So MPLS is one of the many ways to connect two or more distant sites. In other words, it is some kind of a "WAN solution".

But what is the "WAN solution" for MPLS?

Take for example the following basic MPLS diagram. This one is from the book Computer Networking: A Top-Down Approach.

enter image description here

The grey routers are standard routers, while the blue ones are MPLS-capable routers. The black lines are, obviously, the links between the routers.

I understand how MPLS basically works. But how do we connect two distant MPLS routers? (Don't tell me those routers are on the same site.)

T1 and SONET are dead, as one of the top responders here in NESE always say, so I don't think those black lines are T1 nor SONET. DSL and GPON are used in access networks, so I also don't think those are used between such core routers.

What is the WAN solution for MPLS?

  • 1
    T1 and SONET aren't entirely dead, but they certainly have fallen out of favor. As has been said, everywhere, ethernet is everything, everywhere.
    – Ricky
    Commented Mar 1, 2021 at 5:34
  • Did any answer help you? if so, you should accept the answer so that the question does not keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Commented Dec 23, 2021 at 16:42

3 Answers 3


MPLS is a "layer 2.5" protocol -- technically a L2, but nothing natively carries MPLS on the physical layer, it's always wrapped in some other L2 (eg. ethernet.) As such, it can ride over just about any L1 -- T1, T3, OC-x, 10Mbps-400Gbps ethernet, etc. Unless it's your network, you don't know what the provider is using, but it's a safe bet any modern network will be mostly ethernet.

Service providers rarely (never?) extend their MPLS domain beyond their own network -- MPLS stays within a single autonomous system. A customer will not get to tag their own traffic. However, the customer's MPLS tagged traffic can be transported across a service provider network; the SP adds their tag to whatever they receive, switch the traffic across their MPLS fabric, and remove their tag at the customer's other site(s). [this isn't in your example.]


That would be Service Provider's Core Network, so almost all of the Network runs over Fiber Optic Cables between their Central Offices. If remote end is at a location where Fiber Optic Cables cannot run directly, Service Providers may use a different intermediary for the transit traffic.


Ethernet has vastly surpassed competing WAN technologies for performance and cost. On fiber, distances of up to 40 km can be covered using standardized PHYs, with components for up to 80 km being available in bulk. Beyond that, optronic regeneration or direct optical amplifiers (EDFA) can be used.

MPLS usually offers transparent L2 linking, so passing 802.1Q tags for a client requires QinQ aka 802.1ad double tagging.

  • So bottomline, you are saying that those black lines are ethernet over fiber, right?
    – Noob_Guy
    Commented Feb 28, 2021 at 20:12
  • Most of them, yes.
    – Zac67
    Commented Feb 28, 2021 at 21:15

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