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millenium7
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L2 MTU sizes - STILL confused

Wed Mar 14, 2018 5:05 am

I've looked at links to information on MTU and am still left confused about the actual bottom line required MTU sizes
These 2 links come up in a google search and just introduce more confusion
https://mum.mikrotik.com//presentations ... 279781.pdf - page 13
https://mum.mikrotik.com/presentations/US13/kirnak.pdf - page 19

The first shows MTU requirements for MPLS
Image
The first column makes sense. You want an MTU of 1500 at Layer3, MPLS introduces 8 bytes of overhead, add the ethernet frame and you end up with 1522, perfect
What doesn't make sense are the other 2 columns
If you have an MPLS MTU of 1526 why is the L2 MTU also 1526? Wouldn't it be 1540? (1526 + 14 for ethernet = 1540)

In addition to that, aren't the L2 figures 4 bytes short? Because they aren't taking into account the Frame Check Sequence, as shown in the second link
Image
The red lines are wrong aren't they? Shouldn't the L2 MTU in the top section be 1518? and the bottom would then be 1530

Official documentation about ethernet standards is as follows
The frame size of a standard Ethernet frame (defined by RFC 894) is the sum of the Ethernet header (14 bytes), the payload (IP packet, usually 1,500 bytes), and the Frame Check Sequence (FCS) field (4 bytes)
So is this a MikroTik thing where the 'L2 MTU' field in routers/switches doesn't actually include the FCS and thus will always be 4 bytes smaller?
In essence, if I want to send a full 1500 byte IP packet across a MikroTik router with the bare minimum MTU value, do I set MTU as 1500, and then L2 MTU as 1514 (but it actually sends 1518). Or would it be MTU 1500 and L2 MTU 1518?
 
millenium7
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Re: L2 MTU sizes - STILL confused

Wed Mar 14, 2018 7:35 am

Ok from the wiki page here https://wiki.mikrotik.com/wiki/Manual:M ... _frame_MTU
Frame Checksum is not included as it is removed by Ethernet driver as soon as frame reach its destination.
4 bytes must be added to the L2MTU field on mikrotik routers to see the actual MTU
So a full 1500 byte IP packet would need a minimum of 1514 L2 MTU on the mikrotik, but the actual packet that gets transmitted would 1518 because of the FCS (so a switch in between must support actual 1518 MTU)
1518 would work with 1 VLAN and thus become 1522 etc

Still confused about the mentioned MPLS MTU values though


edit: I got the above wrong
L2MTU is not 'full frame'. It DOESNT include the Ethernet header or the Frame Check Sequence trailer. So a standard 1500 byte IP packet will work with an L2MTU of 1504, but if it goes through any switches they need to support either 1518 or 1522 depending on whether or not they include the FCS in their stated figure. Netonix switches I believe they do include the FCS in their MTU values, and they come with 1528 by default (but support up to about 9600?) so if carrying a full 1500 byte PPPoE session over 1 vlan (this would be an L2MTU of 1500+8+4=1512) this would result in a full frame + FCS size of 1530 and will drop frames
Last edited by millenium7 on Mon Mar 19, 2018 10:44 am, edited 1 time in total.
 
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Anumrak
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Re: L2 MTU sizes - STILL confused

Wed Mar 14, 2018 10:32 am

Normal mpls header is 4 bytes. Two mpls headers here recognized as vpls(transport and client labels).
L2MTU is physical interface mtu. Vlan header here on picture is 802.1Q header which is 4 bytes, that's correct.

What is not correct, that untagged mpls explained here with vlan header, what is illogical.

So, more correct data are:

Untagged VPLS: 1500(IP) + 14(pure ethernet) + 4(vpls) + 4(mpls) = 1522 bytes of L2MTU of physical interface.
Tagged VPLS: 1500 + 4(802.1Q(VLAN)) + 14 + 4 + 4 = 1526 bytes of L2MTU of physical interface.
QinQ VPLS : 1500 + 4 + 4 + 14 + 4 + 4 = 1530 bytes of L2MTU of physical interface.

