I my network I see a very strange behaivour.
I have a chain of 10 device, six are Mikrotik and 4 Cisco.
First is Cisco, in the middle are Mikrotik and at the end Cisco..
Some Vpls beetween Cisco and Cisco work, but Mtu that have on encapsulated traffic isn’t 1500 but 1498.
The Vpls are correctly configured and mtu conf are coehrent.
The only strange things is that 1 mirkotik in the middle of network have problem with MTU.
This CCR1072 have 2 interface: 1 to Cisco, 1 to another Mikrotik, and also bridge interface for loopback purpose.
the mtu that I have configured is 1546 but if I try to ping the ip address configured on himself interface receive the error on MTU.
The real mtu are 1500, but on the inerface I have set 1546!
Note that there are 3 different MTU values that you can configure
L3 MTU on physical interface - this one will not affect MTU of VPLS traffic
MPLS MTU - by default it is 1508 and will not be able to pass unfragmented VPLS
L2MTU - this one determines how big Layer2 frame can be sent over the interface.
L3 mtu, all interface have 1546
L2 mtu, all interface have 1580
Mpls mtu have as mtu 1546.
We create a vpls tunnel from remote end that must encapsulate double tagged traffic.
I set up tunnel with advertised mtu at 1508 but when try to ping from remote end into vpls tunnel
I reach only 1496.
So, I think that I see this problem at L3 layer, mpls layer can be have also some problem of fragmentation packet
that generate drops on final tunnels and I cant’ have full mtu also with double tagged traffic.
We use L3 and mpls connection on our dark fiber network to create interconnection (transport network).
Over this network we create vpls tunnel for overlay services.
I see that mismatch at L3 side on transport network, but also on services deployed inside a tunnel we see issue with mtu,
as the problem with L3 and mpls mtu reflects on vpls tunnel.