Re: [mpls] Question about 0x8848 Ethertype

[Eric Rosen <erosen at cisco.com>]

Nitin's interpretation matches the intention of the text.

If the ethertype of an ethernet frame is 8848:

- The top label is to be looked up an interface-specific label space that is
  populated by the context labels in use on the LAN.  Each such label must
  uniquely correspond to an LSR on the LAN.

- As a result of making this lookup, you identify a second label space, one
  that is specific to the LSR that transmitted the frame.  This label space
  is populated by the labels that have been upstream-assigned by that LSR.

If the ethertype is 8847, the top label is an ordinary downstream-assigned
label, looked up in a label space populated by the downstream-assigned
labels of the LSR that receives the frame.

The above mentions three different label spaces; all label uniqueness
requirements are relative to a particular label space.

> How will a node receiving a unicast packet can identify whether the top
> label is upstream assigned or downstream assigned?

Per the standards, there is no way of using upstream-assigned labels in
unicast ethernet frames.