MPLS10S04-Advanced MPLS Technology

8/14/2019 MPLS10S04-Advanced MPLS Technology

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MPLS v1.04-2 2001, Cisco Systems, Inc.

Objectives

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

Describe the concept of Label Switch Paths andthe impact of route summarization on LSP

Understand the basics of MPLS Traffic Engineering

Understand the data-plane loop detection in MPLSand how it relates to IP TTL

Explain the benefits and drawbacks of IP TTLpropagation

Understand the data-plane loop detection in theATM environment and how it affectstroubleshooting tools such as traceroute

Explain the impacts of configuring MPLS innetworks running BGP

Design simplified BGP networks based on MPLStechnology


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Label Switch Paths inUnicast IP Routing

2001, Cisco Systems, Inc. MPLS v1.04-3


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Objectives

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

Explain the concept of Label SwitchPath

Describe how the LSP is built inunicastIP routing

Describe the impact of IP aggregationon Label Switch Paths


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LSP Tunnels

An LSP tunnel is a sequence of LSRs that forwardlabeled packets of a certain forwardingequivalence class.

MPLS unicast IP forwarding builds LSP tunnelsbased on the output of IP routing protocols.

LDP or TDP advertises labels only for individualsegments in the LSP tunnel.

LSP tunnels are unidirectional.

Return traffic uses a different LSP tunnel (usually

the reverse path, as most routing protocolsprovide symmetrical routing).

An LSP tunnel can take a different path from theone chosen by an IP routing protocol (MPLStraffic engineering).


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LSP Tunnel Building Example

IP routing protocol determines the path.

LDP or TDP propagates labels to convert thepath to an LSP tunnel.

LSP:

A

B

D

G

I

A

B

C

D

E

F

G

H

I

IP routing

protocol

updates


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9/58MPLS v1.04-9 2001, Cisco Systems, Inc.

ATM LSRs must not aggregate because

they cannot forward IP packets.

Aggregation should not be used whereend-to-end LSP tunnel are required

(MPLS Virtual Private Networks [VPNs]).

Effects of IP Aggregation onLSP Tunnels (cont.)



Summary

After completing this section, youshould be able to perform the

following tasks:Explain the concept of Label SwitchPath

Describe how the LSP is built in unicastIP routing

Describe the impact of IP aggregationon Label Switch Paths



Review Questions

What is an LSP?

Which mechanism determines thepath?

What happens when IP aggregation

(summarization) is used?


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Explicit Label SwitchPaths (Traffic

Engineering)




Objectives

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

Explain the concept of explicit LabelSwitch Path

Describe how an explicit LSP can be

used for traffic engineeringDescribe the needs for runningLDP/TDP across explicit LSP



Explicit LSP Tunnels

LSP tunnels are usually determined byIP routing protocols.

MPLS traffic engineering can be usedto diverge from the IGP-determinedpath.

Constraint-based Routing using LabelDistribution Protocol (CR-LDP) orRSVP with extensions for trafficengineering is used to establish LSPtunnels.

LSP tunnels can also be configuredmanually.



E

D

C

BA F

MPLS Traffic EngineeringExample

IGP and LDP or TDP create an LSP tunnelbased on the shortest path determinedby IGP.

10.0.0.0/16

10.0.0.0/16

L=pop

10.0.0.0/16

10.0.0.0/16

L=16

10.0.0.0/16

10.0.0.0/16

L=23

10

.0.0.0

/16

10

.0. 0.0

/16

L=

2310.0.0.0/16

10.0.0.0/16

L=44

10.0.0.0

/16

10.0.0.0

/16

L=31



E

D

C

BA F

MPLS Traffic EngineeringExample (cont.)

RSVP creates a TE tunnel between routers Aand E.

The new link can be included in the IGPshortest path calculation.

RSVP uses downstream-on-demand labeldistribution.

The tunnel creation is initiated from router A.

1.2.3.4/32

L=pop

1.2.3.4/32

L=19

1.2.3.4/32

L=54

1.2.3.4



E

D

C

BA F


IGP and LDP or TDP create a new LSP tunnel basedon the shortest path determined by IGP.

This LSP tunnel is going across the MPLS TE LSPtunnel.

