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CS 520 Fall 2003 - Lecture 15CS 520 Fall 2003 - Lecture 15MPLS and its ApplicationsMPLS and its Applications

(with modifications for CS(with modifications for CS520)520)Philip MatthewsPhilip Matthews

Nortel NetworksNortel Networks

April 2000April 2000

(Material prepared by(Material prepared by

Dr. Bilel Jamoussi andDr. Bilel Jamoussi and

Peter Ashwood-Smith)Peter Ashwood-Smith)

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What This Lecture is About

What MPLS is

What MPLS is good for

MPLS protocol/mode comparisons & opinions

Some applications of MPLS

MPLS future

Good tutorial information at www.mplsrc.com.

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What This Lecture is NOT About

General networking

Product comparisons Marketing

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Tutorial Outline

Overview

Label EncapsulationsLabel Encapsulations

Label Distribution ProtocolsLabel Distribution Protocols

Constraint-Based RoutingConstraint-Based Routing

MPLS and VPNs

Network Survivability

SummarySummary

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What is MPLS?

MPLS is an IETF Standardized mechanism forcontrolling packet routing.

Framework and Architecture

Define the scope, the various components and their interactions

Encapsulations

Labels used at the data plane to make forwarding decisions

Signaling Protocols Distribution of Labels to establish Label Switched Paths

Routing Protocol Traffic Engineering Extensions Distribution of Bandwidth and other link attributes

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Label Substitution what is it?

Have a friend go to B ahead of you. At every road they

reserve a lane just for you. At every intersection they post

a big sign that says for a given lane which way to turn and

what new lane to take.

LANE#1

LANE#2

LANE#1 TURN RIGHT USE LANE#2

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Label Switched Path (LSP)

#1 RIGHT #2

#1 RIGHT #2

#2 LEFT #1

#2 LEFT #1

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What is a LABEL?

A property that uniquely identifies aflowon a logical or physical interface

Labels may be platform wide unique or more

commonly interface wide unique.

But are not globally unique, so label values can be

reused in many places.

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Label Switched Path

#7

#99

#9

#3 Right #7

#99 RIGHT #9#7 LEFT #99

#9 LEFT #4072

#3IP

#4072 IP

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Another context: An Optical Label Switched Path labels are

colors of light

RED RIGHT BLUE

WHITE RIGHT ORANGEBLUE LEFT WHITE

ORANGE LEFT RED

IP

IP

RED

BLUE

WHITE

ORANGE

RED

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MPLS Labels

Hop-by-hop or source routing to establish labels Hop-by-hop can use OSPF, RIP, etc. to define labels

Source routing lets sources define routes and their labels

Multiple levels of labels (stacks of labels withinlabels).

Allows groups of flows to carry the same label for part of a

route.

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ROUTE AT EDGE, SWITCH IN CORE

IP ForwardingLABEL SWITCHINGIP Forwarding

IP IP #L1 IP #L2 IP #L3 IP

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Routers Do Both Routing and Switching

Routing

Deciding the next hop based onthe destination address.

A Layer 3 (L3) function.

Switching

Moving a packet from an inputport to an output port and out.

A layer 2 function.

INPUT PORTS OUTPUT PORTS

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MPLS Turns Routing into Switching

So we can avoid performing the layer 3 function.

Use labels to decide next hops.

What benefit does this provide?

In what situations would this benefit not be verysignificant?

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MPLS: Flexible Forwarding

LSP to IPLABEL SWITCHINGIP to LSP


IP DA

IP: Packets are forwarded based on Destination Address (DA). We can

call this destination based routing.

