Talk:MPLS-TP

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Layers

Somebody please clarify:

The article says: reliable packet-based technology

but then says: low cost L2 technology

Packet is layer three so how can it be both?

Answer:

L2 and L3 refers to the OSI layer definition dating back to the early 80s. This definition classified clearly all the data communication functions into 7 layers. Later this thorough definition was maybe considered an overkill, and fewer definition remained, and less rigorously. In practice in IETF L3 is IP and L2 is Ethernet, while MPLS sometimes was defined as an L2.5, with language abuse IMHO.

In the old OSI definition, L3 is Network, which means multiple addressing and relaying. MPLS and MPLS-TP as profile can perform both addressing and relaying. The fact that MPLS is a connection oriented technology is irrelevant, given that the old X.25 network worked much the same as it does, and was explicitly classified as L3 in OSI.

Again in OSI, frame delineation (e.g., HDLC flags, Ethernet preambles and ATM cell hunting) pertain to L2 or Data Link layer. Lacking all of this, MPLS and MPLS-TP must rely on a lower L2 layer.

On the other hand, MPLS-TP is expected to provide a support to IP, the latter (IP) as an L3 upper layer, hence its attempt to classify it as an L2.

My conclusion: if we apply rigorously the old OSI definition, MPLS and MPLS-TP are L3 technologies, but there is a trend to consider them as L2. If we accept to call it an L2, we must anyway admit the presence of a further supporting L2. —Preceding unsigned comment added by GiovanniFiaschi (talk • contribs) 09:37, 22 October 2009 (UTC)[reply]

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