MPLS (Multi-protocol Label Switching) is the end result of the efforts to integrate Layer 3 switching, better known as routing, with Layer 2 WAN backbones, primarily ATM.. Even though the IP+ATM paradigm is mostly gone today because of the drastic shift to IP-only networks in the last few years, MPLS retains a number of useful features from Layer 2 technologies. One of the most notable is the ability to send packets across the network through a virtual circuit called Label Switched Path, or LSP, in MPLS terminology.
NOTE: While the Layer 2 virtual circuits are almost always bidirectional (although the traffic contracts in each direction can be different), the LSPs are always unidirectional. If you need bidirectional connectivity between a pair of routers, you have to establish two LSPs.
The LSPs in MPLS networks are usually established based on the contents of IP routing tables in core routers. However, there is nothing that would prevent LSPs being established and used through other means, provided that:
NOTE: The other routers along the LSP do not inspect the packets traversing the LSP and are thus oblivious to their content; they just need to understand the signaling protocol that is used to establish the LSP.
With the necessary infrastructure in place, it was only a matter of time before someone would get the idea to use LSPs to implement MPLS-based traffic engineering -- and the first implementation in Cisco IOS closely followed the introduction of base MPLS (which at that time was called tag switching). The MPLS traffic engineering technology has evolved and matured significantly since then, but the concepts have not changed much since its introduction:
NOTE: The first MPLS TE implementations supported
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only static hop-by-hop definitions. These can still be used in situations where you need a very tight hop-by-hop control over the path the MPLS TE LSP will take or in networks using a routing protocol that does not have MPLS TE extensions.
The tight integration of MPLS traffic engineering with the IP routing protocols provides an important advantage over the traditional Layer 2 WAN networks. In the Layer 2 backbones, the operator had to establish all the virtual circuits across the backbone (using a network management platform or by configuring switched virtual circuits on edge devices), whereas the MPLS TE can automatically augment and enhance the mesh of LSPs already established based on network topology discovered by IP routing protocols. You can thus use MPLS traffic engineering as a short-term measure to relieve the temporary network congestion or as a network core optimization tool without involving the edge routers.
In recent years, MPLS traffic engineering technology (and its implementation) has grown well beyond features offered by traditional WAN networks. For example:
NOTE: Thanks to RSVP-TE functionality, the reservations on the path segments common to old and new LSP are not counted twice.
About the author: Ivan Pepelnjak, CCIE No. 1354, is a 25-year veteran of the networking industry. He has more than 10 years of experience in designing, installing, troubleshooting, and operating large service provider and enterprise WAN and LAN networks and is currently chief technology advisor at NIL Data Communications, focusing on advanced IP-based networks and web technologies. His books include MPLS and VPN Architectures and EIGRP Network Design. You can read his blog here: http://ioshints.blogspot.com/index.html.
