A large network that is running VPLS has been working quite well. We currently are not worried about packet fragmentation, however, we can not push the 1500 byte MTU packets though the VPLS in some instances.

VPLS circuits are set to 1508 in MTU in case of VLAN information, the VPLS tunnels do come up. In most cases, we have had the VPLS tunnels up for several weeks or longer, then in the morning, we come in and have calls about customer issues. We check, the VPLS circuit is up, but the largest ping we can get though is 1442. In many cases we can simply remove the MPLS interfaces, and readd them, upon readding the VPLS comes back up and we can ping 1500 byte packets across the link.

The question is WHY does this stop working? We have tried BGP VPLS as well, but still 1442 pings only. Whats even more confusing, is we have another VPLS tunnel that goes THOUGH the affected site, and it does not have the MTU issue!

Suggestions?

A large network that is running VPLS has been working quite well. We currently are not worried about packet fragmentation, however, we can not push the 1500 byte MTU packets though the VPLS in some instances.

VPLS circuits are set to 1508 in MTU in case of VLAN information, the VPLS tunnels do come up. In most cases, we have had the VPLS tunnels up for several weeks or longer, then in the morning, we come in and have calls about customer issues. We check, the VPLS circuit is up, but the largest ping we can get though is 1442. In many cases we can simply remove the MPLS interfaces, and readd them, upon readding the VPLS comes back up and we can ping 1500 byte packets across the link.

The question is WHY does this stop working? We have tried BGP VPLS as well, but still 1442 pings only. Whats even more confusing, is we have another VPLS tunnel that goes THOUGH the affected site, and it does not have the MTU issue!

Suggestions?

Sorry for not being of much help here, but I thought at least I should tell you that I see the same behavior for quite some time now.
On the one affected tower I even replaced the RB450G with an RB1100, and the problem was gone for months.
Thought I had it tackled. But now it is back. No changes have been made recently.
As you stated, tunnels to other sites, going through this one, are NOT affected.
Really strange.

saludos
Bernardo

Yes!

I have this problem too but only so far with 5.12 and 5.13

5.7 or earlier I never saw this. Had to downgrade.

A while can go by and everything is OK, but then suddenly the packets size that fits is suddenly smaller.
I couldn't ever find a way to hot fix it like you did.

Yes! I even have the same issue, packets will go THRU the router to one farther up and it works!

I saw this on a RB600 and RB493 so far.

I had this problem again today on v5.8

Still the same here.
The one affected RB1100 is on 4.17 right now, and the problem persists, but only on ether6.
Re-routing (OSPF) over to ether1 and problem is gone.

saludos
Bernardo

It comes and goes for me, doing what was mentioned above, removing the MPLS interface and re-adding fixed it.

This was on an RB433 this time.

So it appears not to be specific to any build. mipsbe and ppc affected for me so far.

make sure you have added all interfaces used by label switching in "/mpls interface" menu, for some customers thatwas the problem, they for got to add one interface in one router in MPLS cloud.

This was all correct network wide.

It is definitely a bug.

Friday the same thing here.

Suddenly the VPLS-Interface stops working with 1500 Bytes. Checked it with a Customer. The Maximum was 1442 Bytes (+ PPPoE-Overhead) which goes through the tunnel.
Hardware between Customer and Core: 3xRouterboard 1100AHx2 and 1xRouterboard 1100AH (all with Version 5.14)

Double checked everything (all used Interfaces included in MPLS-Interface with MTU 1530).

Nothing has changed from our side. Only our carrier switched our leased line on another fibre-pair in the night before, so that the line was offline for about 3 hours and then the ospf-routing and mpls/vpls comes up again but with a smaller MTU within the VPLS.

We just started to implement a pure MTIK MPLS environement in our core network so please Mikrotik give us a hint how to solve this.

Is VPLS tunnel running over your carriers network?

There are several leased lines from data center to locations over the carrier ethernet. It´s SDH and they say they give us a MTU-Size of 9600.
On friday as we noticed it, we made a ticket and they measured the line with no success.
All of our other leased lines work with VPLS 1500.
after the affected leased line is one mpls-router (with vpls-termination). this one works with 1500 bytes. After that there is a licensed radio to the affected routerboard which dont get the 1500 bytes.
There are three vpls-tunnels. I checked the mtu on all tunnels. one can handle 1446 bytes (including icmp-header), one with 1478 and one 1470. that´s really strange.

In the core there is vpls-terminator and from there are several vpls tunnels to the locations.

