MPLS is a Data Link Layer# #Wide Area Network (WAN) backbone technology that delivers WAN and MAN services. It is one of the alternative to replace the legacy Frame Relay, Asynchronous Transfer Mode (ATM) and Point-to-Point/Leased Line as it can provide their functionalities in a multiservice form. Though it can be used to aggregate and transport Frame Relay, ATM, and IP-based traffic, the former two could be done using Any Transport over MPLS (AToM) technology. It also provides the Virtual Private Network (VPN)# technology.
Architecturally, MPLS is divided into Control plane and Data plane. The former is accountable for the exchange of routing information, including labels, between adjacent devices. The latter will handle the forwarding operations.
There are three primary equipment types within a MPLS network: **customer premises equipment (CPE), provider edge (PE), and provider routers (P). CPE will be on the customer site, and all the traffic leaving the local site is routed through it. PE, could be a high-end switch or router, is at the entry point of the service provider’s network. P locates in the core of the network, which will be responsible on forwarding packets based on the labelling.
Within the WAN that implements MPLS, all packets will be assigned a label by a Router (just before entering the MPLS network), based on IP destinations, IP sources, Quality of Service (QoS)#, VPN membership or engineered specific routes, in order to differentiate from each other in the MPLS network. The packet will then be forwarded to the MPLS core routers, and they will apply appropriate services and forward it based on its label. As the packet reached the destination edge of the MPLS network, the label will be removed from the packet. They are usually 4 bytes (32 bits) long.
Note: The router that is allowed to add MPLS label to the packets is also the only router that needs to process the entire packet header.
Note: Routers know the relationship between a label and path through the network via Forwarding Equivalence Classes (FEC)#.
Because of the rising concerns of the Fabrication# of the label, many entities have been opted for IP Security (IPsec)# to enhance the traffic security while traversing MPLS networks.
Several enhancements are done in MPLS in order to improve the overall traffic flow in the network. MPLS network chooses the shortest path for a traffic flow based on the size and priority of a traffic flow. Therefore, it can honour IP Quality of Service (QoS)#.