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

Mon Sep 13 02:06:47 UTC 2010

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 ?

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 

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

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 ?

-=Francois=-