I readded the mpls interfaces, rebooted all routers to the customers and recreated the vpls tunnels.

sorry for double post.

the first vpls tunnel with 1446 is because of a vpn through which i checked with a server. so this is ok.

for the one with 1478 bytes it looks like this:

server -> vpls-bridge (rb1100AHx2) -> mpls router (rb 1100AHx2) -> vpls-bridge (rb1100AH) -> wireless ap -> wireless cpe

for the one with 1470 bytes:

server -> pppoe-server (mtu/mrru 1492) -> vpls-bridge (rb1100AHx2) -> mpls router (rb 1100AHx2) -> vpls-bridge (rb1100AH) -> dslam -> cpe

i know it´s hard to give some advice for this, but everything worked like a charm before and the other users in the forum noticed this strange behavior before.

Update:

changed Routerboard at customer-site ->nothing changed
changed the middle Routerboard -> nothing changed

tested the MTU-Size more accurate:

1478 Bytes is what i get on all vpls-tunnels at the customer-site. In the middle between data center and customer-site i get 1500 Bytes on all vpls-tunnels, so the leased line is definitly ok.
Between the customer-site and the middle Routerboard is a licensed radio which is capable of jumbo frames so i think thats not the problem.

Now, where are my 22 Bytes

This is getting me crazy, because it worked before without any problems and suddenly it stops.
The other vpls-tunnels on the other sites work like a charm so i dont think the routerboard within the data center is the problem.

Any ideas?

Between the customer-site and the middle Routerboard is a licensed radio which is capable of jumbo frames

What kind of radio is this.

It´s a NEC iPasolink 400 and on the other side a NEC iPasolink 200

Next update.

Replaced all Routers on this line to the customers including the one in the data center. (now 3x RB1100AHx2 with RouterOS 5.19)
All routers are configured manually from the scratch.

Same problem. The VPLS-Tunnel to place 1 works with 1500 Bytes and the VPLS-Tunnel to place 2 only with 1478 Bytes.

This is not working:
Now I changed the config in this way that I create a new VPLS-Tunnel between place 1 and place 2 and attach both tunnels to a new bridge.
The OSPF interface mode are set to PTMP on the used interfaces.

This is working:
So where is my mistake? Why is it working with two tunnels and a bridge between and not with one tunnel directly from the data center?
So the licensed radio doesn´t cause the problem. I checked it with the support of our dealer, it´s capable of jumbo frames.

In another part of the network a have the same constellation (3x RB 1100AHx2 with version 5.14 and 5.19 in a row). The only thing which is different is instead of a licensed radio there is a 5GHz Ubiquiti Link (with the new mac address allowing frames bigger than 1518 Bytes for MPLS). And there is no problem. Same config except the routing part. Between data center and place 1 there is ospf network mode ptmp and between place 1 and 2 there is nbma.


Thank you for some hints. This problems causes a lot of headaches

Yea this happened again for me but went long tine without seeing this bug.

It just quits, not changing any settings, just stops passing traffic, but only in one direction.

Mikrotik any suggestions on this issue?

Do you need more information?

It was so bad for me this time that I couldn't fix it. I had to abandon it for now in favor of l2tp

saw this post and thought I would throw my two pesos in as I do a lot of MPLS work in Cisco and Mikrotik.

Not sure if any of you guys have tried this, but if you want to do EoMPLS/VPLS and pass a 1500 byte IP packet between the two connected hosts on the CE routers, then the MPLS MTU (and hence the max L2MTU) has to be at least 1526 for untagged frames and 1530 for tagged frames. You should be able to ping with the fragment bit set at 1472 bytes to test passing a full 1500 byte packet (you lose 20 bytes for the IP header and 8 bytes for ICMP hence 1472)

Basically with VPLS you have to have additional mtu overhead because you are encapsulating a Layer 2 frame inside of an MPLS packet and it has to have an ethernet header to deliver once it gets there so there's 14 bytes plus a 4 byte VPLS header and the standard 8 bytes for MPLS....which gets you to 1526. So whatever the max L2 MTU is on your interface is, subtract 26 bytes and you will get the VPLS payload

Also, if you're setting different MPLS mtus on each router, then you can potentially run into issues. it's better to set them all to the lowest common denominator in the path.

The problem we have with Mikrotik is that its ticking along fine for even months at a time, then quits, without change.

Yesterday it spread like a virus thru my whole network, 14 routers all stopped passing even a 1480 byte packet.

