Basics of MPLS

1© 2003 Cisco Systems, Inc. All rights reserved.MPLS Training - Basics 1

Basics of MPLS

Cisco Advanced Services EDCS-336890

222© 2003 Cisco Systems, Inc. All rights reserved.MPLS Training - Basics

• Introduction

Name, Location and Current profile


Agenda

• Business Drivers for MPLS

• MPLS Capabilities

• MPLS Concepts

• Basic MPLS Forwarding

• Basic MPLS Applications

Hierarchical Routing

IP+ATM Integration

• Summary and Benefits of MPLS

Business Drivers for MPLS

444© 2003 Cisco Systems, Inc. All rights reserved.


Old World New World

Changing Telecom Landscape

Infrastructure

Traffic

Services Focus

Private Networks

Business Networks

OSS

In-House

FR-Based VPNs

Voice-Centric

Transport

Circuit-Switched

Network-Based Service-Based

Outsourced

IP Value-Added

IP-Based VPNs

Data-Centric

Packet-Switched


Customer Requirements

Businesses are building on IPBusinesses are building on IP Businesses need Businesses need privateprivate

IP servicesIP services

CustomersSuppliersPartners

TelecommutersMobile Users

RemoteOffices

IP Intranet IP Extranet


ValueValue--AddedAdded

IP ServicesIP Services

Problem:How to Build a Network

that Can Deliver theseServices andand SLAs

Service Provider Requirements

Growth: IP ConnectionGrowth: IP Connection

Internet, Intranet, Internet, Intranet,

ExtranetExtranet

Multimedia

Content

Hosting

Service Portfolio

Revenue: TransportRevenue: Transport

Frame Relay, ATM, Frame Relay, ATM,

Managed ServicesManaged Services

ManagedIntranets

PrivateVoice Networks

Profit: IP VASProfit: IP VAS

Hosting, Voice, Hosting, Voice,

Video, ASPVideo, ASP’’ss


The Barriers

• Carriers’ customers want IP services:

They need connectionless IP services

They need more flexible IP quality of service guarantees

They need more privacy than the Internet provides

• Frame Relay and ATM services are available:

They provide connection-oriented service

They have inflexible point-to-point bandwidth guarantees

But they have good privacy


The Solution - MPLS

• MULTI-PROTOCOL LABEL SWITCHING

• A mechanism that delivers the best of both worlds:

PRIVACY and QOS of ATM, Frame Relay

FLEXIBILITY and SCALABILITY of IP

• Foundation for IP business services

Flexible grouping of users and value-added services

• Low cost managed IP services

scales to large and small private networks


MPLS Basics

• Traditional routing

Each router holds entire routing table and forwards to next hop (destination based routing)

• MPLS combines L3 routing with label swapping and forwarding

• MPLS Forwarding

Label imposed once at ingress routerAll forwarding decisions then made on label only – no routing table lookups

MPLS Capabilities



Key Cisco MPLS Capabilities

IP/ATM Integration Traffic Engineering

VPN’s

IP Multicast IP CoS

RSVP


• Put routers around the edge of an ATM network

• Connect routers using Permanent Virtual Circuits

• This does not provide optimal integration of IP and ATM

Traditional IP over ATM


IP+ATM Integration

• Internal routing scalability

Limited adjacencies

• External routing scalability

Full BGP4 support, with all the extras

• VC merge for very large networks


Traffic Engineering

Route chosen byIP routing protocol

Route specified bytraffic engineering

• Why traffic engineer?

Optimise link utilisation

Specific paths by customer or class

Balance traffic load

• Traffic follows pre-specified path

• Path differs from normally routed path

• Controls packet flows across a L2 or L3 network


MPLS VPNs

• Private, connectionless IP VPNs

• Outstanding scalability

• Customer IP addressing freedom

• Multiple QoS classes

• Secure support for intranets and extranets

• Simplified VPN Provisioning

• Support over any access or backbone technology

VPN C

VPN A

VPN B

VPN C

VPN AVPN B

VPN C

VPN A

VPN B

VPN CVPN A

VPN B

Connection-Oriented VPN Topology

VPN C

VPN A

VPN B

VPN C

VPN AVPN B

VPN C

VPN A

VPN BVPN C

VPN AVPN BConnectionless

VPN Topology


Single carrier networkSingle carrier network

supporting multiple supporting multiple

customer IP VPNscustomer IP VPNs

Separately engineeredSeparately engineeredcustomer private IP

networks

BGP+ MPLSNetwork

vs.

