Internet Access

MPLS Introduction


MPLS is Multiprotocol Label Switching, operates at an OSI Model layer that is generally considered to lie between traditional definitions of Layer 2 (Data Link Layer) and Layer 3 (Network Layer), and thus is often referred to as a “Layer 2.5” protocol. It was designed to provide a unified data-carrying service for both circuitbased clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames. MPLS can be a powerful tool for any network. It’s not just for the buzzword compliant or the crazy telco-heads.

What is the Advantage of Label Switching?
Originally, it was intended to reduce IP routing lookups.
• When CIDR was introduced, it had unintended consequences.
• CIDR introduced the concept of “longest prefix matching” for IP routing.
• Longest prefix match lookups have historically been very difficult to do.
• The classic software algorithm for routing lookups was called a PATRICIA trie, which required many memory accesses just to route a single packet.
• Exact matches were comparatively much easier to implement in hardware.
• Most early hardware routing “cheated” by doing the first lookup in software, then did hardware-based exact matching for future packets in the “flow”.
• Label switching (or “tag switching”) lookups use exact matching.
• The idea was to have only the first router do an IP lookup, then all future routes in the network could do exact match “switching” based on a label.
• This would reduce load on the core routers, where high-performance was the most difficult to achieve, and distribute the routing lookups across lower speed edge routers. Modern ASICs have eliminated this issue… Mostly.
• Today, commodity ASICs can do many tens of millions of IP routing lookups per second, relatively cheaply and easily.
• However, they still make up a significant portion of the cost of a router.
• Exact matching is still much cheaper and easier to implement.
• A layer 2 only Ethernet switch (which does exact matching) may be 1/4th the cost and 4x the capacity of a similar device with layer 3 capabilities.
• So why do people still care about MPLS? Three reasons:
• Implementing Traffic-Engineering
• The ability to control where and how traffic is routed on your network, to manage capacity, prioritize different services, and prevent congestion.
• Implementing Multi-Service Networks
• The ability to deliver data transport services, as well as IP routing services, across the same packet-switched network infrastructure.
• Improving network resiliency with MPLS Fast Reroute.