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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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