Build once,Sell once

Build once,Sell once

Build once,Sell many

Build once,Sell many

MPLS VPNs


Core

Provider Edge

Edge

LSR

18© 1999, Cisco Systems, Inc. www.cisco.com0905_04F9_c1

TechnologiesCPE

Edge

LSR

• Provider Edge LSR

CAR (packet classification and rate limiting)

MPLS enabled

Precedence Mapping

• Core LSR

WRED (sets drop policy for congestion management)

WFQ/MDRR (queuing policies)

Fast MPLS switching

MPLS end to end class of service

MPLS Concepts



MPLS concepts

• MPLS: Multi Protocol Label Switching

• Packet forwarding is done based on labels

• Labels are assigned when the packet enters into the network

• Labels allocated from a LOCAL POOL in the router

• Labels are on top of the packet

• MPLS nodes forward packets/cells based on the label value (not on the IP information)


MPLS concepts

• MPLS allows:

Packet classification only where the packet enters the network

The packet classification is encoded as a label

In the core, packets are forwarded without having to re-classify them

No further packet analysis

Label swapping


MPLS Components

• Edge Label Switching Routers (ELSR)

label previously unlabeled packets - at the beginning of a Label Switched Path (LSP)

strip labels from labeled packets - at the end of a Label Switched Path

• Label Switching Routers (LSR)

forward labeled packets based on the information carried by labels


MPLS Components

LSR LSR

LSR LSR

ELSR

ELSR

P Network(Provider Control)

PECE CEPE

ELSR

ELSR

C Network(Customer Control)

C Network(Customer Control)

P


Functional Components

• Forwarding component:

uses label information carried in a packet and label binding information maintained by a Label Switching Router to forward the packet

• Control component:

responsible for maintaining correct label binding information among Label Switching Routers


Forwarding Component

• Label Forwarding Information Base (LFIB)

each entry consists of:

incoming labeloutgoing labeloutgoing interfaceoutgoing MAC address

LFIB is indexed by incoming label

LFIB could be either per Label Switching Router, or per interface



• IOS Label Forwarding Code is based on Cisco Express Forwarding (CEF)

Maintenance of label rewrite structures in LFIB

Recursive route resolution

IP to label switching (label imposition) path



• Forwarding algorithm:

Extract label from a packet

Find an entry in the LFIB with the INCOMING LABEL equal to the label in the packet

Replace the label in the packet with the OUTGOING LABEL(from the found entry)

Send the packet on the outgoing interface (from the found entry)



• Carrying label information:

as part of the MAC header:

- VCI/VPI in ATM

via a “shim” between the MAC and the Network Layer header

• Label information can be carried over any Link Layer


Label Header (Shim)

Label

1 2 3 4 5 6 7 8

EXP S

TTL

Bit

2

3

4

1

Byte

LabelEXP

STTL

Label Value (20 bits)Class of Service (3 bits)

Bottom of Stack (1 bit)Time to Live

•Can be used over Ethernet, 802.3, or PPP links•Require 2 new Ethertypes/PPP PIDs•One for unicast, one for multicast

•Four octets per label in stack


PPP

Ethernet

Frame Relay

Label IP header

Label

Label

IP Header

IP Header Data

ATM Header Label

IP Header

Data

ATM Header

Packet over SONET/SDH

Ethernet

Frame Relay PVC

ATM PVC’s

Subsequent cells

Data

Data

Data

GFC DataVPI VCI PTI CLP HEC

IP Header

GFC DataVPI PTI CLP HECVCI

Label

Label

Subsequent cells

ATM label switching

FR

AME

CE

LL

Label Encapsulation


Control Component

• Labels can be distributed by several protocols

TDP/LDP – from IGP routes

RSVP – for traffic engineering paths

BGP – for VPN routes

• Responsible for binding between labels and routes:

Create label binding (local)

