mpls10s03-label assignment and distribution

8/14/2019 MPLS10S03-Label Assignment and Distribution

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2001, Cisco Systems, Inc.

Module 3Label Assignment

and Distribution


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MPLS v1.03-2 2001, Cisco Systems, Inc.

Objectives

Upon completion of this chapter, you will beable to perform the following tasks:

Describe the need for LDP/TDP in MPLSnetwork.

Describe the LDP/TDP neighbor discovery andsession establishment procedures.

Explain the needs for different LDP/TDP labeldistribution modes.

Explain the difference between independentand ordered control.

Describe various LDP/TDP retention modes(conservative and liberal).

Explain the functions and benefits of


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LDP Role in Unicast

IP Routing

2001, Cisco Systems, Inc. MPLS v1.03-3


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Objectives

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

Explain the need for the LabelDistribution Protocol (LDP) in unicastIP routing MPLS application.

Describe the LDPs interaction withother Label Switch Router (LSR)components.


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MPLS Unicast IP Routing

MPLS introduces a new field that is used forforwarding decisions.

Although labels are locally significant, theyhave to be advertised to directly reachable

peers.

One option would be to include thisparameter in existing IP routing protocols.

The other option is to create a newprotocol to exchange labels.

The second option has been used becausethere are too many existing IP routing

protocols that would have to be modified tocarry labels.


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MPLS Unicast IP RoutingArchitecture

LSR

Control Plane

Data Plane

Routing Protocol

LDP

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


7/92MPLS v1.03-7 2001, Cisco Systems, Inc.

MPLS Unicast IP Routing:Example

LSR

Control Plane

Data Plane

OSPF:

RT:

LIB:

FIB:

LFIB:

OSPF: 10.0.0.0/810.0.0.0/8 1.2.3.4

10.0.0.0/8 1.2.3.4

10.0.0.0/8 1.2.3.4

L=5 10.1.1.1

10.1.1.1 10.1.1.1



MPLS Unicast IP Routing:Example

LSR

Control Plane

Data Plane

OSPF:

RT:

LIB:

FIB:

LFIB:

OSPF: 10.0.0.0/810.0.0.0/8 1.2.3.4

10.0.0.0/8 1.2.3.4

10.0.0.0/8 1.2.3.410.1.1.1

LDP: 10.0.0.0/8, L=3

L=5 10.1.1.1

10.0.0.0/8 Next-hop L=3, Local L=5LDP: 10.0.0.0/8, L=5

L=3 10.1.1.1

L=3 10.1.1.1L=5 L=3

, L=3



Summary

After completing this section, youshould be able to perform the

following tasks:Explain the need for the LDP in unicastIP routing MPLS application.

Describe the LDPs interaction withother LSR components.



Review Questions

Why is the LDP/TDP needed?

What is the forwarding equivalence

class in MPLS unicast IP forwarding?Where is this forwarding equivalenceclass taken from?

What is the output of the LDP/TDP?


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Typical LabelDistribution in Packet-

mode MPLS




Objectives


Describe label allocation in packet-mode MPLS environments.

Describe TDP/LDP label distribution in

packet-mode MPLS environments.Explain how the MPLS data structuresare built based on label allocation anddistribution.



Label Allocation in a Packet-Mode MPLS Environment

Label allocation and distribution in apacket-mode MPLS environment followsthese steps:

IP routing protocols build the IP routingtable.

Each LSR assigns a label to every

destination in the IP routing tableindependently.

LSRs announce their assigned labels to all

other LSRs.



Building the IP Routing Table

IP routing protocols are used to build IProuting tables on all LSRs.

FIBs are built based on IP routing tables withno labeling information.

Network Next-hop

X B

Routing table of A

Network Next-hop

X C

Routing table of B

Network Next-hop

X D

Routing table of C

Network Next-hop

X C

Routing table of ENetwork Next hop Label

X B

FIB on A

A B C D

E

Network X



Allocating Labels

Every LSR allocates a label for everydestination in the IP routing table.

Labels have local significance. Label allocations are asynchronous.

Network Next-hop

X C

Routing table of B

A B C D

E

Network X

Router B assigns label 25 to

destination X.



Network Next-hop

X C

Routing table of B

A B C D

E

Network X

Router B assigns label 25 to

destination X.

LIB and LFIB Setup

LIB and LFIB structures have to be initialized onthe LSR allocating the label.

Network LSR label

X local 25

LIB on B Local label is stored in LIB.

