MTU vs bandwidth?

RouterOS General
1

Hello, We have RB2011UiAS.

Our local ISP gave us very fast Internet connection 300down/300up.

When I connect ethernet cable from ISP right to my laptop and set PPPoE connection in Network Settings I get 300/300 with no problem.

But when I connect this cable to Mikrotik, then to switch (1Gbps) and from switch to my laptop I can only receive 200/200.

I don’t know why connection through Mikrotik is slower than directly from ISP to my laptop.

When I first set PPPoE on Mikrotik there was a problem with some websites. So I googled and set two rules

add action=change-mss chain=forward new-mss=1436 out-interface=pppoe-out1 \
    protocol=tcp tcp-flags=syn tcp-mss=1441-65535

add action=change-mss chain=forward in-interface=pppoe-out1 new-mss=1436 \
    protocol=tcp tcp-flags=syn tcp-mss=1441-65535

With those rules every websites work but maybe because of those rules we have slower connection?

We send very big files so we need this extra 100Mbps.

2

Try set to 1492 all

3

I assume you are doing NAT to get out to the internet?

I have a couple of RB2011s and by using NAT they max-out at 150-200mbit (also I have a bit complicated configuration with vlans, bonding etc - but really simple firewall rules < 10rules).
The bottleneck is the CPU itself since it cannot handle more than that. NAT takes up a lot of resources at such high speeds.

Also set your PPPoE Client interface MTU to 1492 as dadoremix correctly pointed out.

TCP-MSS should be set automatically if you are using ‘default’ or ‘default-encryption’ profiles on the pppoe interface (unless you have changed them).
No need to create your own rules for that. The automatically created rules by Mikrotik work just fine. I haven’t had an issue with those since version 4? I don’t even remember how far back :stuck_out_tongue:

There’s also the solution of fasttrack on the latest versions of Mikrotik but I haven’t tested that myself. You should also check that out.
Here’s more info about fasttrack http://forum.mikrotik.com/t/fasttrack-new-feature-in-6-29/87517/1

4

Yes, I am using NAT and Vlans too.

Right now on my PPPoe-out interface I have:

MAX MTU: 1480
MAX MRU: 1480
MRRU: 1600

Should I only change MAX MTU to 1492 or should I also change the other two options?

5

Yes, change both to 1492.
Don’t set MRRU, you don’t need it (unless you do MLPPP? )

But, since you are using NAT then most likely your bottleneck is the RB2011 itself as its CPU is only 600MHz and cannot handle that much traffic.

You can confirm that by checking the CPU usage when you max out at 200mbit.
If it’s at 100% then there’s your problem.

I guess the only solution without changing hardware would be to try fasttrack.

6

Yup, You are right. WIth 200down or up CPU reaches 100%. I will test Fast Track some day