since i update MT RB5009 from 17.4.2 to 17.4.3
i have an issue with mtu ( invalid mtu 1492 on pppoe-out1 from fe80::200:5eff:fe00:103)
What is the causing of this problem?
with v17.14.2 i didn't get this issue

PPPoE server may (erroneously) advertise incorrect MTU (in your case it seems as a viable number, sometimes the value is crazily high). At some version, ROS started to log such advetisements, but it otherwise ignores it.

In your particular case you may want to try setting 1492 as MTU on your pppoe-out1 interface ... it might just work (it mostly depends on implementation on ISP side).

17.14.2

???????????


Ask your provider to provide correct MTU of 1500 on pppoe.

example of an ISP that knows and does its job well code

C:\Windows>mtupath -4 google.it

MTU path scan to google.it (142.251.143.163), ttl=64, limit=48
# 16 processing - best MSS 1472 (estimated MTU 1500) [pPPPPpPppPpppppp]
# 01 nearest minimum MTU on local interface

        #1 MSS IN RANGE     1 <==  1471 ==>  1472
        #2 MSS EXCEEDED  1473 <== 14911 ==> 16384

C:\Windows>mtupath -6 google.it

MTU path scan to google.it (2a004002:411::2003), ttl=64, limit=48
# 16 processing - best MSS 1452 (estimated MTU 1500) [uUUUUuUuuUuUuUuu]
# 01 nearest minimum MTU on local interface

        #1 MSS IN RANGE     1 <==  1451 ==>  1452
        #2 MSS EXCEEDED  1453 <== 14931 ==> 16384

Example on windows.
https://www.iea-software.com/products/mtupath/

I can't change actual MTU and i make MTU test and i found 1430 is the best value but i don't know where to put it in mikrotik Max MTU, Max MRU or MRRU

You set the MTU value for the pppoe-out interface by putting the value in both the Max MTU and Max MRU boxes. Try putting 1492 there. Or if your ISP supports RFC 4638 you can put 1500 in both input boxes.


When you test the MTU with the ping command, please note that the value that you place next to "-l" is the MTU to be tested MINUS 28. So to test if MTU 1500 works, you have to test with


if you write -l 1492, you are actually trying a packet size of 1492 + 28 = 1520 bytes. When you tested with -l 1430, you were testing for MTU 1458 and it worked because it's less than 1460.

Regarding the error message from your original post:

radvd: invalid mtu 1492 on pppoe-out1 from fe80::200:5eff:fe00:103

fe80::200:5eff:fe00:103 is the IPv6 link local address of the ISP end of the PPPoE connection. It is performing IPv6 router advertisement (RA), and is also including the MTU 1492 value in the RA message. RADVD (Router Advertisement Daemon) on your RouterOS router is listening to those messages, because it probably either has "Accept Router Advertisements" set to Yes or "IPv6 Forward" unchecked in this dialog:


Your router saw the advertised MTU 1492 in the RA message, which doesn't match the current MTU of 1460 of the pppoe-out1 interface, and wrote the warning in the log. And because your ISP advertises an MTU of 1492 in its IPv6 RA, it means that MTU 1492 should be supported on pppoe-out1 too. So you should try to put 1492 in both Max MTU and Max MRU as I wrote in the previous post.