Label Action Next hop

25 pop C

LFIB on B Outgoing action is pop, as B

has received no label for X

from C.



A B C D

E

Network X

Label Distribution

The allocated label is advertised to all neighborLSRs, regardless of whether the neighbors areupstream or downstream LSRs for the

destination.

Network LSR label

X local 25

LIB on B

X = 25X = 25

X=25



Receiving LabelAdvertisement

Every LSR stores the received label in its LIB. Edge LSRs that receive the label from their

next-hop also store the label information in the

FIB.

X = 25X = 25

Network LSR label

X B 25

LIB on A

Network LSR label

X B 25

LIB on C

Network LSR label

X B 25

LIB on E

Network Next hop Label

X B 25

FIB on A

A B C D

E

X=25

Network X



Interim Packet Propagation

Forwarded IP packets are labeled only on thepath segments where the labels have already

been assigned.

IP: X Lab: 25 IP: X


X B 25

FIB on A

IP lookup is performed inFIB: packet is labeled.


25 pop C

LFIB on B

Label lookup is performed

in LFIB: label is removed.

A B C

E



Further Label Allocation

Every LSR will eventually assign a label forevery destination.

Network LSR label

X B 25

local 47

LIB on C

Label Action Next hop47 pop D

LFIB on C

A B C D

E

Network X

Router C assigns label

47 to destination X.X=

47

X = 47



Receiving LabelAdvertisement

Every LSR stores received information in itsLIB.

LSRs that receive their label from their next-hop LSR will also populate the IP forwarding

Network LSR labelX B 25

C 47

LIB on E

Network LSR label

X local 25

C 47

LIB on B


X C 47

FIB on B

Network Next hop LabelX C 47

FIB on E

A B C D

E

Network XX=

47

X = 47



Populating LFIB

Router B has already assigned a label to X andcreated an entry in the LFIB.

The outgoing label is inserted in the LFIB afterthe label is received from the next-hop LSR.

Network LSR label

X local 25

C 47

LIB on B


X C 47

FIB on B


25 47 C

LFIB on B

A B C D

E

X=

47

X = 47

Network X


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Packet Propagation Acrossan MPLS Network

IP: X IP: X

Ingress LSR Egress LSR

A B C

E

Lab: 25 Lab: 47


X B 25

FIB on A

IP lookup is performed inthe FIB, packet is labeled.


47 pop D

LFIB on C


in the LFIB, label is removed.


25 47 C

LFIB on B


in the LFIB, label is switched.


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Per-Platform Label Allocation


25 75 D

LFIB on B

X = 25

X=25

An LFIB on a router usually does not contain

an incoming interface. The same label can be used on any interface

per-platform label allocation.

LSR announces a label to adjacent LSR onlyonce even if there are parallel links betweenthem.

Network X

B

C

DA

B fit d D b k f


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Benefits and Drawbacks ofPer-Platform Label Allocation

Benefits:

Smaller LFIB

Quicker label

exchange

Drawbacks: Insecureany

neighbor LSR can sendpackets with any label

in the LFIB


25 75 D

LFIB on B

Network X

B DA

Label for X is announced

only to A.

X = 25

Lab:

25

Lab: 47

A third-party router can still send

packets toward network X, even though

the label was not announced to it.


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-MPLS Label Allocation and

Distribution

Packet-mode MPLS label allocation anddistribution follows these rules:

Every LSR assigns a label for every destinationin the IP routing table.

Labels are assigned once per LSR (per-platform).

Every LSR advertises its label assignments toall neighbors.

Every LSR stores all advertised labels in theLIB.

Labels received from next-hop LSRs are used to

populate label information in the FIB and theoutgoing label in the LFIB.


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Review Questions

Where are received labels stored?

Which forwarding table is used toforward a labeled packet?

Which forwarding table is used toforward an unlabeled packet?

What happens if the next-hop label is

not in the LFIB table?

How many labels are usually assignedto one destination network?


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Convergence inPacket-mode MPLS



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Objectives

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

Identify TDP/LDP convergence issues.Describe the interaction betweenrouting protocol convergence and LDPconvergence.

Describe the packet-mode MPLSconvergence after link failure and linkrecovery.


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Steady State Description

After the LSRs have exchanged the labels,LIB, LFIB and FIB data structures arecompletely populated.