MPLS:

Map packets to LSP based on (Source Address, Destination Address,

protocol, port, DSCP, interface, etc.) Forward packets based on the Label

IP DA IP DA IP DA IP DA

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MPLS-based Solutions

Enable QoS in IP Networks Support Diffserv + connection-oriented QoS

IP Traffic Engineering Use constraint-based routing to adapt to latest network

loading and QoS performance

Virtual Private Networks Use controllable tunneling mechanisms

L2/L3 Integration

Integrate with L1 and L2 technologies like Optical CrossConnects (OXCs) and ATM

Resilient Network Design Automatic Failover and Backup

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BEST OF BOTH WORLDS

PACKET

Forwarding

CIRCUIT

SWITCHING

MPLS + IP forms a middle ground that combines

the best of IP and the best of circuit switching

technologies.

MPLS + IP forms a middle ground that combines

the best of IP and the best of circuit switching

technologies.

MPLS+IP

IP ATM

HYBRID

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MPLS Terminology

LDP: Label Distribution Protocol

LSP: Label Switched Path

LER: Label Edge Router(edge of an areathat supports MPLS)

LSR: Label Switching Router (inside an areathat supports MPLS)

FEC: Forwarding Equivalence Class

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LERs and LSRs

LSP to IPLABEL SWITCHINGIP to LSP


LER LERLSRs

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Forwarding Equivalence Classes

FEC = A subset of packets that are all treated the same way by a router

The concept of FECs provides for a great deal of flexibility andscalability. Traffic can be aggregated or differentiated in many ways

In conventional routing, a packet is assigned to an FEC at each hop (i.e.

L3 look-up), in MPLS it is only done once at the network ingress.

Packets are destined for different address prefixes, but can be

mapped to common path

Packets are destined for different address prefixes, but can be

mapped to common path

IP1

IP2

IP1

IP2

LSRLSR

LER LER

LSP

IP1 #L1

IP2 #L1

IP1 #L2

IP2 #L2

IP1 #L3

IP2 #L3

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Two types of Label Switched Paths:

Hop by hop (Vanilla LDP)

Explicit Routing (LDP+ER)

#18

#427

#819

#216

#14#612

#5 #99#311

#963

#462

#77

Label Switched Path (Two Types)

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#216

#612

#5#311

#14

#99

#963

#462

- A Vanilla LSP creates MPLS paths for standard IProuting (from IP routing tables).

- A Vanilla LSP is actually part of a tree from every source

to that destination (unidirectional).

#963

#14

#99

#311

#311

#311

LABEL SWITCHED PATH (vanilla)

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STANDARD IP

47.1

47.2

Network 47.3

Dest Out

47.1 1

47.2 2

47.3 3

1

23

Dest Out

47.1 1

47.2 2

47.3 3

Dest Out

47.1 1

47.2 2

47.3 3

1

2

1

2

3

Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.

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47.1

47.247.3

IP 47.1.1.1

Dest Out

47.1 1

47.2 2

47.3 3

1

23

Dest Out

47.1 1

47.2 2

47.3 3

1

2

1

2

3

IP 47.1.1.1

IP 47.1.1.1IP 47.1.1.1

Dest Out

47.1 1

47.2 2

47.3 3

STANDARD IP

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Label Switched Path (LSP)

IIntf47.1

47.247.3

1

2

31

2

1

2

3

3

IntfIP 47.1.1.1IP 47.1.1.1

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IIntf

MPLS Label Distribution

47.1

47.247.3

1

2

3

1

2

1

2

3

3

IntfMapping: 40

Request: 47.1

Mapping

:50

Reques

t:47.1

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Benefits and Limitations

Why might this approach be better than normal IP

forwarding that does not use MPLS? Remember, all packets still travel the same paths.

What else might we be able to do with MPLS that couldbe even more powerful?

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#216

#14

#462

- ER-LSP follows the route that source chooses. In otherwords, the control message to establish the LSP (label

request) is source routed.

#972

#14 #972

A

B

C

Route=

{A,B,C}

EXPLICITLY ROUTED OR ER-LSP

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IIntf47.1

47.247.3

1

2

3

1

2

1

2

3

3

IntfIP 47.1.1.1IP 47.1.1.1

EXPLICITLY ROUTED LSP ER-LSP

Explicitly Routing LSP

that does not follow

the standard IP path.

