Zyxel P870 full bridge to Mikrotik RB1200 MTU size

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Forums › LAN (ethernet/Wifi/routers/Bluetooth) › Zyxel P870 full bridge to Mikrotik RB1200 MTU size

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#111868 17-Nov-2012 17:28

I have a Zyxel P870 connected to VDSL2. With my laptop doing the PPPoE authentication the MTU is measured (using ping -f -l ...) at 1480 bytes and speedtest (speedtest.net) is good, 49/6. However with the RB1200 doing the PPPoE authentication and nat routing to laptop the laptop measures the MTU at 1300 bytes and speedtest is poor, 6/6 at best.

I didn't expect the throughput to drop so dramatically and was wondering if someone could enlighten me as to why the MTU reduces by 180 bytes and resultant fragmentation reduces the throughput dramatically.

Thanks

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#718832 17-Nov-2012 17:49

What do you have the MTU set to in the Mikrotik?

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#718851 17-Nov-2012 18:30

MTU on PPPoE interface is set to 1480. LAN interface MTU is on 1500.

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#718870 17-Nov-2012 19:23

Are you blocking ICMP on the RB1200? If so, don't as this can cause issues with MTU path discovery etc.

To confirm if it is that, contact your ISP and ask them what your negotiated MTU is when using the RB1200.

Came across a similar-ish issue a few weeks ago when someone was using a Draytek 2750 or 2950, (the linux based one), it's mechanism for choosing a MTU is buggy. The fix for that was to edit the ppp.conf I think it was, and disable the auto negotiate feature and set a fixed value.

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#718880 17-Nov-2012 19:52

First off you need to establish what MTU your ISP is using. 1492 is pretty much stock standard for EUBA ADSL2+ and VWS connections so you should be setting your ISP's recommended MTU in the Mikrotik.

I've deployed heaps of Mikrotik's over VDSL2 with Zyxel modems and they work flawlessly.

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#718891 17-Nov-2012 20:14

ISP is Snap. icmp not blocked.

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#718932 17-Nov-2012 23:05

I believe snap! use a MAX MTU of 1456 on their EUBA/VDSL, but might be worth posting in their forum subsection to confirm that.

Still give them a call though and see what value is actually being negotiated, it's easy for them to tell if you can't from your end.

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#718948 17-Nov-2012 23:42

Snap said the value should be 1492. I've asked if they can confirm negotiated value.

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#718995 18-Nov-2012 11:28

Response was that there is no negotiation of MTU value as is set on end device. 1492 being ideal size.

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#719008 18-Nov-2012 12:05

RouterOS doesn't negotiate a MTU (or MSS) for a PPPoE client, it uses the value you enter.

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#719302 19-Nov-2012 09:20

With MTU changed to 1492 the throughput has increased to 16/6. Fragmentation still occurs using ping -f on laptop at 1272 bytes.

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#719989 20-Nov-2012 11:05

After realising that Cisco VPN client was rewriting the MTU value on laptop I removed it. Matching MTU of 1492 or 1500 on both Mikrotik lan and laptop interfaces doesn't really help increase throughput though. Max I can get is 20/8 with 15 ms ping.

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#724930 29-Nov-2012 22:24

So nobody has any idea as to why the nat routing performs so badly, cpu only gets to 4%. Fibre is at the gate and I was thinking that Mikrotik RB1200 was the solution, will have to find another one.

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#724975 30-Nov-2012 05:54

No idea. I've got heaps of Mikrotik's deployed with VDSL2 and have no such issues.

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#729480 10-Dec-2012 08:28

Issue related to use of ether10 as WAN port, http://forum.mikrotik.com/viewtopic.php?f=2&t=61671. Issue resolved but I would consider RB1100AH next time.

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#729858 10-Dec-2012 16:18

Spyware: Issue related to use of ether10 as WAN port, http://forum.mikrotik.com/viewtopic.php?f=2&t=61671. Issue resolved but I would consider RB1100AH next time.

Any Luck in your current situation.. YOu tried 1500 for both LAN and your PPPOE Connection, If you cant turn fragmentation off try setting the frag limit to 64, if you can.

What do you get when you do PPPOE in the modem/rouer, any diffrent, and how long have you got VDSL for.

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