Network Next-hop

X C

Routing table of B


X C 47

FIB on B

Network LSR label

X local 25

C 47

E 75

LIB on B


25 47 C

LFIB on B

A B C D

E

Network X


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Link Failure Actions

Routing protocolneighbors and LDPneighbors are lost aftera link failure.

Entries are removed

from various datastructures.

Network Next-hop

X C

Routing table of B


X C 47

FIB on B

Network LSR label

X local 25

C 47

E 75

LIB on B


25 47 C

LFIB on B

"A B C D

E

Network X

"

R ti P t l


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Routing ProtocolConvergence

Routing protocols rebuildthe IP routing table andthe IP forwarding table.

Network LSR label

X local 25

C 47

E 75

LIB on B


25 47 C

LFIB on B


X E

FIB on B

Network Next-hop

X E

Routing table of B

A B C D

E

Network X

"


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MPLS Convergence

The LFIB and labelinginformation in the FIB arerebuilt immediately afterthe routing protocolconvergence, based on

labels stored in the LIB.

Network LSR label

X local 25

C 47

E 75

LIB on B

Network Next-hop

X E

Routing table of B


25 75 E

LFIB on B


X E 75

FIB on B

A B C D

E

Network X

"

MPLS C


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MPLS ConvergenceAfter a Link Failure

MPLS convergence in packet-modeMPLS does not affect the overall

convergence time.MPLS convergence occursimmediately after the routing

protocol convergence, based onlabels already stored in the LIB.


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Link Recovery Actions

Routing protocolneighbors arediscovered after linkrecovery.

Network LSR label

X local 25

C 47

E 75

LIB on B

Network Next-hop

X E

Routing table of B


25 75 E

LFIB on B


X E 75

FIB on B

A B C D

E

Network X

IP R ti C Aft


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IP Routing Convergence AfterLink Recovery

IP routing protocolsrebuild the IP routingtable.

The FIB and the LFIB arealso rebuilt, but the label

information might belackin .

Network LSR label

X local 25

C 47

E 75

LIB on B


25 75 E

LFIB on B


X E 75

FIB on B

Network Next-hop

X E

Routing table of B

C C

pop C

A B C D

E

Network X

MPLS Con ergence


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MPLS ConvergenceAfter a Link Recovery

Routing protocol convergence optimizesthe forwarding path after a link recovery.

The LIB might not contain the label from

the new next hop by the time the IPconvergence is complete.

End-to-end MPLS connectivity might be

intermittently broken after link recovery.Use MPLS traffic engineering for make-before-break recovery.


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Summary

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

Identify TDP/LDP convergence issues.

Describe the interaction betweenrouting protocol convergence and LDP

convergence.

Describe the packet-mode MPLSconvergence after link failure and link

recovery.


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Review Questions

What is the impact of LDP/TDPconvergence time on overall

convergence when a link is lost?

What is the impact of LDP/TDPconvergence time on overallconvergence when a link is restored?


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Typical Label Distributionover LC-ATM Interfaces and

VC-Merge



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Objectives


Describe ATM-related issues that dictateadditional needs for label-distributionprocedures overLC-ATM interfaces.

Describe TDP/LDP label distribution in cell-mode MPLS environments.

Identify the need for per-edge-LSR VCallocation over ATM network.

Describe the functions, benefits and

Issues of Cell Mode MPLS


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Issues of Cell-Mode MPLSEnvironments

An MPLS label is encoded as the virtualpath identifier/virtual channel identifier(VPI/VCI) value in cell-mode MPLSenvironments.

Each VPI/VCI combination represents avirtual circuit in ATM.

The number of virtual circuits supported

by router and switch hardware isseverely limited.

Conclusion: labels in cell-mode MPLSare a scarce resource.


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Building the IP Routing Table

IP routing protocols are used to build IProuting tables on all LSRs.

The routing tables are built as if the ATMswitches were regular routers.

Network Next-hop

X C

Routing table of A

Network Next-hop

X C

Routing table of B

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

Network X

A

E

B

C D

Building the IP Forwarding


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Building the IP ForwardingTable

Unlabeled IP packets cannot be propagatedacross LC-ATM interfaces.

Forwarding tables are not built until the labelsare assigned to destinations in IP routing

tables.

Network Next-hop

X C

Routing table of A

Network Next-hop

X C

Routing table of B

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

Network X

A

E

B

C D


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Requesting a Label

Labels have to be explicitly requested over LC-ATM interfaces.

A router requests a label for every destinationin the routing table with the next hopreachable over an LC-ATM interface.

