Multi Protocol Label Switching (MPLS)

Currently the segmentation between routers and switches is very narrow - Multi Protocol Label Switching (MPLS) introduction. Switches were initially configured to help segment a LAN into multiple collision domains, and thereby permit you to extend the reach of a particular LAN topology. In the past few years’ switches have moved higher up the ladder in the network. When switching is employed in a LAN to link individual client and server computers, the process is known as micro segmentation, because the broadcast domain has been abridged to just the switch and the computer attached to a port. Switches at this level generally operate using the hardware (MAC) addresses of the attached computers. To speed up the processing of routing packets at high-volume rates, a newer technology has been evolving over the past few years and goes by the name of Multi-Protocol Label Switching (MPLS) (Pepelnjak & Guichard, 2000).

This report will explicate on the basics, advantages/ limitation of Multi-Protocol Label Switching (MPLS).

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Traditional routers have a large quantum of overhead processing they must perform to get a packet to its destination. Each router along the packet’s path must open up and analyze the layer three header information before it can resolve on which port to output the packet and to send it to its next hop on its journey. If a packet passes through more than just a few routers, that’s a substantial amount of processing time. It is essential to keep in mind the fact that IP is a connectionless protocol (Guichard, Jim et al., 2005). Decisions must be made about a packet’s travel plans at each stage of its travel through the network. The solution to this problem lies in newer technology-high speed switching. Specifically, Multi-Protocol Label Switching, which is discussed in the next section, combines the best of routing techniques with switching techniques.

When you look at concepts such as ATM or Frame Relay, which are connection-oriented protocols, this isn’t the case. Instead, virtual circuits (either permanent or switched) are established to connect to endpoints of a communication path so that all cells (as in the case of ATM) or frames (as in the case of Frame Relay) normally take the same path through the switched network (Pepelnjak & Guichard 2000).

Layer 3 switching moves switching up the ladder by one rung by switching network frames based on the OSI Network layer address. A layer 3 switch is basically a router, but it executes most of its operations in application-specific integrated chips (ASICS) and performs its packet processing much faster than does a traditional router, which uses a microprocessor (much like a computer CPU) for this function (Borthick, 1997).

Multi-Protocol Label Switching (MPLS)

MPLS is a method that takes the best of both worlds and creates a concept that allows IP packets to travel through the network as if IP were a connection-oriented protocol. Employing special routers called Label Switching Routers (LSRs) executes this. These routers link a traditional IP network to an MPLS network. A packet enters the MPLS network through ingress LSR, which attaches a label to the packet, and exits the MPLS switched network through an egress LSR. The ingress LSR is the router that executes the required processing to ascertain the path a packet will need to take through the switched network. This can be performed employing traditional routing protocols such as OSPF. The path is identified by the label that the ingress router attaches to the packet. As you can see, the ingress router must execute the traditional role that a router fills. It must perform a lookup in the routing table and decide to which network the packet needs to be sent for eventual delivery to the host computer (Guichard, Jim et al., 2005).

Figure 1: Generic Label Format

(Source: MPLS and VPN Architectures, 2003)

However, as the packet travels through the switched network, it is only necessary for the switch to take a quick look at the label to make a decision on which port to output the packet. A table called the Label Information Base (LIB) is employed in a manner similar to a routing table to determine the correct port based on the packet’s label information. The switch doesn’t perform IP header processing, looking at the IP address, the TTL value, and so on. It just expends a small amount of time doing a lookup of the label in the table and outputting the packet on the correct port. When the packet reaches the egress LSR, the router abstracts the label, and then the IP packet is processed in the normal manner by traditional routers on the destination network (Minei & Lucek, 2005).

MPLS/Tag-Switching Architecture

MPLS depends on two principal components: forwarding and control. The control component is creditworthy for maintaining correct label-forwarding information among a group of interconnected LSRs. The forwarding component employs labels carried by packets and the label-forwarding information maintained by an LSR to perform packet forwarding (Minei & Lucek, 2005).