All routers set to the lowest supported MTU.

Perhaps I have been mis-understanding the MTU of my x86 router after doing some reading.

It reports an IP MTU of 1500, great but does not report an L2MTU and I read that you should assume then in this case the IP MTU is the L2MTU, and therefore my MPLS MTU is 1500.?

Ive always had it at 1512 (or at one time 1508).

I dont know what the max size of packets my switch can handle, its not mentioned anywhere (FS726T)

The lowest L2MTU routerboard I have on the network is 1522.

An MPLS MTU of 1512 means that your router won't pass any more than 1512 bytes (including headers via MPLS)

So if you take 1512 and subtract the 26 bytes needed for VPLS, that leaves you with a payload of 1486 (1482 if you're using dot1q tags) which is 14 bytes short of being able to carry a standard 1500 byte IP packet without fragmenting it (fragmenting the packet is bad juju and you never want to do it if you can help it)

If the maximum L2MTU (which is a completely different setting than IP or MPLS mtu) is only 1522 bytes, then you won't be able to carry a 1500 byte IP packet over MPLS/VPLS. Likewise, all the Layer 2 links in the path need to have a 1526 or higher L2MTU.

As to the issue of MPLS forwarding stopping on all routers, here are some things i've seen in production:

1) CPU utilization - this can kill MPLS quickly if it tears down the routing adjacencies
2) Routing - MPLS relies on an IGP (typically OSPF) to build the LDP forwarding table, if the routing dies or something changes it, MPLS can go bye bye
3) MTU differences in the label switched path - if there is an LSR switching traffic that is on a lower MTU segment than the endpoints, it can cause issues.
4) Routing summarization - if you summarize routes with a low enough mask, it can cause issues with an LSR that sits in the summarized subnet. MPLS needs knowledge of specific subnets to build the LDP forwarding information base

saw this post and thought I would throw my two pesos in as I do a lot of MPLS work in Cisco and Mikrotik.

Not sure if any of you guys have tried this, but if you want to do EoMPLS/VPLS and pass a 1500 byte IP packet between the two connected hosts on the CE routers, then the MPLS MTU (and hence the max L2MTU) has to be at least 1526 for untagged frames and 1530 for tagged frames. You should be able to ping with the fragment bit set at 1472 bytes to test passing a full 1500 byte packet (you lose 20 bytes for the IP header and 8 bytes for ICMP hence 1472)

Basically with VPLS you have to have additional mtu overhead because you are encapsulating a Layer 2 frame inside of an MPLS packet and it has to have an ethernet header to deliver once it gets there so there's 14 bytes plus a 4 byte VPLS header and the standard 8 bytes for MPLS....which gets you to 1526. So whatever the max L2 MTU is on your interface is, subtract 26 bytes and you will get the VPLS payload

Also, if you're setting different MPLS mtus on each router, then you can potentially run into issues. it's better to set them all to the lowest common denominator in the path.

Thanks for your reply.

As I´m using only RB1100AHx2 in the core the Layer 2 MTU on the interfaces is set to the defaults on 1598 Bytes. The path to the other routers is capable of Jumbo Frames (9600 Bytes).
All MPLS Interfaces are set to 1530 Bytes (tagged frames planned in the future). The VPLS is set to 1500 Bytes. It works all over the network except the part seen above.

I know that a ping must be 1472 Bytes + 28 Bytes for the header without fragmentation. This works within the network.
But in the one part there is only the chance to ping with 1478 Bytes (including the 28 Bytes overhead).

What I don´t understand:
Why is the VPLS-Tunnel isn´t working from core over another MPLS-Router to the customer site with 1500 Bytes. But if I create two VPLS-Tunnels (one from core to the firste router and another one from the first router to the customer site) and bridge them together it works with 1500 Bytes.
Last week everything was fine with the first constellation and then it suddenly stops working. This must be a bug. I didn´t change anything.

What I´ve done:
- Changed all routers from core to the customer site
- created a new configuration from the scratch
- updated all routers on the path from 5.14 to 5.19

Now with the two tunnels it works, but this isn´t a solution for me. Just a workaround.

@ Inssomniak

Your switch (FS726T) should support jumbo frames. This is what i read in the manual.
http://www.manualowl.com/m/Netgear/FS72 ... 78?page=25

With the x86 hardware i don´t know how the MTU behaves.
But as IPANet Engineer said. MPLS MTU 1512 Bytes is too low to get VPLS with 1500 Bytes. And also 1522 Bytes L2MTU is too low.

