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This lesson, which focuses on understanding MPLS label imposition, is fourth in a series that will help you learn...

skills required for the MPLS portion of the Cisco CCIP certification. These skills are tested in the CCIP 642-611 - Implementing Cisco MPLS Exam.

Read the entire CCIP MPLS certification series
MPLS and Cisco CCIP certification -- an introduction by Ed Tittel

Cisco CCIP MPLS certification, introduction

Lesson 1: Understanding MPLS

Lesson 2: Routing and control architecture

Lesson 3: Distribution of labels

Lesson 4: Label imposition

Lesson 5: Configuring MPLS

Lesson 6: Configuring MPLS and VRF

As we discussed in Lesson 3: Distribution of labels, MPLS label switch routers (LSRs) forward packets based on the labels that are attached to those packets. The path the packet takes across the LSRs is referred to as the label-switched path. Labels are assigned to IP routes or forward equivalency classes (FECs) in the routing table, and the LSRs build a label forwarding information base (LFIB) based on the label distribution that is facilitated by the label distribution protocol.

This is called label-FEC binding. The important concept to understand is how the labels are assigned. Let's use the following example:

(i# = interface number)
R1 (i1)-- (i2)R2(i3) --(i4)R3(i5) --(i6)R4(i7) --(i8)R5(i9) --(i10)R6
R1 = customer edge (CE)
R2 = providor edge (PE)
R3 = providor only (P)
R4 = P
R5 = PE
R6 = CE

R3 and R4 are not participating in any edge client routing. The only way they can forward packets is via labels.

Let's assume that R1's origination is the route When R2 (the PE router) learns the route, it will assign a label to the packet. This is called "label imposition" or "label push."

Now, the PE router will advertise this label to its neighbors using the label distribution protocol. Remember from the last article that this label distribution is governed by the label distribution scheme which is one of the following:

  • Unsolicited downstream: This occurs when a downstream label switch router (LSR) advertises its label bindings to its neighbors automatically.
  • Downstream on demand: This occurs when an upstream LSR requests a label binding from its downstream neighbor.
  • Independent control: This occurs when a new route (or FEC) shows up in the LSR routing table. The LSR will bind a label to the FEC and advertise it to its neighbors at any time.
  • Ordered control: ATM only.

The most common distribution method is unsolicited downstream with independent control. What this really means is that as soon as an IP route/FEC appears, the labels are distributed for the route automatically. So in this case, R2 will advertise the label to R3. R3 will impose a new label and advertise to R4. R4 will impose a new label and advertise to R5.

The table below shows the label bindings for the example:
R1 (i1)-- (i2)R2(i3) --(i4)R3(i5) --(i6)R4(i7) --(i8)R5(i9) --(i10)R6

Router Ingress Interface Ingress Label FEC Egress Interface Egress Label
R2 i2 NA i3 25
R3 i4 25 i5 30
R4 i6 30 i7 35
R5 i8 35 i9 NA

Each router along the path will assign a label to the FEC/route and assign an interface to forward it along. The interface is determined by the IP routing protocols.

This allows the P routers in the middle (R3 and R4) to forward IP packets from private networks across a public backbone.

The next article, Lesson 5: Configuring MPLS, will discuss how MPLS routers support VPNs, as well as the commands for configuring VPNs.

About the author:
Robbie Harrell (CCIE#3873) is the National Practice Lead for Advanced Infrastructure Solutions for SBC Communications. He has more than 10 years of experience providing strategic, business, and technical consulting services to clients. Robbie resides in Atlanta, and is a graduate of Clemson University. His background includes positions as a Principal Architect at International Network Services, Lucent, Frontway and Callisma.

This was last published in July 2007
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