Upgraded to 6.37 this morning and noticed that even with max-mtu=1500 on the pppoe-client interface that the MTU changes to 1480 after between 3-5 seconds (seen using monitor command on interface). Thought it was a problem with 6.37 so downgraded back to 6.36.3 but that exhibited the same problem. …

The Mtu seems auto-adjusted by the discovery mechanism. If interfaces or tunnels You use does not allow 1500, You must set MRRU. Mikrotik say to leave unchanged mtu and mru to defaults See also http://forum.mikrotik.com/t/eoip-tunnel-mtu-and-pppoe-serverr/87493/1

This large LCP request is RFC4638 compliant. It is sent to validate that your link is indeed capable of transferring the large (larger than ethernet standard) packet. Make sure your BRAS supports RFC4638 and that your ethernet MTU is large enough along the path between router and BRAS. (so on y…

The interface MTU on the Mikrotik and DSL modem (Vigor 130) is set to 1508 bytes so the path to the BRAS should be capable of sending > 1492 byte packets. I see in the PPPoE discovery that the Mikrotik advertises PPP-Max-Payload for RFC 4638 negotiation (05dc == 1500): Frame 1: 38 bytes on wire (3…

I have a similar setup, MikroTik router, Draytek Vigor 130 VDSL2 modem, but my provider uses a Juniper BRAS. It works OK here: /interface pppoe-client print detail Flags: X - disabled, R - running 0 R ;;; PPPoE-link XS4ALL-internet name=“pppoe-xs4all-inet” max-mtu=1500 max-mru=1500 mrru=1500 …

The VLAN insertion is done on the Vigor (not on the Mikrotik). I’ve tried with larger MTU (1520 which is the maximum the Vigor supports) with no change to behaviour. I’m sure you are correct - the path between me and the exchange is probably short 4 bytes (most likely a VLAN tag) but getting BT to …

Are you sure that BT support the 1500 byte MTU? You can try with the Vigor making the PPPoE connection and see if that works at 1500 byte MTU. When you do not want IPv6 that may even be the entire solution. (set the Vigor to have the MikroTik as DMZ host with “Active True IP”)

Yeah, BT do support 1500 byte MTU. It’s worked in the past - not sure when it stopped - I only noticed when I upgraded to 6.37 but was the same on downgrade. Most likely, BT have changed something in their network.

Today its 27 feb 2025 and still this problem persists with mikrotik what a joke!!!

1500 Byte MTU with PPPoE and RFC4638 works just fine. The problems described 10 years ago in this thread where probably not even related to ROS itself.

lol… the reason it drops to 1480 is because MikroTik sends a message out at the requested MTU and if it does not receive a response it will lower the MTU as that packet got silently discarded and therefore that MTU is not supported. It doesn’t keep on sending 1 byte at a time to lower it. So often y…

PPPoE and MTU > 1488

RouterOS General

himurae February 28, 2025, 8:37am 12

Please see my post my isp supports RFC 4638 also l2mtu doesnt work on chr/x86 version but vyos/opnsense/ipfire all work on same setup with 1492 mtu except mikrotik why???

http://forum.mikrotik.com/t/pppoe-mtu-always-defaults-to-1480-instead-of-1492/181991/1