FCS of ethernet don't count in MTU. It's just a normal operation, which is have to be done.
 
millenium7
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Re: L2 MTU sizes - STILL confused

Wed Mar 14, 2018 10:45 am

I'm of the understanding that MPLS needs 2 labels if not directly connected? so 8 byte overhead for MPLS not 4 is that correct?
And in that case, does VPLS replace one of the labels (8 byte overhead), or add to it (12 bytes)?

The reason I need to know exact MTU is we have multiple vendors for radio's and switches and I need to make sure the actual hardware MTU is adequate while I plan a major redesign of network segments. Netonix for instance has a default MTU of 1528 and that is the full MTU 'including' FCS, so would be L2MTU of 1524 on a Mikrotik = not enough to carry VPLS over a VLAN. Almost all of the Netonix switches are currently set to default and thus need to be changed

The mikrotik wiki document is also ridiculously confusing with the advanced example at the very bottom https://wiki.mikrotik.com/wiki/Manual:M ... uterBoards
How does 14+4+20+1480 = 1504?
And in the MPLS cloud its showing 14+4+4+4+14+4+20+1480 but somehow that works out to be 1530? no makey sensey

I'm also confused about L2MTU vs MPLS MTU. I understand what L2MTU is, but surely if MPLS MTU is set to i.e. 1530 then you MUST set L2MTU to be 14/18/22 bytes more? (untagged/tagged/q-in-q)
 
millenium7
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Re: L2 MTU sizes - STILL confused

Wed Mar 14, 2018 10:47 am

Additionally i'm a bit confused about the MPLS MTU size needing to be changed at all?
We currently have 1500 byte VPLS interfaces working just fine with the MPLS MTU size set to the default of 1508 (L2MTU at 1600)
 
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Anumrak
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Re: L2 MTU sizes - STILL confused

Wed Mar 14, 2018 10:59 am

Additionally i'm a bit confused about the MPLS MTU size needing to be changed at all?
We currently have 1500 byte VPLS interfaces working just fine with the MPLS MTU size set to the default of 1508 (L2MTU at 1600)
MPLS MTU have to include vpls header(one more mpls header), that's why mpls mtu bigger. And L2MTU is more bigger than MPLS.
 
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Anumrak
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Re: L2 MTU sizes - STILL confused

Wed Mar 14, 2018 11:02 am

I'm of the understanding that MPLS needs 2 labels if not directly connected? so 8 byte overhead for MPLS not 4 is that correct?
And in that case, does VPLS replace one of the labels (8 byte overhead), or add to it (12 bytes)?

The reason I need to know exact MTU is we have multiple vendors for radio's and switches and I need to make sure the actual hardware MTU is adequate while I plan a major redesign of network segments. Netonix for instance has a default MTU of 1528 and that is the full MTU 'including' FCS, so would be L2MTU of 1524 on a Mikrotik = not enough to carry VPLS over a VLAN. Almost all of the Netonix switches are currently set to default and thus need to be changed

The mikrotik wiki document is also ridiculously confusing with the advanced example at the very bottom https://wiki.mikrotik.com/wiki/Manual:M ... uterBoards
How does 14+4+20+1480 = 1504?
And in the MPLS cloud its showing 14+4+4+4+14+4+20+1480 but somehow that works out to be 1530? no makey sensey

I'm also confused about L2MTU vs MPLS MTU. I understand what L2MTU is, but surely if MPLS MTU is set to i.e. 1530 then you MUST set L2MTU to be 14/18/22 bytes more? (untagged/tagged/q-in-q)
And in the MPLS cloud its showing 14+4+4+4+14+4+20+1480 but somehow that works out to be 1530? no makey sensey

It works beacause it seems heavy traffic don't flow, because IP mtu 1500 is max tcp payload of 1480 and udp 1492. If you open a site, you tcp payload will be not longer than 200-500 bytes. That's why it works for now. Not every tcp or udp segment equal maximum of it's payload.
 
millenium7
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Re: L2 MTU sizes - STILL confused

