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Fix MTU problem on PPPoE link by Mike Everest

Q: Hi, I purchased 2 Mikrotik RB750s a while ago, in July and I have not spent a huge amount of time getting them working but have had some difficulty when I have sat down to use them.

I have had no problems with using the router as a straight router on an ethernet network. So I can set a DHCP enabled public interface or a static IP and it just works.

I have an ADSL 8Mb/384Kb connection at home through Exetel, its a residential plan and cannot seem to get the combination right to get it working. I can see in Winbox and the terminal (SSH or from Winbox) that it is connected and has an IP, but I cannot get internet traffic to work, or if it does, its very spontaneous and unreliable. Not really what I was after, obviously.

Now if I am simply not applying a setting thats fine, I would just like some assistance knowing what that setting is. I have tried configuring through both the web interface and Winbox and have yet to get the settings right. I have scoured the forums on Mikrotiks website to not much avail and have even search through Exetel and Whirlpool forums for some help.

I have seen a couple of people mention MTU numbers in relation to Exetel, but I am loath to fiddle with a standard as it may only be right for one network. I wish to get these devices working as I want to push them to my clients as a high powered device with low cost.

A: I am assuming that your problem is related to using the router as the pppoe client connected to a modem set up in bridge mode?


It is common, as I guess you have already identified, for MTU issues to cause some problems with some ISPs, and this is related to the way that the ISP has implemented the customer network.


You can confirm whether this is a contributing factor by using the ping tool in winbox (click ‘tools’ then selecty ‘ping’).  On the ‘advanced’ tab of ping tool, enable ‘do not fragment’ and make the packet size 1500.  If ping works ok, then MTU is probably not an issue.  If the result is time-out or ‘fragmentation-required’ then reduce the calue of packet size until pings work ok.  Sometimes just 1480 is enough – other times as low as 1300 is needed.


You should find the largest MTU size that will work with your connection rather than some arbitrary small number that is known to work because reducing the MTU size will also reduce your maximum (upload) throughput.  When you have discovered the optimum value, set the outbound MTU using the following shell command:


/ip firewall mangle add out-interface=pppoe-out protocol=tcp tcp-flags=syn action=change-mss new-mss=1460 chain=forward


You will need to substitute your optimum MTU value for ‘new-mss’ and substitute your PPPoE client interface name for the ‘out-interface’ value.


Feel free to get back to me for further assistance or advice if needed!

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