The MPLS-OPS Archive

["Vinay Bannai" <bannai@pacbell.net>]

Actually, I think if the service claims to provide L2VPN (without mucking
around with the payload) then you might need
1518 (original payload + ethernet header + CRC) + 4 (control word) + 2 * 4
(labels) + 14 (ethernet encap header) + 4 bytes (encap CRC)

The MTU can be 1548 bytes.

Vinay Bannai
Luminous Networks

-----Original Message-----
From: sthaug@nethelp.no [mailto:sthaug@nethelp.no]
Sent: Sunday, August 10, 2003 10:18 AM
To: mpls-ops@mplsrc.com
Subject: RE: [MPLS-OPS]: L2 VPN over MPLS


I wrote:

> > The service is L2 VPN (Martini ) so 2 label plus the control word .
>
> And if you want to use your L2VPN to transport Ethernet, the Ethernet
> frame itself may be up to 1518 bytes (14 bytes header + 1500 bytes data
> + 4 bytes CRC). Thus you need a minimum MTU of 1518 + 2*4 (labels) + 4
> (control word) = 1530 bytes.
>
> This is easy enough to verify if you have a suitable lab setup with
> for instance POS links (Cisco default MTU 4470). Setting your MPLS
> MTU ("tag-switching mtu" in Cisco speak) to 1530 bytes will let full
> sized Ethernet frames through with EoMPLS - setting it to 1526 will
> block them.

I need to correct myself here: The CRC of the original Ethernet frame
is not included in the EoMPLS frame. However, my original Ethernet
frame also included a 4 byte VLAN tag - so the calculation is still
correct (conveniently :-).

Steinar Haug, Nethelp consulting, sthaug@nethelp.no

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