Curso MPLS

7/27/2019 Curso MPLS

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1CQFE rev17 Russ Davis 1999, Cisco Systems, Inc.

1

Basic MPLS

2000, Cisco Systems, Inc.

2202

1358_06_2000_c2


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2CQFE rev14 Russ Davis 1999, Cisco Systems, Inc. www.Cisco.com

Agenda

Multi Protocol Label Switching

MPLS Concepts

LSRs and labels

Label assignment and distribution

Label Switch Paths

ATM LSRsLoop detection/prevention

LDP concepts

Configurations


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RSP

VIP

IP

First Packet

Subsequent Packets

Distributed Fast Switching

Available withintelligent IPs

Distributed

cache


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Cisco Express Forwarding

Main components

Forwarding Information Base (FIB)

Adjacency table

No process switching of packets


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Cisco Express Forwarding

Forwarding Information Base

Adjacency Database


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FIB Table

Shadow copy of the IP routing table

Classless

Routing protocol independent One for each route in IP routing table

Each entry has one or more path

Each path has nexthop IP address andnexthop interface

Each path points to an adjacency


7/697CQFE rev14 Russ Davis 1999, Cisco Systems, Inc. www.Cisco.com

Adjacency Table

Maintains IP address toMac-rewrite mapping

Populated by ARP table, Frame Relaymap table and ATM map table, etc.

Mac-rewrite of the nexthop is allthats required to switch packet


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CEF Operation

FIB entry created when routesare added to IP routing table

If connected, new FIB entry pointsto the corresponding adjacency

Ready to switch packets

Non-connected prefix requiresmore work



MPLS Concepts

Separation of routing and forwarding

Forwarding is done independently Integrates the control of IP routing with

simplicity of layer 2 switching

Helps in deployment of complicatedrouting services



MPLS Concepts

Separation of control and forwarding

Control is build by standard routingprotocols (OSPF,ISIS,BGP)

Forwarding is build by LDP independentof routing protocols



MPLS Concepts

Forwarding Equivalence Class(FEC)

Group of IP packets forwarded withsame treatment over the same pathirrespective of the final destination

Packet is assigned to FEC based on itsnetwork layer destination address



Label

Label is a short fixed length, locally significantused to identify a FEC

Label is assigned on the network layer addressbut it is not encoding of the network layer

addressLabel has local significance between LSRs

MPLS Concepts



MPLS Concepts

Labeled Packet

Packet into which a label has beenencoded

Label could reside inside network ordata link layer (ATM)



MPLS Concepts

UpstreamWhere the packet source is

Downstream

Towards the destination



Upstream and Downstream LSRs

Rtr-C is the downstream neighbour of Rtr-

B for destination 171.68.10/24

Rtr-B is the downstream neighbour of Rtr-A for destination 171.68.10/24

LSRs know their downstream neighboursthrough the IP routing protocol

Next-hop address is the downstream neighbour

171.68.10/24

Rtr-BRtr-A Rtr-

C

171.68.40/24



MPLS Concepts

Label Assignment

Assignment is done by the LSR downstream

Downstream LSR informs the upstreamLSR of the binding



MPLS Concepts

Label Distribution Protocols (LDP)

Set of procedures by which one LSRinforms other about the label for acertain FEC

LSR that distribute label informationbetween each other are known as LabelDistribution Peers



MPLS Concepts

MPLS architecture does not assume

a single protocolExisting protocols have been extended(MP-BGP), (MPLS-RSVP)

New protocols have also been defined(MPLS-LDP), (MPLS-CR-LDP)


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MPLS allows:

Packet classification only where the packetenters 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 Concepts


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MPLS conceptsPacket forwarding: FEC and Next-Hop

MPLS make use of FECs

MPLS nodes assign a label to each FEC

Packet classification (into a FEC) is donewhere the packet enters the core

No sub-sequent packet classification inthe MPLS network


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Address

Prefix and mask

171.68.10/24

...

Next-Hop

171.68.9.1

...


171.68.10/240 1

Interface

Serial1

...

IP packetD=171.68.10.12IP packet

D=171.68.10.23

Router-A forwards packets withdifferent destination addresses usingthe same route, same next-hop andsame interface

Rtr-A


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Packets are classified into FECs

For each FEC the next-hop iscalculated

In IP routing each hop:

re-classify the packet into one FEC

recalculate the next-hop of the FEC


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Next-Hop


171.68.10/240 1

Router-A forwards labelledpackets by looking at the labelvalue against the label table. Nopacket classification into FEC isdone.