Explicitly Routing LSP

that does not follow

the standard IP path.

This entry gives the

longest prefix match.

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ER LSP - Advantages

Operator has routing flexibility

Can establish LSPs based on policy, QoS, etc.

Can have pre-established LSPs that can be used in case of failures.

Can use routes other than shortest path

Can compute routes based on dynamic constraints(available bandwidth, delay, etc.) in exactly the same manner

as ATM based on a distributed topology database.

(traffic engineering)

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ER LSP - Discord!

Two signaling options proposed in the standards: CR-LDP, RSVPextensions:

CR-LDP = Label Distribution Protocol (LDP) + Explicit Routing

RSVP-TE = Traditional Resource Reservation Protocol (RSVP) + Explicit Route +

Scalability Extensions

RSVP was established several years ago to be able to reserveresources along a path.

To ensure QoS by making sure each flow had enough resources.

Had significant scalability problems.

ITU has decided on LDP/CR-LDP for public networks.

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Tutorial Outline

Overview

Label Encapsulations

Label Distribution Protocols

Constraint Based Routing

MPLS and VPNs


Summary

Upper Layer Consistency Across

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Upper Layer Consistency AcrossLower Layers

EthernetATM

Frame

Relay

MPLS is multiprotocol below (link layer)

Provides for consistent operations, engineering across multiple

technologies

Allows operators to leverage existing infrastructure

Co-existence with other protocols is provided for

WDM

GigEthernet Optical Cross

Connect (OXC)

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MPLS Encapsulation - PPP & LAN Data Links

Layer 2 Header(eg. PPP, 802.3)

Network Layer Headerand Packet (eg. IP)

MPLS Shim Headers (1-n)

1n

Network layer must be inferable from value of bottom label of the stack

MPLS on PPP links and LANs uses Shim Header Inserted

Between Layer 2 and Layer 3 Headers

(other technologies use different approaches)

MPLS on PPP links and LANs uses Shim Header Inserted

Between Layer 2 and Layer 3 Headers

(other technologies use different approaches)

Label Exp. S TTL

Label: Label Value, 20 bits (Values 0 through 16 are reserved)

Exp.: Experimental, 3 bits (was Class of Service)

S: Bottom of Stack, 1 bit (1 = last entry in label stack)

TTL: Time to Live, 8 bits

4 Octets

Label Stack

Entry Format

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Tutorial Outline

Overview



Constraint-Based Routing

MPLS and VPNs


Summary

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Label Distribution Protocol (LDP)

Constraint-based Routing LDP (CR-LDP)

Extensions to RSVP

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Label Distribution Protocol (LDP) - Purpose

Label distribution ensures that adjacent routers have

a common view of bindings of FECs to labels

Routing Table:

Addr-prefix Next Hop

47.0.0.0/8 LSR2

Routing Table:


47.0.0.0/8 LSR2

LSR1 LSR2 LSR3

IP Packet 47.80.55.3

Routing Table:


47.0.0.0/8 LSR3

Routing Table:


47.0.0.0/8 LSR3

Common understanding of which FEC the label is referring to!

Label distribution can either piggyback on top of an existing routing protocol,

or a dedicated label distribution protocol (LDP) can be created.

Label distribution can either piggyback on top of an existing routing protocol,

or a dedicated label distribution protocol (LDP) can be created.

Label Information Base:

Label-In FEC Label-Out

17 47.0.0.0/8 XX



17 47.0.0.0/8 XX

Step 1: LSR creates binding

between FEC and label value

For 47.0.0.0/8

use label 17

Step 2: LSR communicates

binding to adjacent LSR



XX 47.0.0.0/8 17



XX 47.0.0.0/8 17

Step 3: LSR inserts label

value into forwarding base

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Labels are Downstream Assigned

Note that label assignments are decided at the

downstream node and communicated to the upstreamnode.

Why does it need to be done this way?