10.0.0.0/16

10.0.0.0/16

L=pop

10

.0.0.0

/16

10

.0. 0.0

/16

L=

2310.0.0.0/16L=44



E

D

C

BA F


FIB:

10/8 44, 541.2.3.4 54

FIB:

10/8 231.2.3.4 19LFIB:

541916 23

FIB:

10/8 441.2.3.4popLFIB:

19pop

FIB:

10/8 231.2.3.4LFIB:

4423

FIB:

10/8 popLFIB:

23pop10.1.1.14454

10.1.1.14419

10.1.1.144

10.1.1.123

10.1.1.1

10.1.1.1



Explicit LSP Tunnels

As seen in the previous example,MPLS TE can be used to implement

load balancing across unequal paths.Explicit paths are almost transparentto LDP and TDP.

LDP or TDP uses directed hellopackets to find nonadjacentneighbors.



Summary


following tasks:Explain the concept of explicit LabelSwitch Path

Describe how an explicit LSP can beused for traffic engineering

Describe the needs for running LDP/TDPacross explicit LSP



Review Questions

What is the purpose of using explicitLSPs?

Which technology makes use of

explicit LSPs?

How does LDP/TDP find neighborsacross an MPLS/TE tunnel?

Which protocols can be used toestablish MPLS/TE tunnels?

What type of label propagation dothese protocols use?


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Loop Detection inPacket Mode MPLS



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Objectives

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

Describe loop detection in packet-modeMPLS

Explain the implications of IP TTLpropagation into the TTL field of the label

header

Explain the interactions between IP TTLpropagation and traceroute diagnostic

tools


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Loop Detection

LDP or TDP relies on loop detectionmechanisms built into IGPs that areused to determine the path.

If, however, a loop is generated (that is,misconfiguration with static routes),the TTL field in the label header is usedto prevent indefinite looping of packets.

TTL functionality in the label header isequivalent to TTL in the IP headers.

TTL is usually copied from the IPheaders to the label headers (TTLpropagation).


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MPLS Domain

Normal TTL Operation

Cisco routers have TTL propagation enabled bydefault.

On ingress: TTL is copied from IP header to label

header.

IP TTLLabel

TTL

5 44

TTL is

decreased and

copied into

label header.

43

Only the TTL in

the top-of-stack

entry is

modified.

2

The TTL is

decreased and

copied back into the

TTL field of the IP

header.

1

CBA D


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MPLS Domain

CBA D

Loop Detection

Labeled packets are dropped when theTTL is decremented to zero.

IP TTLLabel

TTL

43

Routing loop

5 44

42

41

4

0


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Disabling TTL Propagation

TTL propagation can be disabled.

IP TTL value is not copied into thelabels and label TTL is not copied back

into IP TTL.Instead, the value 255 is assigned tothe label header TTL field on theingress LSR.

Disabling TTL propagation hides corerouters in the MPLS domain.

Traceroute across an MPLS domaindoes not show any core routers.


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CBA D

Traceroute with Disabled TTLPropagation (1)

The first traceroute packet (ICMP orUDP) that reaches the network isdropped on router A.

An ICMP TTL exceeded message issent to the source from router A.

IP TTLLabel

TTL

1

0

traceroute 10.1.1.1

TTL exceeded

traceroute 10.1.1.1

1 10 ms A.acme.com

i i


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CBA D

Traceroute with Disabled TTLPropagation (2)

The second traceroute packet thatreaches the network is dropped onrouter D.

An ICMP TTL exceeded message issent to the source from router D.

IP TTLLabel

TTL

2 12541255 1

0

traceroute 10.1.1.1

1 10 ms A.acme.com

TTL exceeded

traceroute 10.1.1.1

1 10 ms A.acme.com

2 10 ms D.acme.com

Eff t f Di bli TTL


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Effect of Disabling TTLPropagation

Traceroute across an MPLS domaindoes not show core routers.

TTL propagation has to be disabled on

all LSRs.Mixed configurations (some LSRs withTTL propagation enabled and some withTTL propagation disabled) could result

in faulty traceroute output.TTL propagation can be enabled forforwarded traffic onlytraceroute fromLSRs does not use the initial TTL value

of 255.


