16-MPLS Concepts

8/7/2019 16-MPLS Concepts

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2000, Cisco Systems, Inc.

MPLS ConceptsMPLS ConceptsModule 16Module 16


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2000, Cisco Systems, Inc. www.cisco.com MPLS TechnologyChapter 1-2

AgendaAgenda

Basic MPLS Concepts

MPLS Labels and Label Stack

MPLS Applications


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Basic MPLS ConceptsBasic MPLS Concepts

2000, Cisco Systems, Inc. www.cisco.com Chapter#-3


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Basic MPLS ConceptsBasic MPLS Concepts

Multi-protocol Label Switching is a new forwarding

mechanism in which packets are forwarded based on labels.

Labels may correspond to IP destination networks (equal to

traditional IP forwarding).

Labels can also correspond to other parameters (QoS,

source address, ...).

MPLS was designed to support forwarding of other protocols

as well.


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MPLS ExampleMPLS Example

Only edge routers have to perform a routing lookup.

Core routers switch packets based on simple label lookups and swaplabels.

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


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MPLS ArchitectureMPLS Architecture

MPLS has two major components:

Control plane exchanges layer-3 routing information andlabels

Data plane forwards packets based on labels Control plane contains complex mechanisms to exchange routing

information (OSPF, EIGRP, IS-IS, BGP,...) and labels (TDP, LDP,BGP, RSVP, ...).

Data plane has a simple forwarding engine.

Control plane maintains the contents of the label switching table (labelforwarding information base or LFIB).


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MPLS ArchitectureMPLS Architecture

Routers functionality can be divided into two major parts:control plane and data plane

Data plane

Control plane

OSPF: 10.0.0.0/8

LDP: 10.0.0.0/8Label 17

OSPF

LDP

LFIB

LDP: 10.0.0.0/8Label 4

OSPF: 10.0.0.0/8

417

Labeled packet

Label 4

Labeled packet

Label 17


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MPLS Modes of OperationMPLS Modes of Operation

MPLS technology is intended to be used anywhereregardless of layer-1 media and layer-2 protocol.

MPLS uses a 32-bit label field which is insertedbetween layer-2 and layer-3 headers (frame mode).

MPLS over ATM is using the ATM header as thelabel (cell mode).


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Label FormatLabel Format

MPLS uses a 32-bit label field that contains thefollowing information:

20-bit label

3-bit experimental field

1-bit bottom-of-stack indicator 8-bit time-to-live field (TTL)

LABEL EXP S TTL

0 19 22 23 3120 24


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Frame Mode MPLSFrame Mode MPLS

Frame

headerIP header Payload

Layer 2 Layer 3

Frame

headerLabel IP header Payload

Layer 2 Layer 2 Layer 3

Routing lookup

andlabel

assignment


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Label Switch RouterLabel Switch Router

Label Switch Router (LSR) primarily forwards labeled packets (labelswapping).

Edge LSR primarily labels IP packets and forwards them into MPLSdomain, or removes labels and forwards IP packets out of MPLS

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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Architecture of LSRsArchitecture of LSRs

LSRs, regardless of the type, perform the following threefunctions:

Exchange routing information

Exchange labels

Forward packets or cells

The first two functions are part of the control plane.

The last function is part of the data plane.


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Architecture of LSRsArchitecture of LSRs

LSRs primarily forward labeled packets or cells (ATMLSRs)

LSR

Control plane

Data plane

Routing protocol

Label distribution protocol

Label forwarding table

IP routing table

Exchange ofrouting information

Exchange oflabels

Incoming

labeled packets

Outgoing

labeled packets


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Architecture of Edge LSRsArchitecture of Edge LSRs

Note: ATM edge LSRs can only forward cells

Edge LSR

Control plane

Data plane

Routing protocol

Label distribution protocol


IP routing table

Exchange ofrouting information

Exchange oflabels

Incominglabeled packets

Outgoinglabeled packets

IP forwarding table

IncomingIP packets

OutgoingIP packets


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MPLS Labels and Label StackMPLS Labels and Label Stack



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MPLS Label FormatMPLS Label Format

MPLS uses a 32-bit label field that contains the followinginformation:

20-bit label (a number)

3-bit experimental field (usually used to carry IP precedencevalue)

1-bit bottom-of-stack indicator (indicates whether this is the lastlabel before the IP header)

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 LabelsMPLS Labels

Labels are inserted between the layer-2 (frame) header andthe layer-3 (packet) header.