What flexibility does this approach provide?

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Label Distribution Protocol (LDP) Constraint-based Routing LDP (CR-LDP)

Extensions to RSVP

T ffi E i i R i

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Traffic Engineering Requirements

Constraint-Based Routing is one method of Traffic Engineering. Traffic

Engineering seeks to engineer the best use of capacity.

RFC 2702:

Strict & Loose ER

Specification of QoS

Specification of Traffic Parameters

Route Pinning

Preemption Failure Recovery

C t i t B d R ti i LDP (CR

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Constraint Based Routing using LDP (CR-LDP)

Built on existing LDP messages over TCP.

Defines an Explicit Route:

Detailed path that can traverse any links supporting CR-LDP.

Defines a set of constraints for LSP computation andadmission:

Expectation and Allocation of resources:

Peak burst & rate, Committed burst & rate,

Excess burst, Frequency, Weight.

Preemption Level:

Setup and Holding Priority with respect to other LSPs.

Resource Class:

Color of traffic inclusion, exclusion rules for links.

CR LDP P ti

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CR-LDP Preemption

Preemption may or may not be a good idea in aparticular context recall our discussion inprevious lectures.

A CR-LSP carries an LSP priority. This priority canbe used to allow new LSPs to bump existing LSPs

of lower priority in order to steal their resources. This is especially useful during times of failure and

allows you to rank the LSPs such that the mostimportant obtain resources before less important

LSPs. These are called the setup-Priority and a holding-

Priority and 8 levels are provided.

CR LDP P ti

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CR-LDP Preemption

When an LSP is established, its setup-Priority iscompared with the holding-Priority of existing

LSPs, any with lower holding-Priority may be

bumped to obtain their resources.

This process may continue in a domino fashionuntil the lowest holding-Priority LSPs either clear

or are on the worst routes.

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#216

#14

#462

#972A

B

C

Route=

{A,B,C}

Preemption A.K.A. Bumping

This LSP must be

preempted.

Now this one canproceed.

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Label Distribution Protocol (LDP)

Constraint-based Routing LDP (CR-LDP)

Extensions to RSVP

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Tutorial Outline

Overview Label Encapsulations


Constraint Based Routing MPLS and VPNs


Summary

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- IP will over-utilize best paths and under-utilize

not-so-good paths.

Dest=a.b.c.d

Dest=a.b.c.d

Dest=a.b.c.d

IP Follows a Tree to the Destination

a.b.c.d

HOP BY HOP (A K A Vanilla) LDP

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#216

#14

#612

#5 #99#311

#963

#462

- Ultra fast, simple forwarding a.k.a switching- Follows same route as normal IP datapath

- So like IP, LDP will over-utilize best paths and

under-utilize less good paths.

HOP-BY-HOP (A.K.A Vanilla) LDP


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Two types of Label Switched Paths:

Hop by hop (Vanilla LDP)

Explicit Routing (LDP+ER)

#18

#427

#819

#216

#14

#612

#5 #99#311

#963

#462

#77


CR LDP

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CR = Constraint based Routing

Example: USE: (links with sufficient resources) AND

(links of type someColor) AND

(links that have delay less than 200 ms)

&&

=

CR-LDP

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Traffic Engineering

A

B C

D

Traffic engineering is the process of mapping traffic demand onto a networkTraffic engineering is the process of mapping traffic demand onto a network

Demand

Network

Topology

Purpose of traffic engineering:

Maximize utilization of links and nodes throughout the network Engineer links to achieve required delay, grade-of-service Spread the network traffic across network links to minimize impact of failure Ensure available spare link capacity for re-routing traffic on failure Meet policy requirements imposed by the network operator

Traffic engineering is key to optimizing cost/performance

MPLS Traffic Engineering Methods

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MPLS Traffic Engineering Methods

MPLS can use the source routing capability to

steer traffic on desired paths

An operator may manually configure LSRs alongthe desired paths.

What are limitations of manual configuration?