Network Next-hop

X C

Routing table of A

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

Network X

A

E

An ATM switch can allocate an

incoming label only if it already

has a corresponding outgoing

label.

RQ X RQ X

A switch requests a

label from its next hop.

RQ X

C D


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Allocating a Label

Network Next-hop

X C

Routing table of A

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

The egress ATM edge

LSR allocates a label

and replies to the

request.

An ATM LSR can allocate an incoming

label after receiving an outgoing label. It

replies with the allocated label to the

incoming request.


1/56 2/82 D

LFIB on C


2/82 1/37

LFIB on DThe LFIB is actuallythe ATM switching

matrix.

Network LSR label

X E 1/37

local 2/82

LIB on D

Network X

A

EC D

RQ X RQ X RQ X

X=1/37X=2/82X=1/56


1/37 pop

LFIB on E

Network LSR label

X local 1/37

LIB on E

Processing Label


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Processing LabelAllocation Reply

Network Next-hop

X C

Routing table of A

The ingress ATM edge LSR requesting a label

inserts the received label in its LIB, FIB, and

(optionally) LFIB.

Network LSR label

X C 1/56

LIB on A


X C 1/56

FIB on A

Network X

A

EC D

RQ X

X=1/56

Allocation Requests from


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A

E

B

C D

Network LSR label

X D 2/82

local 1/56local 1/43

LIB on C

Allocation Requests fromAdditional Upstream LSRs

Network Next-hop

X C

Routing table of A

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

Every upstream LSR will

request a label for downstream

destinations from an ATM LSR.

The ATM LSR could reuse an

already allocated downstream

label for the second upstream

label.

Network Next-hop

X C

Routing table of B

X=1/43

RQ X


1/56 2/82 D

1/43 2/82 D

LFIB on C


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A

E

B

C D

Cell Interleave Issue

If an ATM LSR reuses a

downstream label, cells from

several upstream LSRs might

become interleaved.


1/56 2/82 D

1/43 2/82 D

LFIB on C


X C 1/43

FIB on B


X C 1/56

FIB on A

1/431/43

1/561/56 2/82 2/82

Solution #1allocate a separate downstream label for everyupstream request.

Solution #2prevent cell interleave by blocking incoming cellsuntil a whole frame is collected.


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A

E

B

C D

Additional Label Allocation

Network Next-hop

X C

Routing table of A

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

The ATM LSR requests a new

label from downstream LSRs

for every upstream request.

The ATM egress router has to

allocate a unique label for every

ATM ingress router for every

destination.

Network Next-hop

X C

Routing table of B

RQ X

X=1/43 X=1/38X=2/87

RQ X (2) RQ X (2)

Network LSR label

X local 1/37

local 1/38

LIB on E


1/37 pop

1/38 pop

LFIB on E


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A

B

C DA

Virtual Circuit Merge

VC merge is a solution in which incoming cellsare blocked until the last cell in a framearrives.

All buffered cells are then forwarded to thenext-hop ATM LSR.

1/431/43

1/561/56 2/822/822/822/82


1/56 2/82 D1/43 2/82 D

LFIB on C

Benefits and Drawbacks


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Benefits and Drawbacksof VC Merge

Benefits of VC mergeThe Merging ATM LSR can reuse thesame downstream label for multipleupstream LSRs.

Drawbacks of VC merge

Buffering requirements increase onthe ATM LSR.

Jitter and delay across the ATMnetwork increase.

The ATM network is effectivelytransformed into a frame-based

network.

Per-Interface Label


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Per-Interface LabelAllocation

The LFIB on an ATM switch (ATM switching

matrix) always contains the incoming interface. Labels have to be assigned for individual

interfacesper-interface label allocation.

The same label can be reused (with a differentmeaning) on different interfaces.

Incoming I/F VPI/VCI Outgoing I/F VPI/VCI

ATM 0/0 1/73 ATM 1/3 1/39

LFIB on C

Network X

RQ X

X=1/73

RQ X

X=1/69

The ATM edge

LSR has to

request a label

over every

interface.

A

E

C

Security of Per-Interface


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Network X

AE

C

Security of Per-InterfaceLabel Allocation


ATM 0/0 1/73 ATM 1/3 1/39

LFIB on C

1/73

Per-interface label allocation is securelabeledpackets (or ATM cells) are accepted only fromthe interface where the label was actually

assigned.