Distributing label binding information among Label Switching Routers


MPLS Forwarding Decisions

• Packets are forwarded based on the label value

• IP header and forwarding decision have been de-coupled for better flexibility

• Other paradigms may be used to forward traffic

• No need to strictly follow unicast destination based routing

• Allows to have distinct forwarding decision based on different control component

Destination unicast routing, Traffic Engineering

Multicast, VPN, QoS

Basic MPLS Forwarding



MPLS: Forwarding


MPLS: Forwarding

Existing routing protocols (e.g. OSPF, IGRP) establish routes


MPLS: Forwarding

Label Distribution Protocol (e.g., LDP) establishes label to routes mappings


MPLS: Forwarding

Label Distribution Protocol (e.g., LDP) creates LFIB entries on LSRs

IN OUT I/F MAC

POP 32 E0/0 aa-00-bb

POP 27 E0/1 aa-00-cc

IN OUT I/F MAC

16 32 S0/0 aa-00-bb

18 27 S0/1 aa-00-cc

IN OUT I/F MAC

16 32 S0/0 aa-00-bb

18 27 S0/1 aa-00-cc IN OUT I/F MAC

16 POP S0/0 aa-00-bb

18 POP S0/1 aa-00-cc


MPLS: Forwarding

IN OUT I/F MAC



IN OUT I/F MAC

16 32 S0/0 aa-00-bb

18 27 S0/1 aa-00-cc

IN OUT I/F MAC

16 32 S0/0 aa-00-bb




Ingress edge LSR receives packet, performs Layer 3 value-added services, and “label” packets


MPLS: Forwarding

IN OUT I/F MAC



IN OUT I/F MAC

16 32 S0/0 aa-00-bb

18 27 S0/1 aa-00-cc

IN OUT I/F MAC

16 32 S0/0 aa-00-bb




LSRs forward labelled packets using label swapping


MPLS: Forwarding

IN OUT I/F MAC



IN OUT I/F MAC

16 32 S0/0 aa-00-bb

18 27 S0/1 aa-00-cc

IN OUT I/F MAC

16 32 S0/0 aa-00-bb




Edge LSR at egress removes remaining label* and delivers packet

* Pentulimate hop popping actually occurs. There may may not necessarily be a label in the packet at the ultimate or egress LSR.


Traditional RoutingRoute Distribution

0

0

Routing Updates (OSPF, EIGRP…)

You Can Reach 128.89and 171.69 thru me

You Can Reach 128.89 thru Me

You Can Reach 171.69 thru Me

11171.69171.69

…………

11128.89128.89

OutOutI/FI/F

AddressAddressPrefixPrefix

1

22171.69171.69

…………

00128.89128.89

OutOutI/FI/F


128.89

171.69

1

…………

00128.89128.89

OutOutI/FI/F


2


11171.69171.69

…………

11128.89128.89

OutOutI/FI/F


22171.69171.69

…………

00128.89128.89

OutOutI/FI/F


Traditional RoutingPacket Routing

2

0

0

1

128.89

Packets Forwarded

Based on IP Address

…………

00128.89128.89

OutOutI/FI/F


1

171.69

Data | 128.89.25.4

Data | 128.89.25.4

Data | 128.89.25.4 Data | 128.89.25.4


InInLabelLabel

OutLabel

11171.69171.69

…………

11128.89128.89

OutOutI/FI/F


MPLS ForwardingIn/Out Label Fields

2

0

0

1

128.891

InInLabelLabel

OutLabel

22171.69171.69

…………

00128.89128.89

OutOutI/FI/F


171.69

InInLabelLabel

OutLabel

…………

00128.89128.89

OutOutI/FI/F



171.69

3636

3131

InInLabelLabel

2929

2727

OutLabel

11171.69171.69

…………

11128.89128.89

OutOutI/FI/F


Frame Based MPLSAssigning Labels

2

0

0

1

128.891

2929

2727

InInLabelLabel

2222

PopPop

OutLabel

22171.69171.69

…………

00128.89128.89

OutOutI/FI/F


Pop Label for 128.89

Use Label 22 for 171.69



Unsolicited Downstream

Label Allocation

PopPop

InInLabelLabel

--

OutLabel

…………

00128.89128.89

OutOutI/FI/F



171.69

3636

3131

InInLabelLabel

2929

2727

OutLabel

11171.69171.69

…………

11128.89128.89

OutOutI/FI/F


Frame Based MPLSPacket Forwarding

2

0

0

1

128.891

2929

2727

InInLabelLabel

2222

PopPop

OutLabel

22171.69171.69

…………

00128.89128.89

OutOutI/FI/F


128.89.25.4Data

Data 171.69.21.7 29Penultimate Hop

(Pop the label)