Thu Mar 15, 2018 1:32 am

Additionally i'm a bit confused about the MPLS MTU size needing to be changed at all?
We currently have 1500 byte VPLS interfaces working just fine with the MPLS MTU size set to the default of 1508 (L2MTU at 1600)
MPLS MTU have to include vpls header(one more mpls header), that's why mpls mtu bigger. And L2MTU is more bigger than MPLS.
Right, so why does the picture say MPLS MTU 1530 and L2MTU also 1530?
if MPLS MTU is set to 1530 then L2 MTU should be 1544/1548/1552 correct?
 
millenium7
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Re: L2 MTU sizes - STILL confused

Mon Mar 19, 2018 11:21 am

Ok I had it wrong, L2MTU in a mikrotik does NOT include the ethernet header. And this is why the MPLS MTU and L2MTU can be the same (as long as you aren't sending MPLS over a VLAN)
And VPLS not only adds 4 bytes, but it also adds another ethernet header
Nowhere in RouterOS do you see the Full Frame MTU so it isn't relevant. But if spec'ing out a switch it absolutely is relevant and this is why i've included it (and why i've been so confused about the whole MTU thing)
I made this spreadsheet to clear things up with plenty of examples

Image

The bottom right one is important to me, since we will eventually want to lease some of our backhaul to other companies, and they may very well want to do PPPoE over a VLAN. So we need to make sure all our mikrotik routers are set to at least 1538 L2MTU and all of our switches can carry at least 1552/1556 (depends if they count the Frame Check Sequence in their figures) for Standard PPPoE

Also if we want to do IP packing. If we have a switch that supports an MTU of 9200 then we can only combine 5 'Oversize PPPoE in Q-in-Q over VPLS' packets (5x 1538 L2 MTU = 7690 + 14 eth + 4fcs = 7708)
But if we lower the L2MTU to 1530 we can fit 6x 1530 = 9180 + 14eth + 4fcs = 9198 and be faster
 
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Re: L2 MTU sizes - STILL confused

Mon Mar 19, 2018 1:38 pm

Slide 18 and 19 from my presentation on MPLS/VPLS/MTU covers this pretty well:
https://mum.mikrotik.com/presentations/US13/kirnak.pdf

I would also recommend actually watching / listening to the presentation, it covers it much more in depth:
https://youtu.be/Q8AF-Srulmk
Last edited by tomaskir on Mon Mar 19, 2018 3:01 pm, edited 1 time in total.
 
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Anumrak
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Re: L2 MTU sizes - STILL confused

Mon Mar 19, 2018 2:20 pm

Additionally i'm a bit confused about the MPLS MTU size needing to be changed at all?
We currently have 1500 byte VPLS interfaces working just fine with the MPLS MTU size set to the default of 1508 (L2MTU at 1600)
MPLS MTU have to include vpls header(one more mpls header), that's why mpls mtu bigger. And L2MTU is more bigger than MPLS.
Right, so why does the picture say MPLS MTU 1530 and L2MTU also 1530?
if MPLS MTU is set to 1530 then L2 MTU should be 1544/1548/1552 correct?
Picture is incorrect.
 
millenium7
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Re: L2 MTU sizes - STILL confused

Tue Mar 20, 2018 10:47 am

Next question I have is in regards to MRRU on PPPoE links

I want to provide full 1500 byte packets to PPPoE customers. I have my VPLS MTU set to 1500, VPLS L2MTU set to 1516, MPLS MTU set to 1538 and L2MTU set to 1538 or higher.
I don't want to go to all this trouble of uniformly setting MTU across our network for maximum speed without any fragmentation issues, and then screwing it up with an incorrect MRRU setting (incase we have clients with routers that don't support oversize PPPoE but do MLPPP)

I'm finding that with with 1500 byte oversize PPPoE I can easily carry a 1500 byte packet across. But using MLPPP, MTU/MRRU at 1500 then 1500 byte packets get fragmented and sent with 1x 1536 bytes and 1x 62 bytes
How exactly does MLPPP work and how much overhead does it add on its own? Essentially what VPLS/MPLS/L2MTU would I need to carry 1500 byte TCP/IP frames without fragmentation with MLPPP?
 
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Re: L2 MTU sizes - STILL confused

Tue Jun 22, 2021 7:30 pm

@millenium7
May I ask if you could share the spreadsheet, that you have prepared? I will try to help you and draw the MLPPP case.

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