1/391/73

The packet or cell arriving

through the proper

interface is forwarded.

A labeled packet or cell coming through

a wrong interface is dropped.


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Summary


Describe ATM-related issues that

dictate additional needs for label-distribution procedures over LC-ATMinterfaces.

Describe TDP/LDP label distribution incell-mode MPLS environments.

Identify the need for per-edge-LSR VCallocation over ATM network.

Describe the functions benefits and

i Q i


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Review Questions

What is ATM switching matrix called inMPLS terminology?

Why is it necessary to have the next-hop label before propagating the locallabel?

What are the benefits of VC-Merge?

What are the drawbacks of VC-Merge?


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MPLS Label Allocation,Distribution and Retention

Modes


Obj i


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Objectives


Describe the difference between per-

interface and per-platform label space.

Describe TDP/LDP unsolicited downstreamand downstream-on-demand labeldistribution.

Describe the difference between orderedand independent label allocation control.

Describe the difference betweenconservative and liberal retention mode.

Identify parameter sets used in Cisco IOS

Label Distribution


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Label DistributionParameters

MPLS architecture defines severallabel allocation and distributionparameters:

Per-interface or per-platform labelspace

Unsolicited downstream and

downstream-on-demand labeldistribution

Ordered or independent LSP tunnelcontrol

L b l S P I t f


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Network X

AE

C

Label Space: Per Interface

The LFIB on an LSR contains incoming aninterface.

Labels have to be assigned for individualinterfaces.

The same label can be reused (with a differentmeaning) on different interfaces.

Label allocation is secureLSRs cannot send


ATM 0/0 1/73 ATM 1/3 1/39

LFIB on C

L b l S P Pl tf


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Label Space: Per Platform


25 75 D

LFIB on B

X = 25

X=25

The LFIB on an LSR does not contain an incominginterface.

The same label can be used on any interface and isannounced to all adjacent LSRs.

The label is announced to adjacent LSRs only onceand can be used on any link.

Per-platforms label space is less secure than per-interface label space.

X = 25

X=25 Network X

B

C

DA

Label Distribution:


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A B C D

E

Network X

Label Distribution:Unsolicited Downstream

The Label for a prefix is allocated andadvertised to all neighbor LSRs, regardless ofwhether the neighbors are upstream or

downstream LSRs for the destination.

Network LSR label

X local 25

LIB on B

X = 25X = 25

X=25

Label Distribution:


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Network X

A

EC D

Downstream-on-Demand

An LSR will assign a label to a prefix onlywhen asked for a label by an upstream LSR.

Label distribution is a hop-by-hop parameterdifferent label distribution mechanisms cancoexist in an MPLS network.

Network Next-hop

X C

Routing table of A

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

RQ X

LSP Control: Independent


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Network X

A

EC D

LSP Control: IndependentControl

Network Next-hop

X C

Routing table of A

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

RQ X

X=1/37


1/37 pop

LFIB on E

An LSR can always assign a label for a prefix,even if it has no downstream label.

Independent control can be used only forLSRs with Layer 3 capabilities.


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Network X

A

EC D

LSP Control: Ordered Control

Network Next-hop

X C

Routing table of A

Network Next-hop

X D

Routing table of C

Network Next-hop

X E

Routing table of D

Network Next-hop

X conn

Routing table of E

RQ X RQ X RQ X

X=1/37X=2/82X=1/56


1/56 2/82 D

LFIB on C


2/82 1/37

LFIB on D


1/37 pop

LFIB on E

An LSR can assign a label only if it has alreadyreceived a label from the next-hop LSR;otherwise it must request a label from the next-hop LSR.

Label Retention: Liberal


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Label Retention: LiberalRetention Mode

Every LSR stores the received label in its LIB,even when the label is not received from anext-hop LSR.

Liberal label retention mode improves

X = 25X = 25

Network LSR label

X B 25

LIB on A

Network LSR label

X B 25

LIB on C

Network LSR label

X B 25

LIB on E

A B C D

E

X=25Network X

Label Retention:


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Conservative Retention Mode

An LSR stores only the labels received fromnext-hop LSRs; all other labels are ignored.

Downstream-on-demand distribution isrequired during the convergence phase.