Rtr-A

In

Lab

5

...

Address

Prefix

171.68.10

...

Out

I/F

1

...

Out

Lab

3

...

In

I/F

0

...

IP packetD=171.68.10.12

Label = 5

Next-Hop

InLab

x

...

AddressPrefix

171.68.10

...

OutI/F

3

...

OutLab

5

...

InI/F

4

...

34

IP packetD=171.68.10.12

Rtr-B

Router-B classify the IP packet into aFEC and assign the correspondinglabel.

IP packetD=171.68.10.12

Label = 3

MP S


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MPLS conceptsMPLS forwarding

MPLS forwarding is performed in the sameway in ATM switches and routers.However,

ATM queuing is given by the label value (VCI)

Router queuing may be given by EXP bits in labelheader

ATM switches do not have capabilities to

analyse layer 3 headers

Labels may be distributed by differentprotocols

LDP, RSVP, PIM, BGP, ...


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MPLS t


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MPLS conceptsATM-LSR

In order to exchange labels, ATM-LSRs need to runan IP routing protocol

ATM-LSRs will act as routers in terms of IP control plane

ATM-LSRs will act as ATM switches in terms of data plane

ATM-LSRs will NOT route packets based on routing table

Packet forwarding is based on label information

Control VC is used to exchange labels

ATM switches use input port,VPI,VCI values and map

them to output port,VPI,VCI values

Label is encoded in same fields

VPI/VCI field used to carry label information

Existing software can work for label swapping

MPLS t


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MPLS conceptsLabels

Label format and length depends onencapsulation

Has to be negotiated between peers overATM interfaces

More than one label is allowed

Label stack: ordered set of labels

MPLS LSRs always forward packets basedon the value of the label at the top of thestack


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MPLS conceptsLabels

ATM Cell Header HEC DATACLPPTIVCIGFC VPI

Label


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Upstream and Downstream LSRs

LSRs assign a label to each FEC

Label distribution may be upstream or

downstream driven Most implementations use downstream

with two variants

Unsolicited DownstreamDownstream on demand

no need for upstream allocation

U li it d D t


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Unsolicited Downstreamdistribution


LSRs distribute labels to the upstream

neighbours

171.68.10/24

Rtr-BRtr-A Rtr-

C

171.68.40/24

Next-Hop

In

Lab-

...

Address

Prefix171.68.10

...

Out

I/F1

...

Out

Lab5

...

In

I/F0

... Next-Hop

In

Lab

5

...

Address

Prefix

171.68.10

...

Out

I/F

1

...

Out

Lab

7

...

In

I/F

0

...

Next-Hop

In

Lab

7

...

Address

Prefix

171.68.10

...

Out

I/F

1

...

Out

Lab

-

...

In

I/F

0

...

Use label 7 for destination171.68.10/24


IGP derived routes

Downstream on demand


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Downstream on demanddistribution


Upstream LSRs request labels todownstream neighbours

Downstream LSRs distribute labels upon

request

171.68.10/24

Rtr-BRtr-A Rtr-

C

171.68.40/24



Request label fordestination 171.68.10/24

Request label fordestination 171.68.10/24

Downstream on demand


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Downstream on demandand VC-Merge

VC-Merge allows correct packet re-assembling

Sequencing of cells by buffering

Receiving (downstream) ATM-LSR cansecurely re-assemble cells into packets

Even cells of different packets use sameVPI/VCI value

Save label space on ATM-LSRs

Downstream on Demand


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Downstream on Demandand ATM

In

Lab

5

8

...

Address

Prefix

171.68

171.68

...

Out

I/F

0

0

...

Out

Lab

3

3

...

In

I/F

1

2

...

171.68

IP

Packet

IPPacket

ATM

cell

5

ATM

cell

8

ATM

cell

8

ATM

cell

8

ATM

cell

5

ATM

cell

3

ATM

cell

3

ATM

cell

3

ATM

cell

3

ATM

cell

3

Downstream LSR do not know howto reassemble correctly cells intopackets. VPI/VCI values are identicalfor all cells

Downstream on Demand


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Downstream on Demandand ATM

In

Lab

5

8

...

Address

Prefix

171.68

171.68

...

Out

I/F

0

0

...