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Summary


following tasks:Describe loop detection in packet-modeMPLS

Explain the implications of IP TTLpropagation into the TTL field of thelabel header

Explain the interactions between IP TTL

propagation and traceroute diagnostic


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Review Questions

How are routing loops prevented inMPLS networks?

What is the purpose of the TTL field?

What is TTL propagation?

What is the result of disabling TTLpropagation?

What can happen when some LSRshave TTL propagation disabled andsome do not?


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Loop Detection inCell-Mode MPLS



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Objectives


Explain the challenges of loopdetection in cell-mode MPLS

Describe how the label-distributionprocedures enable loop detection incell-mode MPLS

List loop detection mechanismsavailable during TDP/LDP label

distribution

Loop Detection in


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Loop Detection inCell-Mode MPLS

VPI/VCI field in the ATM header is usedfor label switching.

ATM header does not contain a TTLfield.

LDP or TDP still primarily relies on IGPsto prevent routing loops.

There is an additional mechanism builtinto LDP or TDP to prevent loops.


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LDP Hop Count TLV

LDP uses an additional type, length,value (TLV) attribute to count thenumber of hops in an LSP tunnel.

The TTL field in the IP header or labelheader is decreased by the number ofhops by the ingress ATM edge LSRbefore being forwarded through an labelVC .

If the TTL field is zero, or less thepacket is discarded.

Maximum number of hops can also be

specified for LDP.

LDP Hop Count


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A D

LDP Hop CountExample

LSR A discovers the length of the LSPtunnel across the ATM domain to LSR Dthrough LDP.

10.0.0.0/16

L=1/35

Hops=1

10.0.0.0/16

L=1/34

Hops=2

10.0.0.0/16

L=1/43

Hops=3

B C

Traceroute Through ATM


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A D

B C

Traceroute Through ATMLSRs Example (1)

The first traceroute packet thatreaches the network is dropped onrouter A.

An ICMP TTL exceeded message is

sent to the source from router A.

IP TTLLabel

TTL

1

-2

TTL is decreased by

3

The new TTL value

would be 2

The packet is

dropped.

traceroute 10.1.1.1

TTL exceeded

traceroute 10.1.1.1

1 10 ms A.acme.com



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A D

B C


The second traceroute packet thatreaches the network is dropped onrouter A.



IP TTLLabel

TTL

2

-1

TTL is decreased by

3

The new TTL value

would be 1

The packet is

dropped.

traceroute 10.1.1.1

1 10 ms A.acme.com

TTL exceeded

traceroute 10.1.1.1

1 10 ms A.acme.com

2 10 ms A.acme.com



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A D

B C


The third traceroute packet thatreaches the network is dropped onrouter A.



IP TTLLabel

TTL

3

0

TTL is decreased by

3

The new TTL value

would be 0

The packet is

dropped.

traceroute 10.1.1.1

1 10 ms A.acme.com

2 10 ms A.acme.com

TTL exceeded

traceroute 10.1.1.1

1 10 ms A.acme.com

2 10 ms A.acme.com

3 10 ms A.acme.com



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TTL is decreased by

3

The new TTL value is

1

The packet is

forwarded.

A D

B C


The fourth traceroute packet thatreaches the network is dropped onrouter D.


sent to the source from router D.

IP TTLLabel

TTL

4

0

1 1 1

traceroute 10.1.1.1

1 10 ms A.acme.com

2 10 ms A.acme.com

3 10 ms A.acme.com

TTL exceeded

traceroute 10.1.1.1

1 10 ms A.acme.com

2 10 ms A.acme.com

3 10 ms A.acme.com

4 10 ms D.acme.com


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LDP Path Vector TLV

Path vector TLV is another safeguardthat prevents loops in LDP.

This TLV is used to carry router IDs of

all ATM LSRs in the path.If an LSR receives an LDP update withits own router ID in the path vector TLV,the update

is ignored.Path vector TLV is similar to BGPs ASpath or cluster list attributes.

Path vector TLV is not present in TDP.

Path Vector


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A D

B C

E

Path VectorExample

The LDP update is dropped because it containsthe router ID of router C in the path vector TLV.

