Why Do We Need MPLS?!?!
I have enjoyed quite a few book recommendations from Brian Dennis and Brian McGahan over the years. That is why I became very intrigued during a recent Open Lecture Series installment when I heard Brian McGahan state that he read an MPLS book cover to cover and it was “mind blowing”. That text - MPLS-Enabled Applications: Emerging Developments and New Technologies.
One of the first questions we should ask, is why MPLS? What issues can it solve? Well, the first IETF MPLS Working Group Meeting occurred in April of 1997. At that meeting, the engineers identified four main items they sought to address with the new technology:
1. Scalability of network layer routing - this refers to the ability to aggregate forwarding information using labels. This is clearly witnessed today in the most popular application of MPLS technologies - the Layer 3 VPN. I love to watch the amazement on student’s faces when in our bootcamps they build an L3VPN for the first time. They are initially bewildered by the BGP-free core that results, and the fact that no router in the topology requires full route information for all of the prefixes in the scenario. For many students, it is at this moment they embrace the inclusion of MPLS in their lab and written exams. OK, OK, maybe embrace is too strong a word. ;-)
2. Greater flexibility in delivering routing services - this refers to the use of labels to identify traffic that is to receive special service and pathing not based strictly on destination-based forwarding. While not a requirement for R&S track students, this gives rise to DiffServ Aware Traffic Engineering and is a concern for SP students.
3. Increased performance - refers to optimization of network performance through a new, simple paradigm of label-swapping. In his excellent book, MPLS Fundamentals for Cisco Press, Luc De Ghein refers to this as the “bogus benefit” of MPLS. He points out that thanks to technologies like Cisco Express Forwarding, the performance gains with MPLS are negligible. But Minei and Lucek quickly point out in MPLS-Enabled Applications that the performance gains of MPLS should be viewed in a greater context than just individual nodes. When one considers traffic engineering and fast rerouting of traffic in the network as a whole, performance gains thanks to MPLS are certainly significant.
4. Simplify integration of routers with cell switched based technologies - in April of 1997, many networks features a core of ATM switches surrounded by routers that were fully meshed with ATM connections. As we know, full meshing router connections gets out of hand real fast, so the idea was to allow the ATM devices to peer with routers. Today the problem has actually changed. More and more networks feature an MPLS core, with ATM switches interconnected via Layer 2. Now the number of adjacencies for ATM switches is the issue. This has all led to work on Generalized MPLS (GMPLS). This is the idea of a common control plane that covers a vast scope of devices such as routers, ATM switches, SONET/SDH equipment, optical cross-connects, etc. More information on this concept can be found in RFC 3945, Generalized Multi-Protocol Label Switching (GMPLS) Architecture.
For R&S candidates in particular, I think understanding the rationale behind MPLS technology is critical before delving into detailed study. I hope you will join me for more blog posts on key MPLS technologies.