MTU question with L2TPV3 setup

[Parves Ataev]

Hi guys,

I have set up a lab (see below) and wondering if someone can shed some light on it. My task is to understand MTU with pseudowire setup between two routers. Two routers connected via point-to point link (In between is our partner ISP), the routers have public ip addresses - ospf is configured< both routers have loopback interfaces and I can ping loopbacks. The switch between has default MTU which is 1500 (partner ISP). We have a client who needs a L2 link between two location ClientSW1-----ClientSW2. I have setup psuedowire L2tpv3 on routers and everything is working as expected, e.g I have setup two SVI VLAN 100 on ClientSW1 and ClientSW2 switches and can ping successfully, meaning i have L2 link between locations. ClientSW1 and ClientSW2 switches have sytem mtu 1600 configured. My question is about MTU. The interesting thing is when I ping from ClientSW1 with source interface VLAN 100 the other side SVI Vlan IP with df bit set and size 1510, I can ping which is a mystery for me. Why i can ping with df bit set and size 1510 without fragmentation if routers and ISP switch has 1500 MTU?

Many Thanks

[Parves Ataev]

Please see the diagram

[Giuseppe Larosa]

Hello Parves,

the ICMP packet is taken as an ethernet frame with vlan-id 100 and carried within an L2TPv3 packet that adds its own encapsulation.

The resulting L2TPv3 packet is fragmented at IPv4 level on the path to the other endpoint, the other device reassembles the packet and extracts the ethernet frame that was in the payload. The ethernet frame with vlan tag 100 is delivered to CE device.

This is why you see these ping successful.

Hope to help

Giuseppe

[Parves Ataev]

Hi Giuseppe,

Thanks for response. Ok i see the idea, but when i send a ping with size say 1511 (anything higher than then 1510) with df bit set, it does not go through. I am having a trouble to understand where are those 10 extra bytes come from. Just to clarify, the link between routers has default MTU 1500.