How Does Routing Work in MPLS?
At the surface level, MPLS functionality turns routers into switches by giving traffic a predetermined path to take based on labels. In this sense, MPLS connections are much stronger and more reliable than traditional packet- or circuit-switched connections.
MPLS allows IP packets to be forwarded at OSI layer 2 (data link) switching level without being passed up to layer 3—the network or routing level. Internet protocol (IP) routing sends traffic on a lengthy path with multiple stops, but with MPLS, traffic is given an MPLS label and sent through a label-switched path (LSP) that’s inserted between the layer 2 and layer 3 headers. With this method, routers need only to interpret the MPLS labels, not the full IP address, of traffic.
There are two types of MPLS routers: label edge routers (LERs) for ingress, which label incoming data; and label switch routers (LSRs) for egress, which send labeled data on its way. There are also intermediate LSRs, which correct the data link on which the packet is being sent, if need be.
These routers combine packets with similar characteristics and place them in the same forwarding equivalence class (FEC) so they can be sent down the same LSP after being given the same label. In a corporate context, this can greatly reduce the types of traffic on a network layer, which helps reduce latency.
What Is MPLS Used For?
MPLS works by creating point-to-point paths that act as circuit-switched connections, but deliver layer 3 IP packets. In this sense, it is best for organizations that have remote branch offices in a large number of widespread locations that need data center access. At least, this was what it was best used for when workers were still going to offices.
Many organizations would run MPLS within a virtual private network (VPN), either to create the aforementioned point-to-point path or for a private LAN service. It provided diversity in that it could be deployed irrespective of what underlying network protocols—ethernet, SDH, ATM, etc.—were being used. The forwarding decision would be unaffected because, again, it only mattered whether the label matched.
What Does MPLS Consist Of?
MPLS has four unique mechanisms that improve connection quality and stability:
- The label: There would be no MPLS without a label attached to each connection.
- Traffic class field: This component prioritizes packets by QoS.
- Bottom-of-stack flag: This tells an egress router that there are no further labels to be put on this connection.
- Time-to-live: This refers to the number of hops data can make before being discarded.
These four subparts make MPLS easier to manage than other less rigid methods of traffic forwarding. It can be likened to tracking a shipment or a package based on a tracking number rather than having to guess, for example, the license plate or VIN of the delivery truck.