Hmm my switch doesn't have that setting. . I did find that it was not on their list of supported list of jumbo frame capable switches.

As for not passing 1500 byte packets I was ok with it. I only wanted it to pass a 1480 byte packet. The VPLS tunnel just carried pppoe traffic with max MTU of 1480 bytes.

So if all I care about is 1480 bytes, I should be able to get away with an MTU of 1506 without fragmenting the pppoe traffic?

1510 if I had tagged traffic?

EDIT:

Ive been doing some experimenting since the total un-called-for collapse of my MPLS network a few days ago. Why it failed like a virus I dont know.

The ONLY way I can get MPLS to work properly between my core x86 router and a directly adjacent mikrotik router is if I set the MPLS MTU to 1500. Then it works, fragmenting or not.

So Im leading to believe the reported IP MTU of the gigabit interface in the x86 router is actually also the L2 MTU.

Increasing this value leads to this: http://forum.mikrotik.com/viewtopic.php?f=2&t=58329

EDIT: more testing.

The intel pro/1000 cards cant do it, their L2MTU is 1504 tops, any attempt to change the MTU in mikrotik crashes the router. This card is essentially useless for MPLS networks with mikrotik.
I took a 10 year old national semiconductor based FE trendnet card and it works fine, go figure.

I ordered another card, unfortunately its a realtek 8169 card and the wiki mentions it might lock up the router, but there isnt a lot of pickings for gigabit ethernet cards, I could not find a low profile Via based one, or a marvell based one which I read is not supporting well past 2.6.31 kernel.

That's too bad about your network card. MPLS is very dependent on MTU and is often the source of much heartache in MPLS networks. To do PPPoE you'll have to account for another 8 bytes of MTU overhead IIRC (2 bytes PP and 6 bytes PPPoE)

Baltic networks sells a MIkroTik router that is a multi-core and does jumbo frames called RouterMAXX. They work very well for MPLS.

http://www.balticnetworks.com/manufactu ... rotik.html

From what I understand though, no intel based pro/1000 card can work. How do you get that routerMAXX to work? (they are based on intel pro/1000 chips from what I read)

The 3 Intel NIC cards I have all support jumbo frames, but mikrotik either wont pass the large packets or when trying to raise the MTU just crashes the router.

Do you have a link for the issue with raising the MTU and that cardset?

The L2MTU on that particular router can be set to 9214 and is what shows up in the router as the L2MTU. I'd have to do some further testing on that platform as I don't have one in front of me but have worked on them before.

Yea a few posts above.

I have reproduced the problem on 2 separeate x86 mikrotik routers with 2 different intel pro/1000 chipsets.

The L2MTU is not reported at all.

You might want to contact Baltic networks and ask them about the RouterMAXX line in regard to this specific issue. I know that I have implemented MPLS and L2 MTUs above 1504 without any issues on that gear, but I don't think i've tried a higher IP MTU.

We have many implementations using Intel Pro1000 cards and 1530 byte and larger frames, they work brilliantly.

We have many implementations using Intel Pro1000 cards and 1530 byte and larger frames, they work brilliantly.

Well then how come mine dont work???
Are you also using MPLS VPLS too?

Yes we are running vpls circuits between data centres. Each vpls tunnel is carrying vlan tagged packets. We are running RouterOS on Dell R210-II with Pro1000 quad port NIC and booting from Kingston 8GB SSD.

We have a lot of these in production all doing MPLS/VPLS with no issues.

Yes we are running vpls circuits between data centres. Each vpls tunnel is carrying vlan tagged packets. We are running RouterOS on Dell R210-II with Pro1000 quad port NIC and booting from Kingston 8GB SSD.

We have a lot of these in production all doing MPLS/VPLS with no issues.

Does that network card report an L2MTU in Mikrotik?

If the card does not report L2MTU you must set MTU to desired value.

If the card does not report L2MTU you must set MTU to desired value.

The cards do not report L2MTU and it cannot be manually set. The values are greyed out in winbox, and at the command line you can enter the command, but it does nothing.

Again if L2MTU is not detected you must set MTU (Layer3 MTU) to desired value.

Again if L2MTU is not detected you must set MTU (Layer3 MTU) to desired value.

Code: Select all

/interface set ether1 mtu=1530

Yea, but when I do that, and I try to put a larger packet thru that interface with MPLS and a VPLS tunnel, the router crashes.