Data 128.89.25.4 27

128.89.25.4Data

171.69.21.7Data Data 171.69.21.7 22

PopPop

InInLabelLabel

--

OutLabel

…………

00128.89128.89

OutOutI/FI/F


128.89.25.4Data

Basic Application – Hierarchical Routing



Hierarchical Routing

Border Routers Run BGP with External Peers

InteriorRouters

All Routers Run IBGP to Learn Exterior Routing Information, and IGP for Interior Topology

Conventional Approach


Hierarchy of Routing Knowledge

• Isolate inter-domain and intra-domain routing

Improved stability

• Reduce interior router table size

Only IGP routes stored at interior nodes

• Improve BGP scaling

Only border nodes need run BGP


Internet Scalability

2

01

128.89136.50156.50

119.10

1

171.69127.18204.162

PopPop

InInLabelLabel

--

OutLabel

…………

--150.10.1.1150.10.1.1

OutOutI/FI/F


EBGP

EBGP

Loopback 150.10.1.1

Loopback 150.10.1.2

1717

1616

InInLabelLabel

2222

PopPop

OutLabel

22150.10.1.2150.10.1.2

…………

00150.10.1.1150.10.1.1

OutOutI/FI/F


--

InInLabelLabel

1717

1616

OutLabel

11150.10.1.2150.10.1.2

…………

11150.10.1.1150.10.1.1

OutOutI/FI/F


I can reach…

128.89,136.50156.50,119.10

via the BGP next hop

150.10.1.1 using onlylabel 16!

0

Basic Application – IP+ATM



MPLS and ATM

• Label Switching Steps:

Make forwarding decision using fixed-length Label

Rewrite label with new value

Similar to ATM cell switching

• Key differences:

Label set up: LDP vs ATM Forum Signaling

Label granularity: Per-prefix


MPLS and ATM

• Common forwarding paradigm

label swapping = ATM switching

• Use ATM user plane

use VPI/VCI for labels

Label is applied to each cell, not whole packet

• Replace ATM Forum control plane with the MPLS control component:

Network Layer routing protocols (e.g., OSPF, BGP, PIM) + Label Distribution Protocol (e.g., LDP)


Label Distribution for ATM

• Uses LDP in “Downstream on Demand” mode

• Referred to as Cell Based MPLS (rather than Frame Based MPLS)

• Label Virtual Circuit (LVC) labels are requested when topology changes

• Precedence can be associated with Label Virtual Circuit (LVC)

• Some LDP extensions for negotiation of ATM specific parameters


171.69

--

--

InInLabelLabel

3030

2727

OutLabel

11171.69171.69

…………

11128.89128.89

OutOutI/FI/F


Cell Based MPLSAssigning Labels

2

0

0

1

128.891

3030

2929

2727

InInLabelLabel

1616

1919

1818

OutLabel

00128.89128.89

22171.69171.69

00128.89128.89

OutOutI/FI/F


1919

1818

InInLabelLabel

--

--

OutLabel

00128.89128.89

…………

00128.89128.89

OutOutI/FI/F


DownstreamOn demand

Label Allocation

Need a Label for 128.89







Cells

1

Labels act as the VCidentifier for ATM

Switches (Label VC or LVC)

Labels change betweenswitches - LVCs arenot end-to-end.

