I have seen a few threads relating to MTU size for MPLS and for VPLS but for my specific scenario.
I plan on running VPLS based BGP.
From what i have read up on the following below is ideal MTU Size
14 + 20 + 4 + 4 + 1480
=1522 MTU
Is this correct?
Well. 1536 is “ideal” or maybe the less ideal. Best is to run bigger l2mtu, just to be sure. My setup:
MPLS mtu = 1590 - I have enough ( all l2 Devices between MPLS routers, i say " must have at least 1600l2mtu)
ip mtu = 1500 (standard)
VPLS - l2mtu = 1525 and ip mtu = 1510 _8 just to be inside at the right end)
Now its possible to play With vlan, qinq etc, without worry about mtu and MPLS.
Hi
Thanks for the reply 
If you want to know exactly how to calculate the MTU and how its done, see the presentation in my sig.
About 15 minutes of it is pure MTU talk.
Thanks for the link tomaskir
This is to calculate minimum MTU, and walk on the edge of a knife. To be sure - Use Maximum l2mtu possible in all l2 net Connected “by” MPLS
Its not edge of the knife, its control.
You control your network, you know what max MTU it uses. You want to limit the MTU, and you want to have things working exactly as intended.
Therefore, setting all MTUs exactly as you want them to be, and therefore enforcing your design and your network constrains is a good thing.
Having random things in your network is bad, designing well and then enforcing your design, that is good.
MTU is one of those things that simply needs to be enforced, or you end up regreting it later.
I still dont agree. - Annyting set to minimum required setting - no matter what - hat can be set “better” - i mean must be a better way to do this. To have Control, does not mean to set mtu to minimum required setting. And s NOT common to have MIMUM setting to all Equipment. A lot of Equipment have 1600 or 9000 as default l2mtu - Also other values like " jumboframes or not jumborframes." - so to say. 1536or whatever needed to fit - does not have to be the best.
If i have a engine - that should have the car to drive at 100km/h - Is it a bad thing to have an engine - capable of making 200km/h? - Who knows when it might be better to have 110km, just for a moment.
I have seen to much stuggeling about MTU and values - Just couse network engineers want to have the minimum required value, and one day change someting. Maybe but a vlan? q in q? Something New and "struggle starts.
Is there ONE negative thing about having - lets say- 1600 l2mtu as a “standard”? - I cannot find one. To have minimum required mtu to all l2 equipent - i can se a bounch of possible future problems.
I still mean - l2mtu - minimum 1600
mpls mtu 1560 (or better 1590)
vpls mtu - 1510 vpls l2mtu 1525
Standard.
Why? Lets say - I deliver a l2 to another ISP. They run this l2 for 4 years, and figure out - we want to use mpls. OK - All my net are capable of delivering 1536 - Its just to change in bothe ends of the VPLS tunnels - and wolla - It work. Also possilbe to deliver MPLS/VPLS over qinq - and even q in q over q if needed. to set a low value just Block future possibilities - or make a big job for the network engineer reconfigure the net.
I do the same with wlan . If a CPE buy 10mb/s - I give lets say 25 - but shape to 10. I_ dont put Maximum possilbe speed in “hardware” to 10mb/s - and have no oppertunity to change into higer speed in the future.
I cannot say that Control is the same as “live on the knife edge” as i spoke about. Control is to make lif easy - and be able to tell YES to any special delivery Customer want.
Its also a what is the goal of mpls. just to replace routes - or running vpls to be able to deliver l2 to cpe/isp as a service. - And what are requirements from other ISP? - as a “lan to lan” VPN - it does not matter - in 99,9% of l2 tunnels - CPE need just 1500 bytes to get their IP frame trough the net - but the first time a ISP want to have something special - like q in q and own MPLS trough the VPLS - thing starts to get a bit complicated to calculate for each line. To have a robust net- capable of deliver bigger frames - give Control. gives oppertunities- and give a roubust good net. - With its own limit- like in my example - limit is 1600bytes in l2. and 1560 (or 1590) in MPLS