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RE: L2 VPN over MPLS

  • From: sthaug@nethelp.no
  • Date: Sun, 10 Aug 2003 19:18:03 +0200
  • Resent-Date: Sun, 10 Aug 2003 13:41:44 -0400
  • To: mpls-ops@mplsrc.com

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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