Reproducible on 2 separate x86 machines with 2 separate types of intel pro/1000 cards.

It is a driver problem. Try v6beta3 there should be newer drivers. If it crashes on beta3 too then get better ethernet card.

It is a driver problem. Try v6beta3 there should be newer drivers. If it crashes on beta3 too then get better ethernet card.

Is beta 3 out? Can I get a copy?

I didnt think an intel pro/1000 GT card was a "bad" card. I dont know what to get thats better than that for a PCI bus.

You can get beta3 if you contact support. Well the card is not bad, but it is quite new and drivers are not optimized in older kernel.

You can get beta3 if you contact support. Well the card is not bad, but it is quite new and drivers are not optimized in older kernel.

Ok thanks emailed support.

I did order a realtek 8169, and I believe that does the job but comment in wiki scares me: "not recommended, may lock up router"

I got in a RTL8169 and it worked fine. It reports an l2mtu 16383

Now Id like to make a call out to all that are using intel pro/1000 cards, that have the l2mtu properly reporting and have VPLS working with MPLS MTU=1526 --
And also that have pro/1000 cards that dont report l2mtu, that adjust the IP MTU past 1500 and have it working without crash.

Models, bus type, etc..

What I have that doesnt work: Pro/1000 GT PCI, Pro/1000 MT Dual port, PCI-X

Anyone else?

Pro1000PT Quad port PCIe

Why are you using old PCI card?

Pro1000PT Quad port PCIe

Why are you using old PCI card?

I couldn't get the PCI-e card in the PCI slot, lol

Can u give me more details? Does it report l2mtu and you are running MPLS MTU 1526 or better?

I couldn't get the PCI-e card in the PCI slot, lol

Haha, fair enough

Screenshot of a working deployment:

Hah Thanks! that image is impossible for me to reproduce in the lab without a router crash.


Anyone else?

Maybe pic-e cards are the difference.

Get a Dell R210-II with Xeon E3 CPU, Intel Pro1000PT Quad Port NIC (I350 should work in later 5.x), then just add a small 8GB SSD for RouterOS.

You should be able to get this configuration from Dell Direct for around USD $800 - $1000 depending on where you are located you can probably get same day warranty on it too.

This is known working config and is being deployed by quite a few people on these forums with success.


I would probably go for the new RouterBoard RB44Ge http://routerboard.com/RB44Ge instead of an Intel NIC on any new builds. This way if you encounter any issues Mikrotik will be more obliged to support you since you are using their hardware

Get a Dell R210-II with Xeon E3 CPU, Intel Pro1000PT Quad Port NIC (I350 should work in later 5.x), then just add a small 8GB SSD for RouterOS.

You should be able to get this configuration from Dell Direct for around USD $800 - $1000 depending on where you are located you can probably get same day warranty on it too.

This is known working config and is being deployed by quite a few people on these forums with success.


I would probably go for the new RouterBoard RB44Ge http://routerboard.com/RB44Ge instead of an Intel NIC on any new builds. This way if you encounter any issues Mikrotik will be more obliged to support you since you are using their hardware

I think I might have to do this, further testing of a realtek card shows it can just quit working, only a disable and re-enable fixes it, I dont like that.
There is a local r210-II for sale here, more than I need (dual SAS 300 gig raid, core i3 chip, DRAC 6 enterprise), new for 800 bux no tax, I can probably strip out the drives and raid controller and sell them and stick in a small sata drive, but I just bought a controller level 6 license and it sits on an IDE drive, that was only a couple months ago and I dont want to buy a new one again

I think I might have to do this, further testing of a realtek card shows it can just quit working, only a disable and re-enable fixes it, I dont like that.
There is a local r210-II for sale here, more than I need (dual SAS 300 gig raid, core i3 chip, DRAC 6 enterprise), new for 800 bux no tax, I can probably strip out the drives and raid controller and sell them and stick in a small sata drive, but I just bought a controller level 6 license and it sits on an IDE drive, that was only a couple months ago and I dont want to buy a new one again

Maybe email support@mikrotik.com and see if they will do a licence transfer (doubtful, but worth a try)

nz_monkey on your example above do you actually have a VPLS tunnel terminating on that box?

EDIT: nvm I looked at the pic again, it was right there. I assume it passes a ping test of 1500 bytes.

Yes. It actually passes 1504 byte packets with no vlan tag, or 1500 byte inside vlans.