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WHEN SHOULD TE BE USED?

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WHEN SHOULD TE BE USED?

When it is not acceptable to simply rank packets andthrow away the least important traffic first.

When traffic is being thrown away but you have otherviable routes that are unused or underutilized.

Dont use TE if it is not necessary. In fact dont useMPLS if vanilla IP is working for you. Use LDP, CR-LDP

and RSVP-TE if/when they are needed.

Reactive traffic engineering

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Reactive traffic engineering

Wait till you have a problem and then patch

around it.

1- Identify a flow to move

Q: how?

2- Establish an LSP on some other route

Q: what route?

How to identify a flow to move?

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How to identify a flow to move?Good Statistics!

A) move the flow that has packets being discarded. Forthis you need to have stats that show {src, dest,

protocol} that are being thrown away. (note this is TE of

leastimportanttraffic)

B) move some other high priority user on the link

somewhere else. For this you need to have stats that

show {src, dest, protocol} of high users. (note, this is TE

ofmoreimportanttraffic)

What route should be used for an LSP?

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What route should be used for an LSP?An non-shortest path!

A) Explicitly route withouthelp of constraint basedrouting.

B) Use constraint not this link so that MPLS can pick

all the other links dynamically but is not allowed to pick

the congested one.

Neither of these approaches will result in shortest paths

and both are hard to administer as things scale up.

Pro-active traffic engineering (plan ahead)

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Pro active traffic engineering (plan ahead)

1- Start with rough idea on {Si, D} B/W

requirements.

2- Establish constraint based tunnels {Si -> D}

3- repeat forever at regular planning intervals(days, weeks, months)

3a- Remeasure {Si -> D} B/W utilization.

3b- Adjust reservations on {Si ->D} to be closer

S1

S2

S6

S3S4

S5

D

MPLS Traffic engineering

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MPLS Traffic engineering

Imperative to be able to monitor flow rates to

the granularity of {source, dest, protocol} .

Use MPLS constraint based routing to assign

paths to flows based on a reservation.

Try to adjust the reservations periodically toreflect changes in utilization.

MPLS aims to do a really good job of placing

routes given the reservations are accurate.

MPLS allows dynamic changes to reservations

so they can slowly converge to reality over

time.

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MPLS Traffic engineeringinteractions with vanilla IP.

There are non trivial interaction issues to deal with

when some of the traffic is traffic engineered (MPLS)

and the rest (vanilla IP) is not.

What problems might occur?

Tutorial Outline

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Tutorial Outline

Overview




MPLS and VPNs


Summary

MPLS Provides Benefits for Establishing

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MPLS Provides Benefits for EstablishingVirtual Private Networks

Virtual Private Network (VPN) Connects two or more separate sites over the Internet

Allows them to function as if they were a single, private network.

Key Features: Security, control over performance, management

ability.

Use of MPLS for VPNs

MPLS can set up one or more LSPs between sites.

Organizations can choose how they use the LSPs.

Can view the LSPs as virtual network links.

Significant debate is in progress on how to use MPLS for VPNs.

Will study VPNs more in a later lecture.

Outline

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Outline

Overview




MPLS and VPNs


Summary

The need for MPLS protection

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The need for MPLS protection

MPLS-based services have been growing

VPNs Migration of ATM and Frame Relay onto MPLS

Traffic Engineering

MPLS-based services are more demanding of resiliency

and reliability Because MPLS promises more reliability.

Thats one reason why customers would use MPLS-based services.

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MPLS protection approaches

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Headend reroute

Recompute LSPs to find a new LSP after a failure.

Initiated by the source or LER.

Just has slow as using OSPF or RIP not good.

Pre-signaled standby LSPs

Backup LSP is signaled in advance from primary ingress to egress.

That does not share any links or LSRs with the primary path.

Called link and node disjoint.

Has of millisecond failover, around 500 milliseconds.

These backup LSPs need to have resources reserved for them, butthese resources are not used very often.

