We configured an L2TP server on VyOS and L2TP client on Teltonika router (using FRR 5.0.2).
We configured an explicit MTU of 1454 on VyOS configuration (parsed as max-mtu on AccelPPP config).
So funny, on VyOS server:
13: l2tp1: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1450 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 3 link/ppp
On remote side:
108: l2tp-client_Exo: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1454 qdisc fq_codel state UNKNOWN group default qlen 3 link/ppp inet 10.0.0.245 peer 192.168.255.1/32 scope global l2tp-client_Exo valid_lft forever preferred_lft forever
VyOS (AccelPPP) it's taking -4 while I have configured:
vyos@oobm# show vpn l2tp remote-access mtu mtu 1454
This avoid OSPF being established because we have MTU mismatch, so we are changing MTU of tunnel using "ip link set ... mtu 1454" but sometimes tunnel gets reconnected because we are using LTE on remote sides.