We are connecting two datacentres with this, running constantly at between 150mbit and 1gbit and as you can see its been pretty reliable

Ok so now Im confused again. BTW the dell r210 is very nice!

But I bench tested all of this in the lab, the onboard broadcom chipsets would set the MTU properly and then MPLS MTU 1526 worked fine, was happy.

But Then I went to put it into production and it didnt work!

I failed to realize that the interface I was running MPLS/LDP over was a VLAN, and it seemed to be where it would stop passing the large packets.
Now how do I get the VLAN to pass the proper traffic? Same way? Adjust its MTU from 1500 to whatever? So Im adjusting the MTU of the physical interface, THEN the vlan interface?

I dont have a PC to check. But im pretty sure the vlan inherits its L2MTU from the parent interface. But yes i do remember adjusting MTU on the couple of installs we have running MPLS inside a vlan.

The 11th/12th gen Dell servers are really nice, i like them a lot more than HP DL/ML G6/7.

The R210 is pretty basic but its nice and short so fits in comms cabinets. It also has great warranty and runs RouterOS really well.

I dont have a PC to check. But im pretty sure the vlan inherits its L2MTU from the parent interface. But yes i do remember adjusting MTU on the couple of installs we have running MPLS inside a vlan.

Im not sure it does on x86. There is no aformentioned L2MTU anywhere on the parent interface, so Im not sure the VLAN knows what to inherit, I cant see it inheriting the IP MTU value?. I think I will have to build a test on the bench again. Lol.

I CAN see that it does indeed inherit the parent interface L2MTU if its on an actual routerboard.

1512

This is a really strange behavior (been searched for errors regarding to this for some hour today).

Thes is somehow a "fix" or hack. to make Things working...

ros code

/ip firewall mangle
add action=change-mss chain=output disabled=no new-mss=1400 passthrough=yes \
    protocol=tcp tcp-flags=syn tcp-mss=1401-65535
add action=change-mss chain=forward disabled=no new-mss=1400 passthrough=yes \
    protocol=tcp tcp-flags=syn tcp-mss=1401-65535

(at both ends of vpls tunnel)

This is a really strange behavior (been searched for errors regarding to this for some hour today).

Thes is somehow a "fix" or hack. to make Things working...

ros code

/ip firewall mangle
add action=change-mss chain=output disabled=no new-mss=1400 passthrough=yes \
    protocol=tcp tcp-flags=syn tcp-mss=1401-65535
add action=change-mss chain=forward disabled=no new-mss=1400 passthrough=yes \
    protocol=tcp tcp-flags=syn tcp-mss=1401-65535

(at both ends of vpls tunnel)

That is definitely covering up an issue with the underlying network.

Ive since determined that it _was_ my underlying network that was really broken, and why it worked in the first place was a bug (without proof) in Mikrotik that allowed it to work that I believe was fixed in very recent versions (MPLS MTU config problem). After careful analysis of my networks L2MTU, some hardware replacements, and proper MTU config across the board I have yet to see this problem happen again.

This is a really strange behavior (been searched for errors regarding to this for some hour today).

Thes is somehow a "fix" or hack. to make Things working...

ros code

/ip firewall mangle
add action=change-mss chain=output disabled=no new-mss=1400 passthrough=yes \
    protocol=tcp tcp-flags=syn tcp-mss=1401-65535
add action=change-mss chain=forward disabled=no new-mss=1400 passthrough=yes \
    protocol=tcp tcp-flags=syn tcp-mss=1401-65535

(at both ends of vpls tunnel)

i got same issue on vpls over bonding , this mangle actually solved it. but just curious , is there any possible issue if we apply this mangle ? like limited throughput or something else ?

Depends on what type of throughput testing your doing. Any TCP traffic will be chopped into a maximum 1400 byte packet, which increases the bandwidth needed due to TCP's congestion mechanisms and such.

UDP however, no.

Its possible to trim it more, and use 1480 etc. Then it would be less impact to performance.

PS, don't use it to smaller than 1280, then ipv6 will stop working.

@Inssomniak

Glad you finally got your MTUs sorted out. It's one of the trickest things to manage when you are building an MPLS based network. It took me several years to understand the role MTUs played across L2, MPLS and L3 when dealing with advanced transport mediums like Metro Ethernet, Wireless backhaul, Sonet, xDSL, etc. Everything has to be just right

Coming from the service provider world and Cisco based MPLS, i've been extremely impressed with the MPLS capabilities of MikroTik and continue to be an advocate for the use of MikroTik.