X = 25X = 25

Network LSR label

X B 25

LIB on A

Network LSR label

X B 25

LIB on C

A B C D

E

Network X

X=25

Cisco IOS Platform MPLS


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Cisco IOS Platform MPLSImplementation

Routers with packet interfaces: Per-platform label space, unsolicited

distribution, liberal label retention,independent control

Routers with ATM interfaces: Per-interface label space, on-demand

distribution, conservative or liberallabel retention, independent control

ATM switches:

Per-interface label space, on-demanddistribution, conservative labelretention, ordered control

Summary


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Summary

After completing this section, youshould be able to perform the followingtasks:

Describe the difference between per-

interface and per-platform label space. Describe TDP/LDP unsolicited downstream

and downstream-on-demand labeldistribution.

Describe the difference between orderedand independent label allocation control.

Describe the difference betweenconservative and liberal retention mode.

Identify parameter sets used in Cisco IOS

Review Questions


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Review Questions

Why and where is downstream-on-demand label propagation used?

What are the benefits and drawbacks

of liberal retention mode?

What are the benefits and drawbacksof per-platform label space?

Why is per-interface label space onATM LSRs needed?


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LDP NeighborDiscovery


Objectives


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Objectives


Describe LDP/TDP neighbor discovery.

Describe LDP/TDP sessionestablishment process.

LDP Session Establishment


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LDP Session Establishment

LDP and TDP use a similar process to establisha session:

Hello messages are periodically sent on allinterfaces enabled for MPLS.

If there is another router on that interface, it

will respond by trying to establish a sessionwith the source of the hello messages.

User Datagram Protocol (UDP) is used for hellomessages. It is targeted at all routers on thissubnet multicast address (224.0.0.2).

Transmission Control Protocol (TCP) is used toestablish the session.

Both TCP and UDP use well-known LDP portnumber 646 (711 for TDP).

LDP Hello Message


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LDP Hello Message

Hello messages are targeted at all routers reachablethrough an interface.

LDP uses well-known UDP and TCP port number 646. The source address used for an LDP session can be set

by adding the transport address TLV to the hellomessage.

A 6-byte LDP identifier TLV identifies the router (first

four bytes) and label space (last two bytes).

Source port=1050

Destination port=646

Source address=1.0.0.1

Destination address=224.0.0.2

IP Header UDP Header LDP Hello Message

Well-know multicast

IP addressidentifying all

routers on the

subnet.

Well-know portnumber used for

LDP.

Transport address=1.0.0.1

Optional TLV

used to identifythe source IP

address for LDP

session.

LDP ID=1.0.0.1:0

6-byte TLV

identifying therouter and

label space.

Label Space


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Label Space

LSRs establish one LDP session perlabel space.

Per-platform label space requires onlyone LDP session, even if there are

multiple parallel links between a pair ofLSRs.

Per-platform label space is announcedby setting the label space ID to zero

(for example, LDP ID=1.0.0.1:0).

A combination of frame-mode and cell-mode, or multiple cell-mode links,results in multiple LDP sessions.

Label Space Negotiation


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Example

One LDP session is established for each announced LDPidentifier (router ID + label space).

The number of LDP sessions is determined by thenumber of different label spaces.

The bottom right example is not common, since ATMLSRs do not use Ethernet for packet forwarding, andframe-mode MPLS across ATM uses per-platform label

space.

ATMATM

ATM

1.0.0.1:10

1.0.0.1:20

1.0.0.1:0

1.0.0.1:0

1.0.0.1:0

1.0.0.1:0

1.0.0.1:10

1.0.0.1:0

LDP Neighbor Discovery


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LDP Neighbor Discovery

1.0.0.1 1.0.0.3

1.0.0.4

MPLS_D

1.0.0.2

TCP(1.0.0.2:104

3 1.0.0.1:646)

UDP: Hello(1.0.0.1:1050 224.0.0.2:646)

UDP: Hello(1.0.0.4:1033 224.0.0.2:646)

UDP: Hello(1.0.0.2:1064 224.0.0.2:646)

UDP: Hello

(1.0.0.1:1051 224.0.0.2:646)

UDP: Hello

(1.0.0.4:1034 224.0.0.2:646)

UDP: Hello

(1.0.0.2:1065 224.0.0.2:646)

UDP: Hello

(1.0.0.1:1052 224.0.0.2:646)

UDP: Hello

(1.0.0.4:1035 224.0.0.2:646)

UDP: Hello

(1.0.0.2:1066 224.0.0.2:646)

TCP(1.0.0.4:1065 1.0.0.1:646)

TCP

(1.0.0.4:1066

1.0.0.2:646)

An LDP session is established from the routerwith the higher IP address.

