[mpls] Question about 0x8848 Ethertype

["Shahram Davari" <davari at broadcom.com>]

Hi,

 

RFC 5332 says:

 

"Ethertype 0x8848, formerly known as the "MPLS multicast codepoint", is to be used only when an MPLS packet whose top label is upstream- assigned is carried in a multicast Ethernet frame."

 

It also says:

 

"It is expected that the LSR will follow the procedures of [RFC5331], pushing on two labels, with the topmost label being a "context label" that is the same for all MPLS packets being transmitted by the LSR onto the Ethernet, but with the second label being different for different LSPs"

 

And RFC 5331 defines context label as:

 

"A "context label" is one that identifies a label table in which the label immediately below the context label should be looked up. A context label carried as an outermost label over a particular multi-access subnet/tunnel MUST be unique within the scope of that subnet/tunnel."

 

Considering the above statements we can conclude that:

 

"Multicast MPLS packets with upstream assigned labels encapsulated in Multicast Ethernet frames in a Multi-access media (LAN) require to use 0x8848 as Ethertype and require to use a context label that is unique over that media to identify the label space for the Multicast MPLS packet"

 

The question is since a context label is unique in a LAN and can be immediately identified as an upstream label, then why do we need 0x8847 Ethertype? Looks like we could just use the context label.

 

Am I missing something?

 

Regards,

Shahram