Out

Lab

3

4

...

In

I/F

1

2

...

171.68

IP

Packet

IPPacket

ATM

cell

5

ATM

cell

8

ATM

cell

8

ATM

cell

8

ATM

cell

5

ATM

cell

3

ATM

cell

4

ATM

cell

4

ATM

cell

4

ATM

cell

3

ATM-LSR requested additional labelfor same FEC in order to distinguishbetween incoming interfaces(Downstream on Demand)

ATM LSRs and


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ATM-LSRs andVC-Merge

In

Lab

5

8

...

Address

Prefix

171.68

171.68

...

Out

I/F

0

0

...

Out

Lab

3

3

...

In

I/F

1

2

...

171.68

IP

Packet

IPPacket

ATM

cell

5

ATM

cell

8

ATM

cell

8

ATM

cell

8

ATM

cell

5

ATM

cell

3

ATM

cell

3

ATM

cell

3

ATM

cell

3

ATM

cell

3

ATM-LSR transmitted cells in sequencein order for the downstream LSR tore-assembling correctly the cells intopackets


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Label retention modes

Liberal retention modeLSR retains labels from all neighbours

Improve convergence time, when next-hop isagain available after IP convergence

Require more memory and label space

May be a problem in ATM-LSRs since a label is aVC

Conservative retention mode (*)

LSR retains labels only from next-hops neighbours

LSR discards all labels for FECs without next-hop

Free memory and label space

Label Distribution


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Label DistributionOrdered vs. Independent

Ordered LSP control

LSR only binds and advertise a label for a particular FEC if:

it is the egress LSR for that FEC or

it has already received a label binding from its next-hop

Independent LSP control

LSR binds a label to a FEC independently from the label it has toreceive from its next-hop

Similar to link-state IP routing (flooding): each router buildrouting table independently

An LSR may label forward packet to a next-hop that does nothave yet label information for that FEC


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Label Distribution Protocols

Several protocols for label exchange

LDP

Maps unicast IP destinations into labels

RSVP, CR-LDP

Used for traffic engineering and resourcereservation

PIMFor multicast states label mapping

BGP

External labels (VPN)


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Label retention modes

Liberal retention modeLSR retains labels from all neighbours

Improve convergence time, when next-hop isagain available after IP convergence

Require more memory and label space

May be a problem in ATM-LSRs since a label is aVC

Conservative retention mode

LSR retains labels only from next-hops neighbours

LSR discards all labels for FECs without next-hop

Free memory and label space

Label Distribution


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Label DistributionOrdered vs. Independent

Ordered LSP control

LSR only binds and advertise a label for a particular FEC if:

it is the egress LSR for that FEC or

it has already received a label binding from its next-hop

Independent LSP control

LSR binds a label to a FEC independently from the label it has toreceive from its next-hop

Similar to link-state IP routing (flooding): each router buildrouting table independently

An LSR may label forward packet to a next-hop that does nothave yet label information for that FEC


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The Label Stack

171.68.10/24

Rtr-A

Next-Hop

In

Lab

5

...

Address

Prefix

171.68.10

...

Out

I/F

1

...

Out

Lab

7

...

In

I/F

0

...

IP packetD=171.68.10.12

Label = 5

Label = 21

IP packetD=171.68.10.12

Label = 7

Label = 21

Rtr-A forwards the labelled packet basedon the label at the top of the label stack


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Label space

LSRs must be able to distinguish betweenlabelled packets

A label corresponds to a particular FEC

LSR can distribute the same label/FECmapping to different neighbours

Same label can be assigned to differentFECs if and only if the LSR candistinguish the interface from which thepacket will arrive

i.e: the LSR can identify who us the upstreamneighbour who insert the label


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Label space

Per interface label space

Label are unique in a per interface base

Used over ATM interfaces

Label = VCs

With interface label space an LSR will acceptlabelled packets form upstream neighboursonly if the labels have been previouslyadvertised to those neighbours.