Cell Based ATM MPLSForwarding Cells

In Label

In Label

11

……

Address Prefix

Address Prefix

128.89128.89

……

OutI/F

OutI/F

00

……

Out Label

Out Label

33

……

In I/F

In I/F

55

……

ELSR LSR

GFC DataVPI PTI CLP HECVCI

Label

33 33 33 33

55 55 55 55

PacketPacket 128.890

1


128.89

In Label

In Label

55

88

……

Address Prefix

Address Prefix

128.89128.89

128.89128.89

……

OutI/F

OutI/F

00

00

……

Out Label

Out Label

33

33

……

In I/F

In I/F

11

22

……

2

Cell Based ATM MPLSMultiple Labels (1)

• If multiple labels were not allocated

Cells of different packets would have same label (VPI/VCI)

Egress router can’t reassemble packets

3333

88

5555

5555

8888

88 3333 3333

PacketPacket

PacketPacket

Cells

Help!Help!1

0


128.89

Cell Based ATM MPLSMultiple Labels (2)

• Multiple labels enable edge router to reassemble packets correctly

In Label

In Label

55

88

……

Address Prefix

Address Prefix

128.89128.89

128.89128.89

……

OutI/F

OutI/F

00

00

……

Out Label

Out Label

33

77

……

In I/F

In I/F

11

22

……

2 3377

88

5555

5555

8888

88 3377 3377

PacketPacket

PacketPacket

Cells

10

Cool!Cool!


128.89

VC Merge

• With ATM switch that can merge VCs

Can reuse outgoing label

Hardware prevents cell interleave

Fewer labels required

For very large networks

In Label

In Label

55

88

……

Address Prefix

Address Prefix

128.89128.89

128.89128.89

……

OutI/F

OutI/F

00

00

……

Out Label

Out Label

33

33

……

In I/F

In I/F

11

22

……

2 3333

88

5555

5555

8888

88 3333 3333

PacketPacket

PacketPacket

Cells

10


VC Merge

Without VC Merge With VC Merge

• VC Merge is used to merge Label VCs (LVCs) going to the same destination

• VC Merge greatly improves IP-over-ATM scalability


MPLS Stack

IP RoutingIP Routing

MPLS SignallingMPLS Signalling

Labels on Layer 2Labels on Layer 2

MPLS: Another View

• The MPLS protocol stack consists of IP Routing and MPLS signalling, together setting up connectivity at layer 2

• Much of the complexity in the MPLS stack is in IP Routing


MPLS and ATM Services

• ATM services for real-time services

P-NNI / UNI Signaling

Voice trunking

Circuit Emulation (CES)

• MPLS for data traffic

Offloads signaling-intensive traffic

Reduces call set-up dependencies

CoCo--Existing on Same PlatformExisting on Same Platform

ATM

IPFR

ATM

FR

IP IP


MPLS is an ATM Control Stack

• MPLS and ATM Forum protocols can co-exist on same the same links and switches

• ATM MPLS sets up Label VCs (LVCs) according to IP Routing information

• Label VCs (LVCs) are not PVCs or SVCs

• LVCs, SVCs and PVCs can coexist on same links in different VPIs

IP RoutingIP Routing

MPLS SignallingMPLS Signalling

L2 VCsL2 VCs

PNNI RoutingPNNI Routing

UNI/NNI SignallingUNI/NNI Signalling

MPLS

Stack

ATM Forum

Stack

Summary and Benefits



Summary

• MPLS allows flexible packet classification and network resources optimisation

• Labels are distributed by different protocols

LDP, RSVP, BGP

• Different distribution protocols may co-exist in the same LSR

• Labels have local (LSR) significance

No need for global (domain) wide label allocation/numbering


Advanced MPLS

• Basic MPLS: destination-based unicast

• Many additional options for assigning Labels

• The Key: separation of routing and forwarding

Destination-Based Unicast Routing

IP Classof Service

Resource

Reservation(Eg. RSVP)

Multicast

Routing (PIM v2)

Explicit and

Static Routes

Virtual

Private Networks

Label Information Base (LIB)

Per-Label Forwarding, Queuing,

and Multicast Mechanisms


Benefits of MPLS

• De-couples IP packet forwarding from the information carried in the IP header of the packet

• Provides multiple routing paradigms (e.g., destination-based, explicit routing, VPN, multicast, CoS, etc…) over a common forwarding algorithm (label swapping)

• Facilitates integration of ATM and IP - from control plane point of view an MPLS-capable ATM switch looks like a router

676767

Basics of MPLS

Documents

Transcript of Basics of MPLS