I am beating my head about trying to be sure I am understanding this. Can anybody guide me on what MTUs my interfaces need to support to transport PPPoE clients to a central PPPoE server via MPLS/VPLS? PPPoE MTU could be either 1480 or 1492 - not settled as yet.
Watch this:
http://tiktube.com/video/KHhE3aEKdDoDEJmpIHJwFzJsDlIoFqoq=
Thre presentation discusses MTU in detail.
Starting with a 1480 MTU on the PPPoE interface you can calculate the required L2 MTU / frame size as follows:
The 1480 MTU on the PPPoE interface will include all of the TCP/IP elements such as IP header etc. If we assume that we need 2 labels total for MPLS/VPLS we would add:
PPP 2 bytes
PPPoE 6 bytes
Ethernet 14 bytes
VPLS/PW 4 bytes
MPLS 4 bytes
Ethernet 14 bytes
This would be an overall frame size of 1524 bytes. The L2 MTU excludes the final Ethernet header so it is 1510 with the assumptions above.
With the same assumptions, a 1492 MTU PPPoE interface would require a L2 MTU of 1522 and would have an overall frame size of 1536.
Note that if you are using VLANs you would also have to consider the 802.1q requirements.
Thanks! Seeing it laid out clearly for my situation helped me a lot!