mpls10s02 mpls concepts

8/14/2019 Mpls10s02 Mpls Concepts

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2001, Cisco Systems, Inc.

MPLS Concepts

Module 2


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MPLS v1.02-2 2001, Cisco Systems, Inc.

Objectives

Upon completion of this chapter, youwill be able to perform the followingtasks:

Identify the drawbacks of traditional IProuting

Describe basic MPLS concepts and LSRtypes.

Understand how different MPLSapplications can coexist on the sameplatform using the same underlying

technology.


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Drawbacks ofTraditional IP

Routing

2001, Cisco Systems, Inc. MPLS v1.02-3


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Objectives

Upon completion of this section,you will be able to identify thedrawbacks of traditional IP routing.


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Traditional IP Forwarding(cont.)

Destination-based routing lookup isneeded onevery hop.

Every router may need full Internet

routing information (more than 100,000routes .

Update:10.0.0.0/8 Up

date:10

.0.0.0

/810

.1.1.1

10.1.1.1

10.1.1.110.1.1.1

Routing

lookup

Routin

g

lookup

Routin

g

lookup


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IP over ATM

Layer 2 topology may be different from Layer 3

topology, resulting in suboptimal paths and linkutilization.

Layer 2 devices have no knowledge of Layer 3 routinginformationvirtual circuits must be manuallyestablished.

Even if the two topologies overlap, the hub and spoke

10.1.1.110.1.1.1

10.1.1.1

10.1.1.110.1.1.1

1

0.1.1.1

10.1

.1.1

10.1

.1.1

10.

1.

1.

1



Traffic Engineering withTraditional IP Forwarding

Most traffic goes between large sites A and B and usesonly the primary link.

Destination-based routing does not provide anymechanism for load balancing across unequal paths.

Policy-based routing can be used to forward packetsbased on other parameters, but this is not a scalablesolution.

Primary

OC-192 linkLarge Site A Large Site B

Small Site C

Backup

OC-48 link



Summary

After completing this section, youshould be able to identify thedrawbacks of traditional IP routing



Review Questions

List major drawbacks of traditional IProuting.

Based on what information do routersforward IP packets?

What mechanism can be used toforward packets based on other

parameters?Why is this mechanism not suitable forlarge networks?


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Basic MPLSConcepts




Objectives

Upon completion of this section, youwill be able to perform the followingtasks:

Describe MPLS architecture.

Describe the MPLS approach to IProuting.

Describe the difference between data

plane and control plane in MPLS.

Describe the difference between packet-mode and cell-mode MPLS.

List Label Switch Router (LSR) types.



Basic MPLS Concepts

MPLS is a new forwarding mechanismin which packets are forwarded basedon labels.

Labels may correspond to IPdestination networks (equal totraditional IP forwarding).

Labels can also correspond to otherparameters, such as quality of service(QoS) or source address.

MPLS was designed to support

forwarding of other protocols as well.



MPLS Example

Only edge routers must perform a routinglookup.

Core routers switch packets based on simple

label lookups and swap labels.

L=5

L=3

10.1.1.110.1.1.1

Routing lookup

and

label

assignment

10.0.0.0/8 L=5

Label

swapping

L=5

L=3

Label

removal and

routing

lookup

L=3



MPLS Versus IP over ATM

Layer 2 devices are IP-aware and run a routing

protocol. There is no need to manually establish virtual

circuits.

MPLS provides a virtual full mesh topology.

10.1.1.1L=5L=3

L=1710.1.1.1

Layer 2 devices run a

Layer 3 routingprotocol and establish

virtual circuits

dynamically based on

Layer 3 information



Traffic Engineering withMPLS

Traffic can be forwarded based on otherparameters (QoS, source, ...).

Load sharing across unequal paths can beachieved.

Primary

OC-192 linkLarge Site A

Large Site B

Small Site C

Secondary

OC-48 link



MPLS Architecture

MPLS has two major components:

Control planeexchanges Layer 3 routinginformation and labels

Data planeforwards packets based on labels

Control plane contains complex mechanisms toexchange routing information, such as OpenShortest Path First (OSPF), Enhanced InteriorGateway Routing Protocol (EIGRP), IntermediateSystem-to-Intermediate System (IS-IS), and BGP,and to exchange labels, such as Tag DistributionProtocol (TDP), label distribution protocol (LDP),BGP, and Resource Reservation Protocol (RSVP).