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

Bottom-of-stack bit indicates if the label is the last label inthe label stack.

Time-to-live (TTL) field is used to prevent indefinite loopingof packets.

Experimental bits are usually used to carry the IP

precedence value.


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MPLS Label StackMPLS Label Stack

Protocol identifier in a layer-2 header specifies that thepayload starts with a label (labels) and is followed by an IPheader.

Bottom-of-stack bit indicates whether the next header isanother label or a layer-3 header.

Receiving router uses the top label only.

Frame

headerLabel 1 IP header PayloadLabel 2 Label 3

S=0 S=0S=1

PID=MPLS-IP


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MPLS Label StackMPLS Label Stack

Usually there is only one label assigned to a packet.

The following scenarios may produce more than one label:

MPLS/VPNs (two labels - the top label points to the egress routers and the

second label identifies the VPN).

Traffic Engineering (two or more labels the top label points to the endpoint

of the traffic engineering tunnel and the second label points to the

destination).

MPLS/VPNs combined with Traffic Engineering (three or more labels).


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MPLS ForwardingMPLS Forwarding

An LSR can perform the following functions:

Insert (impose) a label or a stack of labels on ingress.

Swap a label with a next-hop label or a stack of labels in

the core.

Remove (pop) a label on egress.

ATM LSRs can only swap a label with one label

(VPI/VCI fields change).


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MPLS Forwarding(Frame Mode)

MPLS Forwarding(Frame Mode)

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

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

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

MPLS Domain

10.1.1.1

IP Lookup10.0.0.0/8 label 3

LFIBlabel 8 label 3

IP Lookup10.0.0.0/8 label 5

LFIBlabel 3 label 5

IP Lookup10.0.0.0/8 next hop

LFIBlabel 5 pop

10.1.1.13 10.1.1.15 10.1.1.1


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MPLS ApplicationsMPLS Applications



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Multicast IP RoutingMulticast IP Routing

To support multicast traffic across an MPLS domain we donot need a dedicated protocol.

PIM version 2 with extensions for MPLS is used to

propagate routing information as well as labels.

Forwarding Equivalence Class (FEC) is equal to adestination multicast addresses, stored in the multicastrouting table.

Protocol Independent Multicast


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MPLS Traffic EngineeringMPLS Traffic Engineering

Traffic engineering requires OSPF orIS-IS with extensionfor MPLS/TE as the internal gateway protocol (IGP).

OSPF and IS-IS with extensions hold the entire topology in

their databases. They also have some additional information about network

resources and constraints.

RSVP orCR-LDP are used to establish Traffic Engineering

tunnels and propagate labels. RSVP = Resource Reservation Protocol

CR-LDP = Constraint-based Routed LDP


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Traffic Engineering with MPLSTraffic Engineering with MPLS

Traffic can be forwarded based on other parameters (QoS, source, ...). Load sharing across unequal paths can be achieved.

Secondary

OC-48 link

Large site A

Large site B

Small site C

Primary OC-192

link


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Quality of SerivceQuality of Serivce

Quality of Service is an extension to unicast IProuting that provides differentiated services.

Extensions to TDP or LDP are used to propagate

different labels for different classes.

Forwarding Equivalence Class is a combination of adestination network and a class of service.


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Virtual Private NetworksVirtual Private Networks

Networks are learned via an IGP (OSPF, EBGP, RIPv2 or static) froma customer or via BGP from other internal routers.

Labels are propagated via multi-protocol BGP.

Two labels are used: Top label points to the egress router (assigned through LDP or TDP).

Second label identifies the outgoing interface on the egress router or a routingtable where to perform a routing lookup.

Forwarding Equivalence Class is equal to a VPN site descriptor orVPN routing table.


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Control plane

MulticastIP Routing

MPLS TrafficEngineering

Quality of Service MPLS/VPNUnicastIP Routing

Interaction Between MPLS ApplicationsInteraction Between MPLS Applications

Data plane

Any IGP

LDP/TDP


Unicast IProuting table

PIM version 2

MulticastIP routing table

OSPF or IS-IS

LDP


RSVP

Any IGP

LDP/TDP


Any IGP

LDP

Unicast IProuting tables

BGP


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2000, Cisco Systems, Inc. www.cisco.com MPLS TechnologyChapter 1-31Configuring Frame-Mode MPLS on IOS Platforms-31 2000, Cisco Systems, Inc. www.cisco.com

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16-MPLS Concepts