Pre-signaled Standby LSPs

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Pre signaled Standby LSP s

Planning occurs before failure

Then LSP ingress learns of the failure

Moves traffic to use standby LSP

Ingress must first know about the failure

The farther away the failure, the longer it will take to start the

reroute.


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MPLS Fast Reroute During the establishment of a primary LSP, a protected path for each

possible link or node failure is pre-signaled.

Even faster failover, around 50 milliseconds.

But many more LSPs are established, with resources reserved tothem

Much more to manage.

Midpoint LSRs make their own LSPs to the egress. To go around the immediate downstream link or node that could fail.

LSPs are established using the same criteria used for the primaryLSP.

Since the PLR (Protection Label-Switched Router) is immediatelyconnected to the failed link, it will know about the failure very quicklyfrom the hardware.

Then the backup LSP can be used.

MPLS Fast Reroute

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MPLS Fast Reroute

A merge node joins traffic back onto the primary LSP.

Tutorial Outline

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Tutorial Outline

Overview




MPLS and VPNs


Summary

Summary of Motivations for MPLS

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Simplified forwarding based on an exact match of a fixedlength label

Initial driver for MPLS was based on the existence of cheap, fast

ATM switches

Separation of routing and forwarding in IP networks Facilitates evolution of routing techniques by fixing the forwarding

method

New routing functionality can be deployed without changing the

forwarding techniques of every router in the Internet

Facilitates the integration of ATM and IP

Allows carriers to leverage their large investment of ATM equipment


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Enables the use of explicit routing/source routing in IPnetworks

Can easily be used for such things as traffic management, QoS

routing

Promotes the partitioning of functionality within thenetwork

Move detailed processing of packets to the edge; restrict core to

simple packet forwarding

Assists in maintaining scalability of IP protocols in large networks


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Applicability to both cell and packet link-layers

Can be deployed on both cell (eg. ATM) and packet (eg. FR, Ethernet)

media

Common management and techniques simplifies engineering

But MPLS is much more complex than traditional IPforwarding

Routers need to be able to forward based on labels (in addition to their

normal functions).

LSPs must be signalled and maintained.

Some ISPs have said they are not using MPLS and do not plan to.

This will continue to be true if overprovisioning remains effective. But MPLS is more seriously being considered to carry legacy ATM and

Frame Relay traffic (connection-oriented traffic).

And some of these ISPs are realizing that their customers want MPLS

to provide more assurance about their IP-based services.

S y S

MPLS: Partitioning Routing and Forwarding

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MPLS: Partitioning Routing and Forwarding

Routing

Forwarding

OSPF, IS-IS, BGP, RIP

MPLS

Forwarding Table

Based on:Classful Addr. Prefix?

Classless Addr. Prefix?

Multicast Addr.?Port No.?

ToS Field?

Based on:Exact Match on Fixed Length Label

Current network has multiple forwarding paradigms- classful longest prefix match (Class A,B,C boundaries)

- classless longest prefix match (variable boundaries)

- multicast (exact match on source and destination)

- type-of-service (longest prefix. match on addr. + exact match on ToS) As new routing methods change, new route look-up algorithms are required

- like when CIDR was introduced Next generation routers will be based on hardware for route look-up

- changes will require new hardware with new algorithms MPLS has a consistent algorithm for all types of forwarding; partitions routing/fwding

- minimizes impact of the introduction of new forwarding methods

MPLS introduces flexibility through a consistent

forwarding paradigm

MPLS introduces flexibility through a consistent

forwarding paradigm

Summary

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Summary

MPLS is an important emerging technology.

MPLS/LDP/CR-LDP have been recommended by the ITUfor IP transport on ATM in public networks.

Basic functionality (Encapsulation and basic LabelDistribution) has been defined by the IETF.

Traffic Engineering based on MPLS/CR-LDP is being

developed (protocols and research).

Documents

CS520_Lect_15_MPLS_TE_2003