Monday, September 10, 2001

Tutorial: An Introduction to MPLS

Rick Gallaher, CISSP, is owner of Dragonfly Associates LLC and author of  Rick Gallaher's MPLS Training Guide

September 10, 2001

In this article, we will examine how an MPLS network is constructed and how MPLS data flows.  In future MPLS Tutorials, we will examine:
In order to assist your further study, I have provided an acronym list and a list of related URLs to accompany each article.  


What is this new protocol that leading telecommunication experts claim “will take over the world”?  Well, you can rest your worried mind – IP and ATM are not on death row.  In fact, it is my belief that MPLS will breathe new life into the marriage of IP and ATM.

The best way to describe the function of MPLS is by using an analogy of a large national firm with campuses located throughout the United States.  Each campus has a central mail-processing point through which mail is sent around the world, as well as to its other campuses.  Since its beginning, the mailroom has been under orders to send all intercampus correspondence via standard first-class mail.  The cost of this postage is calculated into the company’s operational budget.

MPLS Multiple Protocol Label Switching; also,
Multiple Protocol Lambda Switching
LER Label Edge Router
LSR Label Switch Router
LIB Label Information Base
LSP Label Switch Path
 FEC Forward Equivalence Class; also, Functional Equivalent Class
MPLS allows for the marriage of IP to layer 2 technologies (such as ATM) by overlaying a protocol on top of IP networks. 
Network routers equipped with special MPLS software process MPLS labels contained in the Shim Header.
Raw IP traffic is presented to the LER, where labels are pushed; these packets are forwarded over LSP to LSR where labels are swapped.
At the egress to the network, the LER removes the MPLS labels and marks the IP packets for delivery. 
bulletIf traffic crosses several networks, it can be tunneled across the networks by using stacked labels.

However, for months now, some departments have been complaining that they require overnight delivery and package-tracking services.  As a manager, you set up a system to send three levels of mail between campuses – first class, priority, and express mail.  In order to offset the increased expense of the new services, you bill the departments that use these premium services at the regular USPS rate plus 10%.

Priority and express mail are processed by placing the package into a special envelope with a distinctive label.  These special packets with distinctive labels assure the package priority handling and tracking capability within the postal network.  In order to avoid slowdowns and bottlenecks, the postal facilities in the network created a system that uses sorting tables or sorting databases to expedite these special packets.  

The Construction of an MPLS Network

In an IP network, you can think of routers as post offices or postal sorting stations.  Without a means to mark, classify, and monitor mail, there would be no way to process different classes of mail.  In IP networks, you find a similar situation.  Figure 1 below shows a typical IP network with traffic having no specified route.

Figure 1: An IP Network

In order to designate different classes of service or service priorities, traffic must be marked with special labels as it enters the network.  Special routers called LER (Label Edge Routers) provide this labeling function (Figure 2).  The LER converts IP packets into MPLS packets, and MPLS packets into IP packets.  On the ingress side, the LER examines the incoming packet to determine whether the packet should be labeled.  A special database in the LER matches the destination address to the label.  An MPLS shim header (Figure 3) is attached and the packet is sent on its way.

Figure 2: IP Network with LERs and an IP packet with Shim header attached

To further explain the MPLS shim header, let's look at the OSI model.  Figure 3 (a) shows OSI layers layer 7 through layer 3 (L7-L3) in red and layer 2 (L2) in yellow.  When an IP packet (layers 2-7) is presented to the LER, it pushes the shim header (b) between layers 2 and 3.  Note that the shim header is neither a part of layer 2 or layer 3; however, it provides a means to relate both layer 2 and layer 3 information.

The Shim Header (c) consists of 32 bits in four parts – twenty bits are used for the label, three bits for experimental functions, one bit for stack function, and eight bits for time to live (TTL).  It allows for the marriage of ATM (a layer-2 protocol) and IP (a layer-3 protocol).

Figure 3: The MPLS Shim Header and Format
A shim header is a special header placed between layer two and layer 3 of the OSI model. The shim header contains the label used to forward the MPLS packets.

In order to route traffic across the network once labels have been attached, the non-edge routers serve as LSR (Label Switch Routers).  Note that these devices are still routers.  Packet analysis determines whether they serve as MPLS switches or routers.

The function of LSR is to examine incoming packets.  Providing that a label is present, the LSR will look up and follow the label instructions, and then forward the packet according to the instructions.  In general, the LSR performs a label swapping function.  Figure 4 shows LSR within a network. 

Figure 4: LSR (Label Switch Routers)

Paths are established between the LER and the LSR.  These paths are called LSP (Label Switch Paths).  The paths are designed for their traffic characteristics; as such, they are very similar to ATM path engineering.  The traffic-handling capability of each path is calculated.  These characteristics can include peak traffic load, inter-packet variation, and dropped packet percentage calculation.

