Hi,
I know it's already been a lot of topics about discussing MTU on MPLS and VPLS but I'm a little bit confused...

My Setup:

Side A:
CCR1036 with:
6x PPPoE clients (Max MTU: 1492, Max MRU: 1492, MRRU: 1600, Interfaces: 6x VPLS over MPLS)
Each VPLS: MTU: 1526, Advertised L2MTU: 1590
MPLS MTU: 1550

Side B:
CCR1036 with:
6x VPLS over MPLS
Each VPLS: MTU: 1526, Advertised L2MTU: 1590
MPLS MTU: 1550
Use of 6x ether interfaces with MTU: 1530, L2MTU: 1590 (physically connected with 6x vDSL modems of my ISP, each one)
Use of 6x bridge interfaces with MTU: 1530, L2MTU: 1590 (each bridge interface includes 1x ether interface + 1x VPLS interface).

Remote Connection of Side A and B:
2x UBNT PowerBeam500 M5 ISO AC with MTU: 1530
Ether interfaces MTU on CCR1036s: 1530, L2MTU: 1590

Reason of this connection:
CCR1036 on side A is the main router of my network with: PPPoE Server (for my clients connection), HotSpot Server (for my clients connection) enabled and Per Connection Classifier method enabled for vDSL lines Load Balance. Because I wasn't able to have vDSL stable lines in place of Side A, but I was able in Side B, I used the above tunnel connection in order to do PCC method based on PPPoE, on my main router (side A).

Moreover I use Change MSS with clamp to pmtu mangle rule on my main router (side a) and every PPPoE client/customer has 1480 actual MTU.

I've already read the below instructions:
- Tomas Kirnak Presentation: https://mum.mikrotik.com/presentations/US13/kirnak.pdf
- Mikrotik Wiki manual: https://wiki.mikrotik.com/wiki/Manual:M ... uterBoards
- Many threads of this forum....

But, I'm confused with the difference between Actual MTU, L2MTU, L3MTU, Full Frame MTU and I'm not sure if I had made the right calculations on my setup above.

Please advise me about the differences of the above MTU and inform me if you find any mistake on my above setup calculations of MTUs.

Thanks in advanced.

L2MTU is ethernet MTU without ethernet header, MTU is IP MTU without IP header, Actual MTU is MTU that is set right now on interface. Full frame MTU is ethernet frame MTU with all headers above itself without ethernet header(14 bytes).

https://wiki.mikrotik.com/wiki/Manual%3 ... uterBoards

All MTU sizes are interfaces MTU without it specific header that it belong to terminate on itself.

Correct me if I wrong

MTU is IP MTU without IP header
Correct me if I wrong

you are.
MTU (or IP MTU) is the packet size, e.g. if it is for IP, it consists of the IP/IPv6 header and the IP payload.
MPLS MTU is the frame size, where the frame consists of one or more MPLS labels (4 bytes each) and the MPLS payload, which may be IP packets or Ethernet frames (in case of RouterOS)
L2 MTU usually is the maximum L2 frame size, including ethernet headers (and dot1q/dot1ad), ethernet payload, that can be passed over an interface - in case of ROS it excludes the Ethernet header - as ROS versions doesn't support L2 encapsulation other than ethernet on physical interfaces with settable MTU anyway. it's kind of weird, as the Layer 2 max transmission unit shall cover the MAC layer headers as well, as IP MTU also contains the IP/IPv6 header - but that's what it is.

L2 MTU > MPLS MTU > IP MTU

of course if you don't use MPLS, the middle one doesn't matter.

also you can calculate your MPLS MTU, that's the main thing ,depends what you want to do.

MTU is IP MTU without IP header
Correct me if I wrong

you are.
MTU (or IP MTU) is the packet size, e.g. if it is for IP, it consists of the IP/IPv6 header and the IP payload.
MPLS MTU is the frame size, where the frame consists of one or more MPLS labels (4 bytes each) and the MPLS payload, which may be IP packets or Ethernet frames (in case of RouterOS)
L2 MTU usually is the maximum L2 frame size, including ethernet headers (and dot1q/dot1ad), ethernet payload, that can be passed over an interface - in case of ROS it excludes the Ethernet header - as ROS versions doesn't support L2 encapsulation other than ethernet on physical interfaces with settable MTU anyway. it's kind of weird, as the Layer 2 max transmission unit shall cover the MAC layer headers as well, as IP MTU also contains the IP/IPv6 header - but that's what it is.

L2 MTU > MPLS MTU > IP MTU

of course if you don't use MPLS, the middle one doesn't matter.

I meant that if PPPoE interface set on 1480 MTU that means this interface terminates PPPoE headers, but interface itself do not count it header size. Try it by yourself. Set PPPoE interface MTU 1480 and ping through it with "don't fragment" bit. Your max ICMP payload will be 1452. That because PPPoE interface count ICMP header(8) and IP(20). That's all. But we know that PPPoE headers are 8 bytes. But ROS don't count it. As you said, max ICMP payload must be 1444(1444+8+20+8=1480). But it won't work.

I meant that if PPPoE interface set on 1480 MTU that means this interface terminates PPPoE headers, but interface itself do not count it header size.

i see. IP headers =/= L2 frame headers, and in case of PPPoE it's a frame header. therefore it is perfectly logical for ROS to not to take them into consideration.
wherever you set "MTU" on routeros, it always shows the L3 MTU (thus the IP MTU) of the respective interface.

the 8 bytes of "PPPoE header" (6 bytes PPPoE session header + 2 bytes PPP header) are outside of the L3 MTU.
so is the ethernet header (1500 vs 1514)
btw, in case of PPPoE over plain 1500 bytes of ethernet payload can have 1492 bytes of IP MTU.

I meant that if PPPoE interface set on 1480 MTU that means this interface terminates PPPoE headers, but interface itself do not count it header size.

i see. IP headers =/= L2 frame headers, and in case of PPPoE it's a frame header. therefore it is perfectly logical for ROS to not to take them into consideration.
wherever you set "MTU" on routeros, it always shows the L3 MTU (thus the IP MTU) of the respective interface.

the 8 bytes of "PPPoE header" (6 bytes PPPoE session header + 2 bytes PPP header) are outside of the L3 MTU.
so is the ethernet header (1500 vs 1514)
btw, in case of PPPoE over plain 1500 bytes of ethernet payload can have 1492 bytes of IP MTU.

Yeah, that's true.

To author:
You need to count headers bytes only beyond your transmitting mpls interface, not vpls tunnel.(vpls is double-header mpls)