[c-nsp] tag-switching mtu question

[David Freedman]

Using the following example:


[ Switch with (Jumbo frame support) ] ---> [ Router ] -- > MPLS network


The router in this case is at a small remote PoP, its a 7200VXR with 
PA-FE interfaces.

A user on the switch wants an xconnect (l2vpn) via one of the VLANs to a 
remote destination.
The user has a requirement to carry an additional two labels of their 
own (8 bytes), both the Jumbo enabled switch and the MPLS core support this.
The user is incapable of using CsC (Carrier supporting carrier) for this 
application.

My question is how to accept the oversized ethernet frames on the Router 
trunk interface, being a PA-FE it is incapable of having its natural MTU 
set, but supports a tag-switching mtu.

I was wondering whether the following configuration was valid:

!
interface FastEthernet0/0
  description to the MPLS core
  tag-switching mtu 1540
  tag-switching ip
!
interface FastEthernet0/1
  description To the jumbo switch
  tag-switching mtu 1540
!
interface FastEthernet0/1.100
  encapsulation dot1q 100
  description Customer Vlan 100
  xconnect 1.2.3.4 vc 100 encapsulation mpls
!


I wouldn't have thought it would be so easy,
assuming it does work , what are the potential pitfalls of doing this ?


Thanks in advance for any responses.

Kind Regards,

David Freedman