MPLS_B

MPLS_A NO_MPLS_C

LDP Session Negotiation


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LDP Session Negotiation

Peers first exchange initializationmessages.

The session is ready to exchange labelmappings after receiving the first

keepalive.

1.0.0.1 1.0.0.2Initialization message

Establish TCP session

Initialization message

Keepalive

Keepalive

MPLS_AMPLS_B

LDP Sessions


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Between ATM LSRs

An IP adjacency between ATM LSRs isestablished through the control virtual circuit(0/32).

The control virtual circuit is used for LDP or TDPas well as for IP routing protocols.

Virtual Switch Interface (VSI) protocol is used topopulate the ATM switching matrix (LFIB) in thedata plane of some ATM switches (Cisco-specificimplementation).

OSPF

OSPF OSPF

OSPF

LDP

LDP LDP

LDP

LFIBLFIB LFIB

LFIB

VSI VSI

0/32 0/320/32

LDP or TDP Discoveryf N dj t N i hb


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of Nonadjacent Neighbors

LDP or TDP neighbor discovery ofnonadjacent neighbors differs from normaldiscovery only in the addressing of hellopackets.

Hello packets use unicast IP addressesinstead of multicast addresses.

Once a neighbor is discovered, themechanism to establish a session is thesame.

Summary


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Summary

After completing this section, youshould be able to perform the

following tasks:

Describe LDP/TDP neighbor discovery.

Describe the LDP/TDP session

establishment process.

Review Questions


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Review Questions

How do routers find LDP/TDP peersreachable through an interface?

Which protocol is used to discover

neighboring LSRs?Which protocol is used for the sessionitself?

How do ATM LSRs establish an LDP/TDPsession?

What is different in an LDP sessionestablishment between non-adjacent peers?


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Penultimate HopPopping


Objectives


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Objectives


Describe penultimate hop popping.

Describe how LSRs request PHP throughTDP/LDP.

Identify when the PHP could be used in

MPLS network.

Describe the benefits and drawbacks ofPHP.

Double Lookup Scenario


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MPLS Domain

Double Lookup Scenario

Double lookup is not anoptimal way of forwardinglabeled packets.

A label can be removed one

hop earlier.

10.0.0.0/8

L=19

10.0.0.0/8

L=18

10.0.0.0/8

L=17

LFIB

18 19

FIB10/8 NH, 19

LFIB

17 18

FIB10/8 NH, 18

LFIB

35 17

FIB10/8 NH, 17

LFIB

19 untagged

FIB10/8 NH

10.1.1.117

10.1.1.118

10.1.1.119

10.1.1.1

Double lookup is

needed:

1. LFIB: remove thelabel.

2. FIB: forward the IP

packet based on IP

next-hop address.

10.0.0.0/8

Penultimate Hop Popping


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MPLS Domain

A label is removed on the routerbefore the last hop within an MPLSdomain.

10.0.0.0/8

L=pop

10.0.0.0/8

L=18

10.0.0.0/8

L=17

LFIB

18 pop

FIB10/8 NH, 19

LFIB

17 18

FIB10/8 NH, 18

LFIB

35 17

FIB10/8 NH, 17

LFIB

FIB10/8 NH

10.1.1.117

10.1.1.118

10.1.1.1

10.1.1.1

10.0.0.0/8

Pop or implicit

null label is

advertised.

One single lookup.



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Penultimate hop popping optimizesMPLS performance (one less LFIB

lookup).PHP does not work on ATM(VPI/VCI cannot be removed).

The pop or implicit null label usesvalue 3 when being advertised to aneighbor.

Objectives


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Objectives


Describe penultimate hop popping.Describe how LSRs request PHPthrough TDP/LDP.

Identify when the PHP could be used inMPLS network.

Describe the benefits and drawbacks ofPHP.

Review Questions


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Review Questions

What is the main benefit of

penultimate hop popping?How does a router know that it has topop the label?

Summary


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Summary

After completing this chapter, you should beable to perform the following tasks:

Describe the need for LDP/TDP in MPLS network.

Describe the LDP/TDP neighbor discovery and

session establishment procedures. Explain the needs for different LDP/TDP label

distribution modes.

Explain the difference between independent andordered control.

Describe LDP/TDP retention modes(conservative and liberal).

Explain the functions and benefits of penultimate-hop-popping.


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2000, Cisco Systems, Inc. www.cisco.co

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