Data plane has a simple forwarding engine.

Control plane maintains contents of the label-

switching table (label forwarding information base,or LFIB).



MPLS Architecture

Router functionality is divided into twomajor parts: control plane and dataplane

Data Plane

Control Plane

OSPF: 10.0.0.0/8

LDP: 10.0.0.0/8

Label 17

OSPF

LDP

LFIB

LDP: 10.0.0.0/8

Label 4

OSPF: 10.0.0.0/8

417Labeled packet

Label 4

Labeled packet

Label 17



MPLS Modes of Operation

MPLS technology is intended to be usedanywhere regardless of Layer 1 mediaand Layer 2 protocol.

MPLS uses a 32-bit label field that isinserted between Layer 2 and Layer 3headers (frame-mode).

MPLS over ATM uses the ATM header asthe label (cell-mode).



Label Format

MPLS uses a 32-bit label field that

contains the following information:20-bit label

3-bit experimental field

1-bit bottom-of-stack indicator8-bit time-to-live (TTL) field

LABEL EXP S TTL

0 19 22 23 3120 24



Frame-Mode MPLS

Frame

HeaderIP Header Payload

Layer 2 Layer 3

Frame

HeaderLabel IP Header Payload

Layer 2 Layer 2 Layer 3

Routing

lookup and

label

assignmen

t



IP Header

Cell_Mode MPLS

Frame

HeaderIP Header Payload

Layer 2 Layer 3

Frame

HeaderLabel IP Header Payload


ATM Adaptation

Layer 5 (AAL5) HeaderLabel Payload


ATM

HeaderCell 1

PayloadATM

HeaderCell 2

VPI/VCI fields

are used for

label switching


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Label Switch Router

Label switch router (LSR) primarily forwardslabeled packets (label swapping)

Edge LSR primarily labels IP packets andforwards them into MPLS domain, or removes

labels and forwards IP packets out of theMPLS domain

MPLS Domain

Edge

LSRLSR

10.1.1.1 L=3 L=5

L=43L=3120.1.1.1

10.1.1.1

20.1.1.1


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ATM Label Switch Router

ATM LSR can only forward cells

ATM edge LSR segments packets into cellsand forwards them into an MPLS ATM domain,or reassembles cells into packets and

forwards them out of an MPLS ATM domain

MPLS Domain

ATM

Edge

LSR

ATM

LSR

10.1.1.1 L=1/3

L=1/620.1.1.1

10.1.1.1

20.1.1.1

L=1/3 L=1/3 L=1/5 L=1/5 L=1/5

L=1/6 L=1/6L=1/9 L=1/9 L=1/9


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Architecture of LSRs

LSRs, regardless of the type, perform thefollowing three functions:

Exchange routing information

Exchange labels

Forward packets (LSRs and edge LSRs)or cells (ATM LSRs and ATM edge LSRs)

The first two functions are part of thecontrol plane.

The last function is part of the dataplane.


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Architecture of LSRs

LSRs primarily forward labeledpackets or cells (ATM LSRs).

LSR

Control Plane

Data Plane

Routing Protocol

Label Distribution Protocol

Label Forwarding Table

IP Routing Table

Exchange of

routing information

Exchange of

labels

Incoming

labeled packets

Outgoing

labeled packets


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Architecture of Edge LSRs

Note: ATM edge LSRs can only forward

cells.

Edge LSR

Control Plane

Data Plane

Routing Protocol

Label Distribution Protocol

Label Forwarding Table

IP Routing Table

Exchange of

routing information

Exchange of

labels

Incoming

labeled packets

Outgoing

labeled packets

IP Forwarding Table

Incoming

IP packets

Outgoing

IP packets


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Summary

After completing this section, you shouldbe able to perform the following tasks:

Describe MPLS architecture.

Describe MPLS approach to IP routing. Describe the difference between data

plane and control plane in MPLS.

Describe the difference between packet-

mode and cell-mode MPLS. List Label Switch Router (LSR) types.

Describe LSR architecture.


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Review Questions

What are the major drawbacks oftraditional IP forwarding and how doesMPLS solve them?

