– The following is an excerpt from the upcoming edition of the:

IPexpert MPLS Operation and Troubleshooting book.

Introduction to Label Distribution

Thus far in this book we have discussed the values and characteristics of labels, we have dealt with their meaning and how they affect the creation of both the control and forwarding planes used in MPLS. Throughout this entire process we have only loosely referred to the exchange of labels between label switching devices. Now it is time to discuss the process and mechanism behind this exchange or advertising process.

There are two methods at our disposal to exchange labels and their associated bindings between devices that are peered. The key word here is “peered”. Whether we are using the Cisco proprietary Tag Switching or the industry standard Label Distribution Protocol (LDP) as the mechanism of choice to communicate label information, it all still boils down simply to peering relationships and the rules governing those peers. For the purpose of our discussion we will deal specifically with LDP from this point forward.

When MPLS is enabled on an integrated services router (ISR) LDP is the default label distribution protocol. It is this protocol that dynamically creates and governs the label switched path in our network. So it must be noted that everything we have discussed up to this point regarding label generation, assignment, and exchange is maintained by this single protocol. Additionally LDP is also responsible for the formation of the peering relationships we mentioned earlier. It is these relationships that facilitate the exchange of our local label bindings between peers.

At this time we should define and innumerate the major operational roles this protocol will be responsible for in terms our discussions in this chapter. Primarily we can outline three such functions:

• Dynamic discovery of adjacent LDP Peers
• Peering and session establishment
• Regulation of peer-to-peer communication and label exchange

We will now initiate an introduction to the concepts associated with these major operational functions in their own individual sections.

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This is an excerpt from the next book in IPexpert’s very popular Protocol Operation and Troubleshooting Series. This volume deals with MPLS by starting at the very foundation and working all the way up to advanced concepts. This section taken from the very first chapter should offer some eye opening insights into how MPLS operates and how to isolate and repair failures affecting its operation…

[Taken from Chapter One: Introduction to MPLS]

The creation of the virtual routing and forwarding instances is the first step in establishing the overall MPLS architecture at the command line interface. This becomes the first component we as administrators have authoritative control over, and defines how an MPLS backbone will build label forwarding tables and actually forward labeled packets. The VRF instances we define on each device directly affect the following MPLS components:

Forward Equivalence Class

A forwarding equivalency class (FEC) is how a group of IP packets that share a specific label will be forwarded. However, it should be pointed out that a more accurate term than “packets” would be IP prefixes or routes due to the fact that these elements can and will more-often-than-not share a particular label. Thus they will be treated equivalent in forwarding. This is not to say that a given FEC cannot reflect treatment for a specific prefix verses a group of prefixes. In fact, a FEC can be as generic or as granular as we as administrators need it to be.

Control Plane

With MPLS, routers determine how to forward a given FEC or labeled packet in the identical fashion employed traditionally to forward IP packets via standard ip routing but the decision is based on the incoming label of a particular packet. This process involves a consultation of the forwarding table to determine the outbound interface that will be used to forward the labeled packet. Then the actual forwarding process will take place. In this discussion routing is the movement of packets (labeled or otherwise) from one network to another, where forwarding is the actual process of migrating a packet (labeled or otherwise) between interfaces on a given device.

The most basic concept that drives the inner workings of MPLS is the dynamic creation of the label forwarding information base (LFIB) from router to router. In a similar fashion as that used by our IGP routing protocols, information exchange takes place between MPLS speakers to create these tables. This process is best described as the formation of the MPLS control plane, and defines the process whereby labels are bound to network prefixes found in the FIB. This process  requires the exchange of label information between devices. We will address the mechanics of this process in depth in the following chapters where we discuss MPLS labels, but at this time we need to understand that MPLS speaking devices will dynamically exchange label information such that they can create their own discreet label information base (LIB). The specific information that is exchanged by this process is the local label assigned by the router itself and the outgoing label that will be used to switch the traffic to a neighboring device.

To summarize, up to this point the router has assigned a label to each prefix found in the RIB. The MPLS process refers to these prefixes as FECs (Forwarding Equivalency Classes), and all prefixes that share the same label will be treated equivalent in how they are forwarded. This information is then advertised to any MPLS peer. The resulting local database of FECs, interfaces and assigned labels is referred to as the LIB.
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In this ongoing series here at blog.ipexpert.com, we are going to answer the most common questions CCIE instructors hear. Here is the latest:

Question: What is the R&S lab environment like? Does it use putty?

Answer: The R&S Lab Exam environment has changed pretty dramatically from when I was taking it. That was waaaaay back in 2005 :-) Today, you are provided with scratch paper and colored pencils and all other materials are electronic. That is right. No more printed documents. All of your tasks and trouble tickets will appear on screen in the exam Graphical User Interface (GUI) environment.

You can expect a 24 inch widescreen monitor, a QWERTY keyboard, and a mouse.

In the GUI, clicking on router icons produces a customized Putty window in which you work.

The documentation is available from a help menu, and also a link on the workstation desktop.

For a tour of the exam GUI interface – check out this video from Cisco:

https://learningnetwork.cisco.com/static/video-wp/CCIE_RS_v4_Lab_Exam_Demo_640x480-v2.htm

Anthony Sequeira CCIE, CCSI
Twitter: @compsolv
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