8 Chapter 1 An Introduction to MPLS

F I G U R E 1 . 7 The MPLS label stack with frame-mode encapsulations

The encapsulations discussed in this section are for frame-mode MPLS only. There is another method called cell-mode MPLS that is discussed in Chapter 3, “MPLS and ATM.” For now, you need only be concerned with frame-mode MPLS.

As you can see in Figure 1.7, regardless of the frame-mode encapsulation method, the placement of the label does not change. You may recall that the Layer 3 header contains the destination field that is used for Layer 3 routing (forwarding). Because the label comes before the Layer 3 header, the router sees it first. The router can now forward packets based on the MPLS label instead of on the Layer 3 header. We say that in MPLS, IP traffic is switched instead of routed.

MPLS Architecture

Now that you know what a label is, let’s learn about the MPLS architecture. Whenever I teach MPLS and I start talking about the MPLS architecture, students usually give me that deer-in-the-headlights look. I don’t want your eyes to glaze over as you read this section. If you get confused at any time during this section, just repeat to yourself, “Labels are bound to routes in the routing table.”

With that said, let me start with an introduction to the MPLS architecture. Essentially, there are two components that make up the architecture of MPLS: control and forwarding.

MPLS Architecture 9

Control

The control plane of the MPLS architecture is responsible for binding a label to network routes and distributing those bindings among other MPLSenabled routers. Again, repeat to yourself, “Labels are bound to routes in the routing table.”

Let’s break the function of the control plane down a little bit further. Since labels are bound to network routes, an MPLS-enabled router needs to have a routing table. To get a routing table, you need a routing protocol. (Or you could use static routes, but I don’t recommend it.) Now that you have a routing table, you need some way to exchange labels. Why do you need a label-exchanging mechanism? Because labels get bound to routes in the routing table. There are two protocols that are supported by Cisco IOS devices to exchange labels: TDP and LDP.

TDP The Tag Distribution Protocol (TDP) is Cisco’s proprietary protocol that is used to bind tags (which are the same as MPLS labels) to network routes in the routing table.

LDP The Label Distribution Protocol (LDP) is the IETF version of Cisco’s TDP. LDP is used to bind labels to network routes. The label information base (LIB) is a mapping of incoming labels to outbound labels, along with outbound interface and link information.

Now, repeat to yourself, “Labels are bound to routes in the routing table.” (Go ahead. Nobody is watching.)

Forwarding Equivalence Class (FEC)

A forwarding equivalence class (FEC) is a grouping of IP packets that are treated in the same way. For example, a destination subnet could correspond to an FEC. When I say that labels get bound to routes in the routing table, I’m referring to an IP prefix being the equivalent of an FEC. So, to be specific, labels are bound to FECs.

FECs can be based on a number of criteria, including IP ToS bits, IP protocol ID, port numbers, etc.

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Forwarding

An MPLS-enabled router switches IP packets instead of forwarding them traditionally. The forwarding component of the MPLS architecture (known as the forwarding plane or data plane) is where information created and maintained from the control plane is actually used. Simply put, think of the forwarding plane as being like a big cache. The routing table is built in the control plane and cached in the forwarding plane. For labels, the LIB is built in the control plane, and only those labels in use reside in the label forwarding information base (LFIB). The LFIB is a subset of the LIB.

An additional component that resides in the forwarding plane is the forwarding information base (FIB). The FIB is built by Cisco Express Forwarding (CEF). The FIB is essentially a cached version of the IP routing table that eliminates the need for a route-cache. For Cisco MPLS or tag switching to work, CEF must be enabled.

Cisco Tag Switching

Cisco’s proprietary tag switching was the precursor of MPLS and is the technology on which the MPLS standard is based.

In Cisco’s tag switching, what we refer to as a label is called a tag. The architecture of Cisco’s tag switching is made up of two main components: the control plane and the forwarding plane. The control plane comprises the following:

Routing protocol

Routing table

Tag exchange using Tag Distribution Protocol (TDP), resulting in the tag information base (TIB)

The forwarding plane is made up of:

FIB

tag forwarding information base (TFIB)

The two technologies are virtually identical.