What functions does an LSR perform?List the types of LSRs.

Name the two modes of MPLS.

Explain the difference between an LSRand an Edge LSR.

Explain the difference between an LSRand an ATM LSR.


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MPLS Labels andLabel Stack



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Objectives

Upon completion of this section,you will be able to perform thefollowing tasks:

Describe the format of MPLS label.Explain the concept of the MPLSLabel Stack.

Describe the way MPLS labels areused in Packet-mode and ATMenvironment.


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MPLS Label Format

MPLS uses a 32-bit label field that

contains the following information: 20-bit label (a number)

3-bit experimental field (usually used tocarry IP precedence value)

1-bit bottom-of-stack indicator (indicateswhether this is the last label before the IPheader)

8-bit TTL (equal to the TTL in IP header)

LABEL EXP S TTL

0 19 22 23 3120 24


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MPLS Labels

Labels are inserted between the Layer2 (frame) header and the Layer 3(packet) header.

There can be more than one label(label stack).

The bottom-of-stack bit indicates if thelabel is the last label in the label stack.

The TTL field is used to preventindefinite looping of packets.

Experimental bits are usually used tocarry the IP precedence value.


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MPLS Label Stack

Usually only one label assigned to a packet.

The following scenarios may produce morethan one label:

MPLS VPNs (two labelsthe top label

points to the egress routers and the secondlabel identifiesthe VPN)

MPLS TE (two or more labelsthe top label

points to the endpoint of the trafficengineering tunnel and the second labelpoints to the destination)

MPLS VPNs combined with MPLS TE (threeor more labels)


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MPLS Forwarding

An LSR can perform the followingfunctions:

Insert (impose) a label or a stack of

labels on ingress.Swap a label with a next-hop label ora stack of labels in the core.

Remove (pop) a label on egress.

ATM LSRs can only swap a labelwith one label (VPI/VCI fieldschange).

MPLS Forwarding


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MPLS Domain

MPLS Forwarding(Frame-Mode)

On ingress a label is assigned and imposed by the IP routingprocess.

LSRs in the core swap labels based on the contents of thelabel forwarding table.

On egress the label is removed and a routing lookup is used to

forward the packet.

10.1.1.1

IP Lookup

10.0.0.0/8 label3

LFIB

label 8 label 3

IP Lookup

10.0.0.0/8 label5

LFIB

label 3 label 5

IP Lookup

10.0.0.0/8 nexthop

LFIB

label 5 pop

10.1.1.13 10.1.1.15 10.1.1.1

MPLS Forwarding


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MPLS Domain

MPLS Forwarding(Cell-Mode)

Labels (VPI/VCI) are imposed during the IP lookup process on ingressATM edge LSRs. Packets are segmented into cells.

ATM LSRs in the core swap labels based on the contents of the ATMswitching table. ATM LSRs cannot forward IP packets.

On egress ATM edge LSRs the labels are removed (cells arereassembled into packets) and a routing lookup is used to forwardpackets.

10.1.1.1

IP Lookup

10.0.0.0/8 label1/3

LFIB

label 8 label1/3

IP Lookup

10.0.0.0/8 Nexthop

LFIB

label 1/5 pop

10.1.1.1

IP Lookup

10.0.0.0/8 label1/5

LFIB

label 1/3 label1/5

1/3 1/3 1/3 1/3 1/5 1/5 1/5 1/5


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MPLS Applications



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Objectives


Identify various MPLS applications.

Describe the overall structure of eachMPLS application.

Explain the interactions betweenseveral MPLS applications running onthe same platform.


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MPLS Applications

MPLS is already used in many differentapplications:

Unicast IP routing

Multicast IP routing

Traffic Engineering (MPLS TE)

QoS

Virtual private networks (MPLS VPN)

Regardless of the application, the functionalityis always split into the control plane and the

data plane: The applications differ only in the control

plane.

They all use a common label-switching data

plane.


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Multicast IP Routing

A dedicated protocol is not needed tosupport multicast traffic across anMPLS domain.

peripheral interface manager (PIM)version 2 with extensions for MPLS isused to propagate routing information

as well as labels.FEC is equal to a destination multicastaddress, stored in the multicast routingtable.


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MPLS TE

MPLS traffic engineering requires OSPFor IS-IS with extensions for MPLS TE asthe IGP.

