VPN TECHNOLOGIES: Multiprotocol Label Switching (MPLS) Operations

Multiprotocol Label Switching (MPLS) remains a vitally important part of any service provider (SP) networks. MPLS is still growing in popularity in enterprise networks as well, particularly in larger enterprise internetworks. This article will give a brief overview of the core concepts with MPLS, particularly its use for unicast IP forwarding and for MPLS VPNs.

MPLS defines protocols that create a different paradigm for how routers forward packets. Instead of forwarding packets based on the packets’ destination IP address, MPLS defines how routers can forward packets based on an MPLS label. By disassociating the forwarding decision from the destination IP address, MPLS allows forwarding decisions based on other factors, such as traffic engineering, QoS requirements, and the privacy requirements for multiple customers connected to the same MPLS network, while still considering the traditional information learned using routing protocols.

MPLS includes a wide variety of applications, with each application considering one or more of the possible factors that influence the MPLS forwarding decisions. For the purposes of the CCIE Routing and Switching exam, we will list two such applications in the first two major sections of this article:

MPLS Unicast IP Forwarding

MPLS can be used for simple unicast IP forwarding. With MPLS unicast IP forwarding, the MPLS forwarding logic forwards packets based on labels. However, when choosing the interfaces out which to forward the packets, MPLS considers only the routes in the unicast IP routing table, so the end result of using MPLS is that the packet flows over the same path as it would have if MPLS were not used, but all other factors were unchanged.

MPLS unicast IP forwarding does not provide any significant advantages by itself. However, many of the more helpful MPLS applications, such as MPLS Virtual Private Networks (VPN) and MPLS traffic engineering (TE), use MPLS unicast IP forwarding as one part of the MPLS network. So to understand MPLS as you would typically implement it, you need a solid understanding of MPLS in its most basic form: MPLS unicast IP forwarding.

MPLS requires the use of control plane protocols (for example, Open Shortest Path First [OSPF] and Label Distribution Protocol [LDP]) to learn labels, correlate those labels to particular destination prefixes, and build the correct forwarding tables. MPLS also requires a fundamental change to the data plane’s core forwarding logic. This section begins by examining the data plane, which defines the packet-forwarding logic. Following that, this section examines the control plane protocols, particularly LDP, which MPLS routers use to exchange labels for unicast IP prefixes.


One of the most popular MPLS applications is called MPLS Virtual Private Networks (VPN). MPLS VPNs allow a service provider, or even a large enterprise, to offer Layer 3 VPN services. In particular, SPs oftentimes replace older Layer 2 WAN services such as Frame Relay and ATM with an MPLS VPN service. MPLS VPN services enable the possibility for the SP to provide a wide variety of additional services to its customers because MPLS VPNs are aware of the Layer 3 addresses at the customer locations. Additionally, MPLS VPNs can still provide the privacy inherent in Layer 2 WAN services.
MPLS VPNs use MPLS unicast IP forwarding inside the SP’s network, with additional MPLS-aware features at the edge between the provider and the customer. Additionally, MPLS VPNs use MP-BGP to overcome some of the challenges when connecting an IP network to a large number of customer IP internetworks—problems that include the issue of dealing with duplicate IP address spaces with many customers.

Other MPLS Applications

This last relatively short section of the chapter introduces the general idea about the protocols used by several other MPLS applications. To that end, this section introduces and explains the concept of a Forwarding Equivalence Class (FEC) and summarizes the concept of an FEC as used by various MPLS applications.

Frankly, this chapter has already covered all the concepts surrounding the term FEC. However, it is helpful to know the term and the FEC concept as an end to itself, because it helps when comparing various MPLS applications.

Generally speaking, an FEC is a set of packets that receives the same forwarding treatment by a single LSR. For simple MPLS unicast IP forwarding, each IPv4 prefix is an FEC. For MPLS VPNs, each prefix in each VRF is an FEC—making the prefix in VRF-A a different FEC from the prefix in VRF-B. Alternately, with QoS implemented, one FEC might be the set of packets in VRF-A, destined to, with DSCP EF in the packet, and another FEC might be packets in the same VPN, to the same subnet, but with a different DSCP value.

For each FEC, each LSR needs a label, or label stack, to use when forwarding packets in that FEC. By using a unique label or set of labels for each FEC, a router has the ability to assign different forwarding details (outgoing interface and next-hop router.)

Each of the MPLS applications can be compared by focusing on the information used to determine an FEC. For example, MPLS traffic engineering (TE) allows MPLS networks to choose to send some packets over one LSP and other packets over another LSP, based on traffic loading—even though the true end destination might be in the same location. By doing so, SPs can manage the flow of data over their high-speed core networks and prevent the problem of overloading the best route as determined by a routing protocol, while barely using alternate routes. To achieve this function, MPLS TE bases the FEC concept in part on the definition of an MPLS TE tunnel.

You can also compare different MPLS applications by listing the control plane protocols used to learn label information. For example, this chapter explained how MPLS VPN uses both LDP and MP-BGP to exchange label information, whereas other MPLS applications use LDP and something else—or do not even use LDP at all. Table 19-5 lists many of the common MPLS applications, the information that determines an FEC, and the control plane protocol that is used to advertise FEC-to-label bindings.

Table 19-5. Control Protocols Used in Various MPLS Applications

Application FEC (Forwarding Equivalence Class ) Control Protocol Used to Exchange FEC-to-Label Binding
Unicast IP routing Unicast IP routes in the global IP routing table Tag Distribution Protocol (TDP) or Label Distribution Protocol (LDP)
Multicast IP routing Multicast routes in the global multicast IP routing table PIM version 2 extensions
VPN Unicast IP routes in the per-VRF routing table MP-BGP
Traffic engineering MPLS TE tunnels (configured) RSVP or CR-LDP
MPLS QoS IP routing table and the ToS byte Extensions to TDP and LDP


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