1500 L3 MTU on a Mikrotik PPPoE Server

RouterOS General
1

Hello everyone,

I've done a fair bit of reading on the forum but I'm struggling to get a concrete answer

I'm trying to get 1500 L3 MTU on a PPPoE session.

The service supports 1508 baby jumbo frames, I only ever get as high as 1492.
[admin@pppoe] /interface pppoe-server> print detail
Flags: X - disabled, D - dynamic, R - running
0 DR name="" user="snip" service="pppoe" mtu=1492 mru=1500 remote-address="snip" encoding="" uptime=28m5sMy interfaces are configured for 1508 where PPPoE arrives, L2MTU is already much higher than 1508 too:
Flags: D - dynamic, X - disabled, R - running, S - slave

NAME TYPE ACTUAL-MTU L2MTU MAX-L2MTU MAC-ADDRESS

1 R ;;; PPPoE Service
ether2 ether 1508 1598 4074 D4:CA:6D:2D:2B:C1
11 DR pppoe-in 1492
13 R PPPoE_X vlan 1508 1594 D4:CA:6D:2D:2B:C1
14 RS PPPoE_X_2 vlan 1508 1590 D4:CA:6D:2D:2B:C1
15 RS PPPoE_X_3 vlan 1508 1590 D4:CA:6D:2D:2B:C1
16 RS PPPoE_X_4 vlan 1508 1590 D4:CA:6D:2D:2B:C1
17 RS PPPoE_X_5 vlan 1508 1590 D4:CA:6D:2D:2B:C1
23 R pppoe bridge 1508 1590 D4:CA:6D:2D:2B:C1In:
/interface pppoe-server server> print
max-mtu=1500 max-mru=1500 mrru=disabledSo all is should be? Why do I only get 1492?

2

RIF WIFI:
Typically, the largest Ethernet frame that can be transmitted without fragmentation is 1500 bytes. PPPoE adds another 6 bytes of overhead and PPP field adds two more bytes, leaving 1492 bytes for IP datagram. Therefore max PPPoE MRU and MTU values must not be larger than 1492.
You are talking about using Baby Jumbo Frame, can i see your port cfg ?

3

No support for: https://tools.ietf.org/html/rfc4638 when being a server.

4

/interface pppoe-server server> print
max-mtu=1500 max-mru=1500 mrru

This is not IP mtu, this is pppoe mtu.

IP mtu is pppoe minus 20 bytes.

5

You need max-mtu=1500 max-mru=1500 set on both client and server for RFC4638 to work. If it is only set on the server, the client will still use 1480 or 1492.