MPLS Stupid Question

CChN

So if MPLS is a layer 2.5 protocol, how exactly, does it encapsulate Ethernet frames which reside at layer 2?

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Comments

bighornsheep

MPLS doesn't encapsulate ethernet frames. In MPLS frame mode, MPLS packets are encapsulated by Ethernet frames.

It's called "layer 2.5" because it can forward layer 3 information such as route advertisements and IP packets end-to-end, but it behaves like a layer 2 protocol in that the packets are "switched" from node to node, and the MPLS header information is only significant on a per-hop basis, which is not the typical layer 3 characteristic.

redwarrior

I don't think that's a stupid question at all and I also think it goes to the heart of why MPLS is so awesome.

MPLS adds labels to packets, frames, or cells if you're using ATM. I guess this could be considered similar to how 802.1q inserts a tag into a frame, but in this case, the label is slapped on the Front end of the datagram. Essentially, the goal of this is to make it not matter at all within the MPLS network what you're using for layer 2 connectivity. The MPLS switches within the MPLS network could care less if whatever is behind the label is a cell, a frame, or an entire IP packet so long as the label tells it where to go.

I believe they call MPLS a layer 2.5 technology because you can use it to label frames and cells, yet not have to encapsulate them entirely in a packet. In this way, you can use it to move layer 2 datagrams through a provider network without having to change them into a layer 3 format. I know adding MPLS labels is called encapsulation and removing them is called decapsulation, but to me, it seems less inclusive than say, encapsulating an ethernet frame into an IP packet.

Any MPLS gurus out there want to chime in? ISCW was a few months ago, so I could be patchy here.

networker050184

bighornsheep wrote: »

MPLS doesn't encapsulate ethernet frames.

Are you sure about that? Google Ethernet over MPLS.

CChN

http://www.cisco.com/en/US/docs/optical/15000r4_6/ethernet/454/guide/eompls.pdf

"EoMPLS provides a tunneling mechanism for Ethernet traffic through an MPLS-enabled Layer 3 core. It encapsulates Ethernet protocol data units (PDUs) inside MPLS packets and using label stacking forwards them across the MPLS network."

bighornsheep

Ok, I was wrong; and you now have your answer. Next time, don't call it a stupid question if you're looking for a guru answer. Back to