All devices in an MPLS network run IP routing protocols on their control plane to construct IP routing tables. In MPLS devices that support IP forwarding (for example, edge LSR), the IP routing tables are used to build IP forwarding tables (FIBs). In MPLS devices that back up only label forwarding (such as ATM switches with MPLS functionality), the IP routing FIB does not exist. Figure 1 shows the IP routing operation of the MPLS control plane (Minei & Lucek, 2005).

Figure 2: LSRs Build the IP Routing Table

(Source: MPLS and VPN Architectures, 2003)

Hierarchical Routing
Essential to MPLS is the notion of attaching between a label and network layer routes. MPLS supports a wide range of forwarding granularities to provide good scaling characteristics while also accommodating diverse routing functionality. At one extreme, a label can be associated (bound) to all routes announced into an IP network from an edge router through BGP. This MPLS functionality can be employed very successfully to construct highly scalable IP networks. The IP routing architecture models a network as a collection of routing domains. Within a domain, routing is provided via interior routing (such as OSPF), and routing across domains is provided via exterior routing (such as BGP). However, all routers within domains that carry transit traffic (such as domains formed by ISPs) must maintain information provided by exterior routing, not just interior routing (Pepelnjak & Guichard, 2000).

MPLS-Based Virtual Private Networks

One of the most popular MPLS applications today is the implementation of virtual private networks (VPNs) with the help of MPLS technology. To support MPLS-based VPNs, Cisco IOS was modified to support a large number of independent IP routing tables within a single router-a global IP routing table and a number of Virtual Routing and Forwarding (VRF) tables. As shown in Figure 3, each VRF has its own set of routing protocols, operating as an independent router from an IP routing perspective (but not from a network management perspective, where the whole router is still managed as one device). The total independence of the VRF tables allows different VPN networks to use overlapping IP address space. For example, VPN-A and VPN-B could both use networks (Pepelnjak, Ivan et al., 2003).

Figure 3. VRF Architecture

(Source: Definitive MPLS Network Designs, 2005)



MPLS Quality of Service


An important MPLS capability is QoS support. Two mechanisms provide a range of QoS to packets passing through a router or tag switch (Pepelnjak, Ivan et al., 2003):

1.      Classification of packets into different classes

2.      Handling of packets via appropriate QoS characteristics (such as bandwidth and loss)

MPLS Disadvantages

MPLS still is in the development stages, so you’ll find that different vendors implement it in different ways. Several Internet draft documents try to produce a standard for MPLS. Other features, such as Quality of Service (QoS) and traffic management techniques are being evolved to make MPLS a long-term solution (Pepelnjak & Guichard 2000).

The specific limitations of MPLS are furnished below (Pepelnjak, Ivan et al., 2003):

1.      An additional layer is added

2.     The router has to understand MPLS


Multiprotocol Label Switching (MPLS) is a technology that combines the benefits of connection-oriented Layer 2 forwarding with the benefits of connectionless Layer 3 Internet Protocol (IP). In MPLS-enabled networks, all network devices become IP-aware. They run IP routing protocols on the control plane while performing the actual packet forwarding based on labels allocated to IP prefixes with a variety of label distribution protocols (Minei & Lucek 2005).














Borthick, S. L. (1997, November). Full (Wire) Speed Ahead – Layer 3 Switching Update. Business Communications Review.

Borthick, S. L. (1999, October). New Foundations for Routing and Switching. Business Communications Review.

Guichard, Jim; Faucheur,Le François & Vasseur,Jean-Philippe.(2005,March).Definitive MPLS Network Designs. Cisco Press.

Keller, D. (2002). The Future of Ideas: The Fate of the Commons in a Connected World. Duke Law Journal.

Minei, Ina & Lucek, Julian.(2005,October).MPLS-Enabled Applications : Emerging Developments and
New Technologies. John Wiley & Sons.

Passmore, D. (2000). Clearing Up MPLS Confusion. Business Communications Review.

Pepelnjak, Ivan & Guichard, Jim. (2000). MPLS and VPN Architectures. Cisco Press: 1st edition.

Pepelnjak, Ivan; Guichard, Jim & Apcar, Jeff (2003). MPLS and VPN Architectures. Cisco Press: 1st edition.

Sevcik, P. J. (1997). Why Circuit Switching Is Doomed. Business Communications Review.



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