MPLS/VPLS + MTU/L2MTU = confused

RouterOS Forwarding Protocols
1

So I know that there are a million posts about this, but no-one (that I have seen) gives a concise answer. So let me ask a very direct, simple question & I hope (& pray) someone gets back with a simple answer.


I want to centralize my services by creating VPLS tunnels to a central router.

Its a WISP network, do I need to increase my MTU (not l2mtu) to something like 1600 or 1700 along the entire path that the VPLS tunnel will travel to provide a 1500 unfragmented packet to my customers?

OR

Do I any need to make sure that the L2MTU is big enough to handle a VPLS tunnel i.e. >=1526 - and therefore leave the MTU at standard of 1500?

2

You don’t need to change MTU. You have to make sure that L2MTU is big enough along the path.

3

wow.

There we go.

Thanks!

Then maybe I can sneak one more question.

If I want to start to operate PPPoE & VLAN’s etc on the tunnel, I just increase the MTU under the MPLS settings?

4

PPPoe over VPLS? If you increase VPLS MTU then you also need to increase L2MTU along the path to fit the larger packet.

5

Another question when using VLAN’s, MPLS, VPLS on a network,If your Max L2MTU is 1526 and MPLS interface is set to 1526, should VPLS be set to 1508 or be set higher ?

6

i think you should decrease it each step, reserving space for relevant header portion(eg add VLAN, MPLS and other things(or combination of) sizes) eg if you use MPLS under VLAN under “anykind interface” you should deduct from L3MTU of “anykind interface” BOTH VLAN and MPLS things.
i guess you trying to avoid PMTU issues and retain L3MTU=1500 -alike behavior ~consistent across network ? if so - just increase it a bit(leaving space for underlying thing headers).

7

This is where it becomes a little confusing, Clients use PPPoE,
(1) The AP (433AH) has a max L2MTU of 1526 and is using (Vlan, MPLS, VPLS) MPLS interface is set to 1526 and VPLS is set to 1508
(2) AP connected to a RB750 that has a max L2MTU of 4074 but is currently set to 1526 ( using MPLS, VPLS ) MPLS interface is set to 1526 and VPLS is set to 1508
(3) RB750 connected to PTP RB912 has a max L2MTU of 4076 and is currently set to 1598 ( using MPLS ) MPLS interface is set to 1526
(4) The other side of PTP has the same setting
http://wiki.mikrotik.com/index.php?title=Manual:Maximum_Transmission_Unit_on_RouterBoards&redirect=no
On the Wiki what is unclear is the actual VPLS setting to used, some of our AP’s don’t have a L2MTU of 1530 for a MPLS network, so the question I would ask is what is the optimum setting I can use for now?

8

Hi,

I think this is a really good presentation about that topic and you may have a look at it.

http://mum.mikrotik.com/presentations/US13/kirnak.pdf

9

This article explains MTU pretty well:
http://wiki.mikrotik.com/wiki/Manual:Maximum_Transmission_Unit_on_RouterBoards