Re: Transporting QinQ across a Layer 2 link locked at 1518 octets AND across a Layer 3 link

[Francois Menard <francois () menards ca>]

Oops two typos - sunday evening casualties.

On 2010-09-12, at 10:06 PM, Francois Menard wrote:

> Folks,
>
> Question #1:
>
> Is it possible for me to put an MPLS router on both ends of a circuit leased from a transport service provider which 
> does not support QinQ (i.e. packets of 1526 bytes), and which requires us to tag traffic onto a well specified set of 
> VLANs (i.e. if we want two VLANs, the service provider tells us which two VLANs to use).
>
> I was thinking of lowering the MTU size on my MPLS router such that I could do QinQ over VPLS, over MPLS, over 
> Ethernet transport locked at @ 1514 octets.
>
> I would imagine that my effective IPv4 payload would be reduced to something like (not taking into account CRC 
> removed in the Ethernet driver)
>
> 1514 minus 8 bytes for MPLS label minus 4 bytes for VPLS control word minus 4 bytes for VLAN tag #1 minus 4 bytes for 
> VLAN tag #2 = 
> 1514 -8 -4 -4 -4 -4 = 1490 octets
>
> So if I set my MTU on my MPLS router at 1490 octets and send QinQ over VPLS over , wouldn't that allow for all of the 
> above mentioned overhead to pile-up without exceeding the 1514 octets size allowed by my transport provider ?

So if I set my MTU on my MPLS router at 1490 octets and send QinQ over VPLS over MPLS over Ethernet, wouldn't that 
allow for all of the above mentioned overhead to pile-up without exceeding the 1514 octets size allowed by my transport 
provider ?

> Question #2:
>
> I have another link, which is restricted by the transport service provider, which is an MPLS-VPN service, and which 
> does not support QinQ, nor supports layer 2 bridging.
>
> An option available to me is to use an EoIP tunnel on a Mikrotik RouterOS router, which maps Ethernet over GRE over 
> IP, causing some 28 octets of overhead. This is proprietary to Mikrotik.
>
> So in this case, assuming that I want to do something as dangerous as:
>
> QinQ over VPLS over MPLS over Ethernet over EoIP (over GRE, over IP)
>
> And accordingly set my MTU to:
>
> 1480 (from above) minus 28 octets (Ethernet over GRE over IP) = 1452 octets 

1490 (from above) minus 28 octets (Ethernet over GRE over IP) = 1462 octets

> So if I set my MTU on my MPLS router at 1452 octets, wouldn't that allow for all of the above mentioned overhead to 
> be successfully transported across an IP layer 3 circuit, effectively ending up with

1462 octets

> QinQ over MPLS over Ethernet over IP ?
>
> What are the consequences of setting the MTU as low as 1452 octets?  What applications end-up breaking ?

1462 octets

> -=Francois=-