MPLS is growing in popularity as a WAN technology used by service providers. This growth in popularity is due in part to MPLS's capability to support multiple protocols on the same network - for example, an MPLS network can accommodate users connecting via Frame Relay or ATM on the same MPLS backbone - and MPLs capability to perform traffic engineering (which allows traffic to be dynamically within an MPLS cloud, based on current load conditions of specific links and availability of alternate paths).
MPLS inserts a 32-bit header between Layer 2 and Layer 3 headers. Because this header is shimmed between the Layer 2 and Layer 3 headers, it is sometimes referred to as a shim header. Also because the MPLS header resides between the Layer 2 and Layer 3 headers, MPLS is considered to be a Layer 2 1/2 technology.
The 32-bit header contains a 20-bit label that is used to make forwarding decisions within an MPLS cloud. The process of routing MPLS frames through an MPLS cloud is referred to as label switching.
Figure 2-3 shows a sample MPLS network. Table 2-1 defines the various MPLS network elements shown in the figure.
| Element | Description |
| CPE | A customer premises equipment (CPE) device resides at a customer site. A router, for example, could be CPE that connects a customer with an MPLS service. |
| CE | A customer edge (CE) router is a customer router that provides the connectivity between the customer network and the service provider network. CE routers use static or dynamic routing protocols but do not run MPLS. The MPLS function occurs in the service provider network. |
| ELSR | An edge label switch router (ELSR) resides at the edge of an MPLS service provider's cloud and interconnects a service provider to one or more customers. |
| PE | A provider edge (PE) router is the MPLS service provider's router that connects to the customer router. PE is another name for ELSR. |
| LSR | A label switch router (LSR) resides as part of a service provider's MPLS cloud and makes frame forwarding decisions based on the labels applied frames. |
| P | A provider (p) router is a service provider internal router that doesn't directly interface with the customer routers. A P router is internal to the service. |
An MPLS frame does not maintain the same label throughout the MPLS cloud. Rather, an LSR receives a frame, examines the label on the frame, makes a forwarding decision based on the label, places a new label on the frame, and forwards the frame to the next LSR. This process of label switching is more efficient than routing based on Layer 3 IP addresses. The customer using a provider's network and the MPLS transport across that network is not normally aware of the details of the exact MPLS forwarding that is done by the service provider.
About The Author
Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, StormWind.com. Anthony is also a VMware Certified Professional.
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