Figure 5 shows the LSP established between MPLS-aware devices.  Because MPLS works as an overlay protocol to IP, the two protocols can co-exist in the same cloud without interference.

Figure 5: LSP (Label Switch Paths)


To review the construction of an MPLS network, the LER adds and/or removes (pops or pushes) labels.  The LSR examines packets, swaps labels, and forwards packets, while the LSP are the pre-assigned, pre-engineered paths that MPLS packets could take.

Right about now, you may be asking whether the advantages of MPLS are worth the extra effort.  Consider for yourself:

Your company uses a database application that is intolerant of packet loss or jitter.  In order to ensure that your prime traffic will get through, you have secured a high-cost circuit, and you have over-provisioned the circuit by 60%.  In other words, you are sending all of your mail as “express mail” for $13.50.

With MPLS, you can have the LER sort your packets and place only your highest priority traffic on the most expensive circuits, while allowing your routine traffic to take other paths.  You have the ability to classify traffic in MPLS terms, and your LER sorts traffic into FECs (Forward Equivalence Classes).  Figure 6 shows the network now broken down into FECs.

Figure 6: An MPLS Network with Two FECs

Data Flow in an MPLS Network

The simplest form of data “flow” occurs when IP packets are presented to the ingress router (acting as the LER)
(Figure 7). 

Figure 7: Ingress LER Attaches a Shim Header

Much like the mail room that classifies mail to your branch location into routine, priority and overnight mail, the Label Edge Router classifies traffic. In MPLS, this classification process is called forward equivalence class, or FEC for short.

The LER are the big decision points. LER are responsible for classifying incoming IP traffic and relating the traffic to the appropriate label.  This traffic classification process is called the FEC (Forward Equivalence Class).
LER use several different modes to label traffic.  In the simplest example, the IP packets are “nailed up” to a label and an FEC using preprogrammed tables such as the example shown in Table 1.

Destination / IPPort
FECNext HopLabel
Push 17Push nothing; native IP)

Table 1: LER Instruction Set

When the MPLS packets leave the LER, they are destined for LSR where they are examined for the presence of labels.  The LSR looks to its forwarding table (called a Label Information Base [LIB] or a connectivity table) for instructions.  The LSR will swap labels according to the LIB instructions.  Table 2 shows an example of a Label Information Base.

Label/In Port In Label/Out Port/OutFECInstruction Next Hop
Table 2: A Label Switch Router’s Label Information Base (LIB)
Figure 8 demonstrates the LSR performing its label-swapping functions.

At the egress of the network, the LER removes the MPLS header and forwards the packet to an IP network.  Label swapping greatly simplifies MPLS packet flow.

The LER performs many packet analysis functions:  mapping layer 2 to MPLS; mapping MPLS to layer 3; and classifying traffic with great granularity.  In addition, the LER decides what traffic becomes an MPLS packet. One decision-making method is called “triggered mode.” Using this method, when a predetermined number of packets are addressed to a single location within a specified time window, the router will determine that there is a "traffic stream," and will reroute the stream of traffic for MPLS processing.

Even further enhancements and flexibility are available to MPLS by using the label-stacking method, as shown in figure 9.

Consider this scenario:  You own Network 1; however, your traffic must proceed across a network that is not owned by your company (Network 2). You must ensure that Network 2 handles your traffic according to your service-level agreements (SLA), but they are not using the same label criteria as your company. 

In this case, you would stack labels and build a tunnel across Network 2. This configuration would preserve the integrity of your network's labels while allowing the other network to operate independently.

Figure 9 Stacked Labels with Tunneled Network


The concept of processing by labels is not new; it has been implemented successfully for the US Postal Service, Federal Express, and many other package-handling systems. In networking, this process has been used in Frame Relay and ATM. What is new is that the ubiquitous, uncontrolled Internet Protocol (IP) is now under a new set of rules where it can be classified, managed, and policed across any type of network.  

A nice feature about MPLS is that it is not a fork lift modification.  In some cases it only requires software modifications to existing IP routers.  For a fraction of the expense of installing a dedicated network, MPLS allows IP traffic to be classified, marked and policed while providing a method of layers two and layers 3 exchanging data.

MPLS does not replace IP – it supplements IP so that traffic and be marked, classified and policed.  With the use of MPLS, end-to-end quality of service can finally be achieved.

More on MPLS

Rick Gallaher, CISSP, is owner of Dragonfly Associates LLC and author of  Rick Gallaher's MPLS Training Guide