Hello
Why setting maximum supported MTU is not used as a default setup?
I’m talking only about my own network, not regarding connection to any other networks, to avoid MTU problem that occurs in case of using vlan\Vxlan\EoIP\VPN?
Considering, that all devises of a subject-network support enlarged MTU?
Lets consider using MTU=2000 as a default setup
Industry standard MTU is 1500. When you want something else, you will have to configure it.
+10k
(Without considering that, if you don’t configure the internal network correctly, you will create nothing but outgoing problems…)
What problems, I wander?)
I have an EoIP links for PPPoE, sometimes over vlans, Im using mikrotik devises 99%. What can go wrong?
In the beginning i've said -"Considering, that all devises of a subject-network support enlarged MTU"
if you don’t understand what @pe1chl wrote to you,
in short, it has to be like this and it has to stay like this,
any other consideration is useless.
If you need a different value, you can change it.
Simple and precise.
You asking about “not used as a default setup” – not what may be best in your environment.
Max MTU on the internet is 1500. https://www.rfc-editor.org/rfc/rfc894
I’m not going to change any outgoing interface mtu, IT IS MENTIONED THAT:
1 All devices support enlarged MTU (All are mikrotik)
2 MTU’s that are going to be changed ARE INSIDE of the controlled network
the question was - “why not just change MTU INSIDE THE NETWORK” to avoid possible problems with fragmentation if anything will change and everything should be done once again (added qinq\VxLAN\VPN\EoIP)
All your answers are not concerning the question.
It is not even clear what your question is, let alone what problem you have.
RouterOS is flexible. Configure it as you like!
Ah, no? and what is “the question”?
Topic tite: “max-MTU Question”
Is the question?
Read reply on post #2, is the perfect answer.
If it is a question,
read reply on post #2, is the perfect answer.
Read reply on post #2, is the perfect answer.
Read reply on post #2, is the perfect answer.
Since 3 people didn’t understand anything else from what you have wrote, then enlighten us by writing better what the question would be…
Max L3 MTU should default to 1500 to minimise idiots from sending jumbo frames to the public internet.
However, for L2 MTU, there’s no reason for it to not be maxed out.
Even if device A<>Devic B have 9000<>9216 L2 MTU, it doesn’t break anything if L3 MTU is equivalent on both example 1500<>1500 or 8000<>8000.
For networks that I have been hired to design, fix or clean up, we always enable jumbo frames intra-AS. So L2 is maxed on all equipment on all vendors. L3 MTU intra-AS is set to 9000 wherever possible.
If some old hardware supports only 3000 MTU, we use 3000 L3 MTU for that particular link between device A and B, where B only does 3000.
PMTUD does its job and correctly sends packets/frames in correct size based on the path. We’ve never had any fragmentation in the networks I deployed large MTU on.
I learnt how to MTU correctly with jumbo frames from this article:
http://forum.mikrotik.com/t/how-to-edge-router-and-bng-optimization-for-isps/150007/1
I’ve also deployed jumbo frames for WAN/Internet when upstream supports it, but generally I try to avoid it as when they use MPLS, and they are stupid, the failover path is below 1400 MTU, so the jumbo frames on the WAN port then breaks PMTUD. So for WAN/Internet, I would use 1500, unless you use custom script to check MTU every 10 seconds and adjust interface MTU based on results.
DarkNate thank you
Everyone else - get lost please
You’re in the “Beginner Basics” forum. And no “beginner” should be messing with MTU. Defaults are fine.
I think you should learn to write your questions more clearly – you’d get better answers. No need to be rude.
MTU is a primary school level computer networking concept. It is “basics” in every sense of the word. One needs to have grown up illiterate without formal education to think otherwise.
If we mean PMTUD, then sure that’s more advanced.
Yeah I assumed a toddler, in @DarkNate tautology.
You are sooooo funny trying to look important and advanced
That is why you think, that everyone is a beginner. In the first post i’ve mentioned that I just try to understand why everyone calculates MTU instead of just setting 2000 for example. I’ve wrote about vlans,Vxlans, EoIPs maybe because I work with it?
It was just a question - “why not?”
It is in the “Beginner Basics” not because I’m a beginner, It is hear because It fits fine here. What other forum fits better? - RouterOS general discussion? Forwarding Protocols?
So please get lost and do something useful
DarkNate thanks ones again
Can you please explain again what your question is and what answer you are expecting?
Because even after re-reading it, that is not at all clear to me.
Default MTU is 1500. That is not going to change. But when you want 2000, you can set that.
Why is that not OK for you?
The point is - Why calculating and even think of it at all, if there is a simple way? Why this approach isn’t used as a default way of handling MTU?
It can bring a lot of problems if you are using any VPN, or vlan, why not set it to maximum as DarkNate does and just forget about it?
One of reasons: different devices support different maximum L2 packet sizes. So different devices will have to use different L3 MTU. But that L2 maximum packet size is true also for switches which may be in path between L3 devices (and switches, being L2 devices, don’t do ICMP). Next: PMTU only works between L3 devices and ICMP reply only gets back from device which first successfully receives large packet and can’t forward it because egress interface has smaller (L3!) MTU. And the whole process doesn’t work inside same L2 network, in such network L3 MTU on all devices has to be the same (device can’t send back ICMP for too large packet if it can’t receive it first).
And tgen, 1500 bytes has been industry standard since ethernet was concieved in late 1970ies. Industry standards don’t get changed at a whim of one user.
So back to answer #2 above.
If you write VPN, since I doubt you mean VPN for Local LAN, obviously L3 MTU size can’t be more than 1500, for what already DarkNate wrote:
At this point is completely useless have 2000 (or 9000) as L2 MTU on WAN interface.
Since default MTU of 1500 is valid only for L3 layer (usually MikroTik devices have 1580 or 1588 as L2 default MTU)…
Not all peripherals have the option to have 9000 or even 2000, it depends on the model and brands…
So it is obvious that by buying a MikroTik device that supports 9000 on L2, it is not said that then all the other peripherals will line up,
so it’s up to you to configure them, changing the defaults from 158x on L2 to the maximum supported,
AND THEN correctly set the MTUs of the L3 which obviously have to be calculated.
So the calculations are always there, and the MTU defaults for L3 are always 1500, and for L2 it’s usually 1500 + L3 headers
If your connection on ISP is, for example, by pppoe, if your ISP do not do something to have a full MTU vs Internet of 1500, you usually have less (ex. 1492)
And you can not do anything about that.
If you use VPN (or others) on that pppoe, you have to calc the correct VPN L3 MTU size (1492 - VPN overhead) or you do fragmentation.
Etc. etc. etc.
So, still valid reply on post #2 (and also what DarkNate wrote)
MTU of 9000 is introduced only with 1Gbps interfaces, and is called “jumbo frames”.
It is only practical to use on a very limited local network like SAN or NAS network between fixed hosts and storage, usually in a data center scenario.
You cannot mix the same MTU on different hosts on the same network, because the hosts using the default 1500 byte MTU will normally not be able to RECEIVE a packet of 9000 bytes from another host.
So a local network with MTU much larger than 1500 is normally not practical, and even the margin above 1500 can be quite thin. The L2MTU can be like 1508 or 1512.