No label spoofing

Useful when interconnecting MPLS domains


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Label space

171.68.10/24Rtr-A

Next-Hop

In

Lab

5

5

Address

Prefix

171.68.10

171.68.10

Out

I/F

2

2

Out

Lab

7

8

In

I/F

0

1

IP packetD=171.68.10.12

Label = 5

IP packetD=171.68.10.12

Label = 7

Same label for FEC 171.168.10 is advertised todifferent upstream neighbours

1

02

IP packetD=171.68.10.15

Label = 5

IP packetD=171.68.10.15

Label = 8


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Label space

171.68.10/24

IP packetD=171.68.10.12

Label = 5

IP packetD=171.68.10.12

Label = 7

1

0 2

IP packetD=171.68.10.15

Label = 5

IP packetD=171.68.10.15

Label = 8

Next-Hop

In

Lab

Address

Prefix

Out

I/F

Out

Lab

In

I/F

5 171.68.10 2 705 171.68.10 2 81

5 171.68.40 3 94

171.68.40/24

3

IP packetD=171.68.40.33

Label = 9

4

IP packetD=171.68.40.33

Label = 5

Same label is assigned to different FECs if LSRis able to distinguish the upstream neighbourwho sent the packet


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Label space

171.68.10/24IP packet

D=171.68.10.12

Label = 6

1

0 2

IP packetD=171.68.10.15

Label = 6

IP packetD=171.68.10.15

Label = 8

Next-Hop

In

Lab

Address

Prefix

Out

I/F

Out

Lab

In

I/F

5 171.68.10 2 70

6 171.68.10 2 81

... ... ... ......

Packet is using a label NOTpreviously advertised by thedownstream neighbour

Label not being previouslyadvertised to that neighbour.The packet is dropped

b l S i h P h ( SP)


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Label Switch Path (LSP)

Each labelled packet

enters the MPLS network in the ingress LSR

exits the MPLS network in the egress LSR

LSP is the sequence of LSRs throughwhich the labelled packets have to gothrough in order to reach the egress LSR

L b l S it h P th (LSP)


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Label Switch Path (LSP)

LSR-ingress to LSR-egress path is thesame for packets of the same FEC

LSPs are unidirectional

Return traffic takes another LSP

IGP domain with a label

distribution protocol

Ingress-

LSREgress-LSR

P lti t H P i


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Penultimate Hop Popping

The label at the top of the stack isremoved (popped) by the upstreamneighbor of the egress LSR

The egress LSR requests the poppingthrough the label distribution protocol

Egress LSR advertises impl ic i t -nul l label

The egress LSR will not have to do alookup and remove itself the label

One lookup is saved in the egress LSR

P lti t H P i


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Penultimate Hop Popping

0 1

Summary routefor 171.68/16

01

171.68.10/24

Next-Hop

InLab AddressPrefix OutI/F OutLabInI/F

4 171.68/16 2 pop0

... ... ... ......Next-Hop

InLab

AddressPrefix

OutI/F

OutLab

InI/F

- 171.68/16 1 40

... ... ... ......

Egress LSR summarises morespecific routes and advertisesa label for the new FEC

Summary route is propagate throughthe IGP and label is assigned by eachLSR

Use label implicit-nullfor FEC 171.68/16


Use label 4 forFEC 171.68/16

Egress LSR needs to do an IP lookup for findingmore specific route

Egress LSR need NOT to receive a labelledpacket

labelled will have to be popped anyway

171.68.44/24

Address

Prefix and mask

171.68.10/24

Next-Hop

171.68.9.1

Interface

Serial1

171.68.44/24 171.68.12.1 Serial2

171.68/16 ... Null

Aggregation and layer 3


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Aggregation and layer 3summarisation

The LSR which does summarisation willbe the end node LSR of all LSPs related to

the summary addressAggregation point

The LSR will have to examine the second

level label of each packetIf no second label, the LSR has to examine

the IP header

No summarisation in ATM-LSRs

Aggregation and layer 3


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Aggregation and layer 3summarisation

0 1 01

171.68.10/24

Next-Hop

In

Lab

Address

Prefix

Out

I/F

Out

Lab

In

I/F

- 171.68/16 1 40

... ... ... ......

Address

Prefix and mask

171.68.10/24

171.68.44/24

Next-Hop

171.68.9.1

171.68.33.3

Interface

Serial1

Serial4

Egress LSR advertises morespecific routes label mappings

Area Border router summarises and advertisesa single label for the summary route

Label mappings for171.68.10/24171.68.44/24


Use label implicit-nullfor FEC 171.68/16

Aggregating LSR need to do an IP lookupfor finding more specific route

!!!! Do not aggregate in ATM-LSRs !!!!

