, Luca Salvatore <Luca at ninefold.com> wrote:

> I have a few sites connected via a VPLS core.  The core devices are all MX 10 routers connected via 10Gb fibre.
> I'm having problems doing file copies (SCP between two Centos VMs).
> 
> The issue is that the file copy never gets anywhere, on the Centos CLI it sits at 0% then says 'stalled'
> To fix this issue I have just set the MTU on the Centos machines to be 1400 - when this is in place the copy works and I get nice speeds.
> 
> I don't believe I should have to modify the MTU though, shouldn't path MTU discovery take care of this?
> 
> For example - I have done some TCPdumps, I can see the sender is sending traffic with the DF bit set, however I don't see any 'ICMP fragmentation needed' coming back from the MX saying the MTU is too big, I assume this should be the case.
> 
> I haven't modified any of the MTU's on the MX, everything is just the default.
> 
> I also have normal layer 3 running over the fibre between the routers and when I use that I don't see any issues, so it must be something to do with VPLS.
> 
> Any thoughts would be greatly appreciated.
> 
> Thanks
> Luca.
> 
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