The MPLS WG Archive

[David Charlap <David.Charlap@marconi.com>]

"Fu, Jingsong" wrote:
> 
> When encasulating a label frame ove ethernet, 0x8847/0x8848 is
> uesed in ethertype to indicate that a frame is MPLS packet. Where
> should be the original ethertype, or is it removed?

The original Ethertype is lost.  The egress router (which pops the label
and transmits an unlabeled packet) is responsible for providing one.

Note that preserving an ethertype is not useful here.  The labeled
packet may leave an Ethernet and get forwarded onto something else.  The
egress router must provide an appropriate layer-2 encapsulation for the
medium it transmits the unlabeled packet on.

Since the egress router must know the type of the MPLS payload in order
to function over any medium, it should know the appropriate ethertype if
the egress interface is Ethernet.

How the egress router learns the type will depend on how the LSP was set
up.  RSVP-TE sends the type in the label-request object (using
ethertypes).  LDP assumes that everything is IP.  I don't know how
CR-LDP does this.

-- David