OSPF and IS-IS with extensions hold theentire topology in their databases.

OSPF and IS-IS should also have someadditional information about network

resources and constraints.

RSVP or CR-LDP is used to establishtraffic engineering tunnels (TE tunnels)and propagate labels.


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Quality of Service

Differentiated QoS is an extension tounicast IP routing that provides

differentiated services.Extensions to TDP or LDP are used topropagate different labels for differentclasses.

FEC is a combination of a destinationnetwork and a class of service.


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Virtual Private Networks

Networks are learned via an IGP (OSPF, EBGP,RIP version 2 [RIPv2] or static) from acustomer or via BGP from other internalrouters.

Labels are propagated via MP-BGP.

Two labels are used:

Top label points to the egress router(assigned through LDP or TDP).

Second label identifies the outgoing interface onthe egress router or a routing table where arouting lookup is performed.

FEC is equal to a VPN site descriptor or VPN

routing table.

Interaction Between MPLS


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Control Plane

Multicast

IP Routing

MPLS Traffic

Engineering

Quality of Service MPLS/VPNUnicast

IP Routing

Interaction Between MPLSApplications

Data Plane

Any IGP

LDP or TDP

Label forwarding table

Unicast IP

routing table

PIM version 2

Multicast

IP routing table

OSPF or IS-IS

LDP

Unicast IP

routing table

RSVP

Any IGP

LDP or TDP

Unicast IP

routing table

Any IGP

LDP

Unicast IP

routing tables

BGP


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Summary

After completing this section, youshould be able to perform thefollowing tasks:

Identify various MPLS applications.

Describe the overall structure of eachMPLS application.

Explain the interactions betweenseveral MPLS applications running onthe same platform.


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Review Questions

Where is MPLS used today?

Where can MPLS potentially be used in

the future?What do the different applications ofMPLS have in common?

What are the differences between thevarious applications of MPLS?


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Differences BetweenTag Switching and

MPLS



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Objectives


Explain the differences between TagSwitching and MPLS.

List the IETF standards definingMPLS.


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MPLS Standardization

MPLS functionality has been available in Ciscorouters since Cisco IOS Release 11.1CT. It wascalled tag switching, and the switching part isequal tostandard MPLS.

The only difference between MPLS and tagswitching is in the label distribution protocol:

Cisco proprietary implementation uses TDP.

IETF specifies LDP as the standard label

distribution protocol. Although TDP and LDP are also functionally

equivalent, they are not compatible.

They can, however, coexist in an MPLS

domain as long as any two peers are usingthe same rotocol.

S S


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MPLS Standards

MPLS is basically a standardizedversion of tag switching.

The following are just some of the manydrafts defining MPLS:

draft-ietf-mpls-arch

draft-ietf-mpls-label-encaps

draft-ietf-mpls-ldp

Many other drafts can be found athttp://www.ietf.org/html.charters/mpls-

TDP LDP
http://www.ietf.org/html.charters/mpls-charter.htmlhttp://www.ietf.org/html.charters/mpls-charter.html


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TDP vs. LDP

MPLS and tag switching are equal onthe data plane.

The only difference is on the controlplane, where tag switching uses Cisco

proprietary TDP and MPLS usesstandard LDP.

TDP and LDP are functionally equivalentbut not compatible.

TDP uses User Datagram Protocol (UDP)and Transmission Control Protocol(TCP) port number 711.

LDP uses UDP and TCP port number


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S


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Summary

After completing this section, youshould be able to perform the

following tasks:Explain the differences between TagSwitching and MPLS.

List the IETF standards defining MPLS.

R i Q ti


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Review Questions

What is the difference between TagSwitching and MPLS on the data plane?

What is the difference between Tag

Switching and MPLS on the controlplane?

How can Tag Switching and MPLS be

combined?Which label switching mechanism is thedefault switching mechanism on Ciscorouters?

S


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Summary

After completing this chapter, youshould be able to perform thefollowing tasks:

Identify the drawbacks of traditional IProuting.

Describe basic MPLS concepts and LSRtypes.

Describe how different MPLS applicationscan coexist on the same platform usingthe same underlying technology.

List the standard bodies that are workingon MPLS technology and the relationship


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