24 Chapter 1 An Introduction to MPLS

The MPLS solution is to use traffic-engineered tunnels that are made possible with label stacking. Figure 1.17 shows two tunnels. On R6, two tunnels, both with a destination of R1, are configured to load-share. The first tunnel takes a path from R6 to R5 to R4 to R1. The second tunnel follows the path from R6 to R3 to R2 to R1. Since MPLS supports unequal cost load balancing, traffic will be load-balanced now across these two tunnels on a per-packet basis. Tunnels are unidirectional, so a second set of tunnels would need to be set up from R1 to R6 to support traffic flow in the opposite direction from the example. Since tunnels are unidirectional in nature, it’s possible for the return tunnel from R1 to R6 to take a completely different path that’s based on the tunnel constraints.

F I G U R E 1 . 1 7 Traffic-engineered network with tunnels

Tunnel 1

Another application for MPLS is VPNs. A discussion of VPNs begins in

Chapter 4, “VPNs: An Overview.”

Summary

There are many problems experienced by service providers when trying to implement end-to-end solutions using two dissimilar technologies: ATM and IP. MPLS evolved out of early attempts at solutions to glue the IP

Exam Essentials 25

and ATM worlds together. Cisco’s proprietary solution, tag switching, later became standardized into what we now know as MPLS.

Frame-mode MPLS uses a 32-bit label stack, referred to as a shim header, because it is placed between the Layer 2 header and the Layer 3 payload. An MPLS-capable router or switch label-switches packets instead of routing them traditionally.

The MPLS architecture consists of two components: the control plane and the forwarding or data plane. These two components make label switching possible. The control plane binds labels to FECs. With CEF, label switching is made possible in the forwarding plane with the FIB and LFIB.

As packets enter the service provider network, an edge-LSR imposes a label. The label is used by every LSR along the LSP to label-switch the packet. By labeling at the network edge, it is possible to classify packets and implement consistent QoS throughout the network. Traffic engineering is made possible with label stacking.

Exam Essentials

Understand the MPLS label stack. The MPLS label stack is a total of 32 bits. The label itself is 20 bits. The label stack is placed between the Layer 2 header and the Layer 3 payload and is referred to as a shim header.

Know the MPLS architecture. The MPLS architecture is divided into two planes: control and forwarding. The control plane is responsible for binding labels to routes, or more specifically, to FECs. The forwarding plane (also known as the data plane) operates like a big cache by maintaining the FIB and LFIB. The control plane builds the bindings and the forwarding plane actually uses those bindings to switch packets. Don’t forget, CEF must be enabled for MPLS to work.

Be able to identify MPLS operation. Packets enter the service provider network as unlabeled IP. An edge-LSR imposes a label and forwards the newly labeled packet to the next LSR along an LSP. Each LSR along the LSP label-switches the packet. The next-to-last router in the path pops the label through a mechanism called penultimate hop popping.

26 Chapter 1 An Introduction to MPLS

Know MPLS applications. First of all, MPLS changes network design by eliminating the need for an overlay. Performance is improved because packets are switched instead of routed. QoS can be implemented end to end by having an edge-LSR classify packets and map a value to the Experimental (EXP) field of the MPLS label stack. Traffic engineering is made possible through label stacking and traffic-engineered tunnels.

Key Terms

Before you take the exam, be certain you are familiar with the following terms:

Review Questions 27

Review Questions

1.What command do you use to display the labels on a Cisco IOS router/ switch using MPLS?

A.show mpls ip

B.show mpls forwarding-table

C.show tag forwarding-table

D.show mpls labels

2.How many octets are there in the MPLS label stack header?

A.1

B.2

C.3

D.4

3.In frame-mode MPLS, the MPLS label stack resides ___________ and

___________. (Choose two.)

A.Before the Layer 2 header

B.After the Layer 2 header

C.Before the Layer 3 payload

D.After the Layer 3 payload

4.How many bits make up the label portion of the MPLS label stack?

A.3

B.16

C.20

D.32

28Chapter 1 An Introduction to MPLS

5.What command do you use to display the labels on a Cisco IOS router/ switch using tag switching?

A.show ip mpls

B.show mpls forwarding-table

C.show tag forwarding-table

D.show mpls labels

6.An MPLS-capable router/switch is called a(n) ___________?

A.LSA

B.LSR

C.LRR

D.TSR

7.Which device in the network only connects to service provider equipment?

A.P

B.PE

C.CE

D.C

8.Which network device typically imposes the labels?

A.P

B.PE

C.CE

D.C

9.What is the process of removing a label by the next-to-last router called?

Review Questions 29

A.Popping

B.Fast switch popping

C.Penultimate hop popping

D.Label disposition

10.Which field of the MPLS label stack is used for Quality of Service (QoS)?

A.Label

B.Experimental

C.S

D.TTL

11.Which of the following is not a suitable application for MPLS?

A.Quality of Service

B.Virtual private networks

C.Routing protocol replacement

D.Traffic engineering

12.In MPLS, VPNs and traffic engineering are made possible by ______. (Choose the most appropriate answer.)

A.Label stacking

B.Label popping

C.Label imposition

D.Label switching

13.Cisco’s proprietary version of MPLS is called ___________.

A.Multi-protocol tag switching

B.Multi-Protocol Label Switching

C.Tag forwarding

D.Tag switching

30Chapter 1 An Introduction to MPLS

14.Which protocol does tag switching use to exchange tags with neighbors?

A.LDP

B.LIB

C.TDP

D.FIB

15.Which protocol does MPLS use to exchange labels with neighbors?

A.LDP

B.LIB

C.TDP

D.FIB

16.For MPLS or tag switching to work, ___________ must be enabled.

A.LFIB

B.LIB

C.FIB

D.CEF

17.To indicate the bottom of a stack, the S bit is set to ___________.

A.0

B.1

C.2

D.None of the above

18.An IP prefix is analogous to a(n) ___________.

A.FIB

B.LFIB

C.FEC

D.CEF

Review Questions 31

19.LSPs are ___________.

A.Unidirectional

B.Bi-directional

C.None of the above

20.An ATM switch that is MPLS-enabled is called a(n) ___________.

A.ATM-LSR

B.Edge-LSR

C.ATMF-LSR

D.Core-LSR