Specific routes171.68.10/24171.68.44/24

1

4

In

Lab

Address

Prefix

Out

I/F

Out

Lab

In

I/F

Pop 171.68/16 Null -0

- 171.68.44/24 1 8-

- 171.68.10/24 1 7-

Route Selection


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Route SelectionHop by Hop Routing

At each hop the LSR selects the LSPwhere to forward the packet

Similar to IP routing where each hop

makes its own route selection

Route Selection


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Route SelectionExplicit Routing

The ingress LSR has the knowledge of thecomplete path (LSP) for the FEC

The ingress LSR specifies all LSR nodes

that are in the path The LSP can be set statically by

configuration

The LSP can be set dynamically usinglink-state topology information

Traffic Engineering makes use of explicitrouted LSPs

Route Selection


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Route SelectionExplicit Routing

LSR-1 request an explicit LSP to LSR-5:

LSR1, LSR-2, LSR-4, LSR-5

The request travels hop-by-hop and when it reaches theegress point labels are advertised back to the ingress LSR



LSR-1LSR-2

LSR-4 LSR-5

LSR-

3

LSR-6

Ingress

Egress

Pop labelfor LSR-5

Use label25

for LSR-5

Need labels forLSP-1 going through

LSR-1LSR-2LSR-4LSR-5

Use label39

for LSR-5

Route Selection


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oute Se ect oExplicit Routing and label stack

LSR-1LSR-2

LSR-4 LSR-5

LSR-

3

LSR-6

Ingress

Egress

Pop labelfor LSR-5

Use label25

for LSR-5

Use label39

for LSR-5

Use label 3for LSR-6

Use label 9for LSR-6

LSR-1 and LSR-5 arenon-adjacent peersfor label exchange

LSR-5 advertisesmappings to LSR-1as LSR-1 was anadjacent neighbor

Route Selection


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Explicit Routing and label stack

LSR-1LSR-2

LSR-4 LSR-5

LSR-

3

LSR-6

Ingress

Egress

LSR-5 --> 25LSR-6 --> 3

IP packet

Label = 3

Label =25

IP packet

Label = 9

IP packet

Label = 3

Label =39

39 Pop3 9

IP packet

Label = 3

ATM LSR


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ATM LSRsLabel Encoding

Label information is carried in VPI/VCIfield with different techniques toencode labels

Loops and TTL


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Loops and TTL

In IP networks TTL is used to preventpackets to travel indefinitely in thenetwork

MPLS may use same mechanism but

not on all encapsulationsTTL is present in the label header for PPP

and LAN headers

Loops and TTL


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Loops and TTL

When a packet enters into the MPLSnetwork

The Ingress LSR does a copy of the layer 3TTL into the Label TTL

Each hop decrements TTL

If TTL = 0 packet is discarded

The Egress LSR does a copy of the label TTLinto the layer 3 TTL when the packet leavesthe MPLS network

Loops and TTL


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Loops and TTL

LSRs exchange label mappings withhop-count information



LSR-1LSR-2

LSR-4 LSR-5

LSR-

3

LSR-6

Ingress

Egress

Use label21

for LSR-6Hops=2

Use label25

for LSR-6Hops=4

Use label39

for LSR-6Hops=3

Pop labelfor LSR-6Hops=1

Loops and Path Vector (LDP)


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Loops and Path Vector (LDP)

Each LSR inserts (append) its ID into

label mappings messages LSR receiving an LDP message will

check the ID list

If its own ID is found the loop isdetected

.

Concepts


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pLDP Messages

Peers exchange LDP messages

Discovery messages

Used to discover and maintain thepresence of new peers

Hello packets (UDP) sent to all-routers-in-

subnet multicast addressOnce neighbour is discovered, the LDP

session is established over TCP

Concepts


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pLDP Messages

Session messages

Establish, maintain and terminate LDP sessions Advertisement messages

Create, modify, delete label mappings

Notification messagesError signalling


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Configuration

CEF should be running

Tag switching you need to configureit in interface mode

tag-switching ip

mpls ip


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Configuration

To enable to type of label distributionprotocol you want to run, this is doneper interface basis

mpls label-protocol [ tdp ldp both]


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Configuring TTL

By default ip ttl is copied into mpls packet

To disable you need the followingcommand

no tag-switching ip propagate-ttl

no mpls ip prapagate-ttl

TTL propagation if disabled has to bedone on both ingress and egress LSR

Date post:	02-Apr-2018
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