10.0.0.0/16

PV=D

10.0.0.0/16

PV=D

,C

E

10.0.0.0/16

PV=D,C,E

10.0.0.0/16

PV=D,C,E,B


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Loop Detection Summary

MPLS primarily relies on loopdetection mechanisms built into

IGPs.Hop count TLV is used to simulateTTL functionality on ATM LSRs withthe help of ATM edge LSRs.

Path vector TLV is used to preventloops in LDP updates.


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Summary

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

Explain the challenges of loopdetection in cell-mode MPLS

Describe how the label-distribution

procedures enable loop detection incell-mode MPLS

List loop detection mechanismsavailable during TDP/LDP label

distribution


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Review Questions

Which mechanisms are used to preventrouting loops in MPLS-enabled networks

using cell-mode MPLS?

Which TLVs in LDP are used to preventloops?

Describe TTL operation in cell-modeMPLS.


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MPLSBGPInteraction



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Objectives


Describe label allocation procedures

for external IP routesExplainlabel sharing between externalroutes and BGP next hops

Describe traditional BGP core designrequirements

Explain the relaxation of core designrequirements made possible by MPLS

List BGP design rules applicable in-


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Label Allocation in Unicast IP

Labels are assigned to FECs.

FEC in unicast IP routing is equal to adestination prefix found in an IP routing

table.

This is true only for IGP-derivedprefixes.

BGP-derived prefixes are assigned thelabel that is used for the BGP next-hopaddress.

Result: all prefixes learned from anexternal BGP neighbor use a single

S D i


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Transit AS

System DesignRequirements

All core routers are required to run BGP.

All core routers require full Internet routinginformation (more than 100,000 networks) tobe able to forward IP packets between ISP1and ISP2.

Core1Border1 Border2Core2

ISP1 ISP2

IBGP IBGP IBGPRR RR

EBGP EBGP

mp e e wor


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Transit AS

Core1Border1 Border2Core2

ISP2ISP1

mp e e worDesign in MPLS-Based

Networks

Only border routers are required to run BGP.

Core routers run an IGP to learn about BGPnext-hop addresses.

Core routers run LDP or TDP to learn aboutlabels for next-hop addresses.

EBGP EBGP

IBGP

MPLS-Based Transit AS


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MPLS-Based Transit ASBuilding FIB and LFIB

All routers are capable of forwarding packets to externaldestinations:

Border (edge) routers label and forward IP packets.

Core routers forward labeled packets.

Core1 Border2Core2

ISP2

Border1

ISP1

10

.0.0

.0/8

1.2.3.4

10.0.0.0/8

10.0

.0.0/8

FIB:

1.2.3.4 serial0/0

LFIB:

1.2.3.4/32

1.2.3.4/32

L=pop

FIB:

1.2.3.4 popLFIB:35 pop

1.2.3.4/32

1.2.3.4/32

L=35

FIB:

1.2.3.4 35LFIB:23 35

1.2.3.4/32

1.2.3.4/32

L=23

FIB:

1.2.3.4 23

LFIB:

64 23

FIB:

1.2.3.42310.0.0.0/8 23

FIB:

1.2.3.4 serial0/010.0.0.0/8 1.2.3.4

MPLS-Based Transit AS


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Core1 Border2Core2

ISP2

Border1

ISP1

MPLS-Based Transit ASPacket Propagation

1.2.3.4

FIB:

1.2.3.4 serial0/0

LFIB:

FIB:

1.2.3.4 popLFIB:

35 pop

FIB:

1.2.3.4 35LFIB:

23 35

FIB:

1.2.3.4 23

LFIB:

64 23

FIB:

1.2.3.4 2310.0.0.0/8 23

FIB:

1.2.3.4 serial0/010.0.0.0/8 1.2.3.4

10.1.1.1

10.1.1.123 10.1.1.135 10.1.1.1

10

.1.1.1

Benefits of MPLS-Based


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Benefits of MPLS BasedTransit AS

Simplified BGP topology (only AS edgerouters are required to run BGP with fullInternet routing).

Core routers do not require a lot of

memory (100,000 networks may requiremore than 50 MB of memory for the BGPtable, IP routing table, and CEFs FIB tableand distributed FIB tables).

Changes in the Internet do not impact corerouters.

Allows private addresses (RFC 1918) to beused in the core if TTL propagation isdisabled (traceroute across the AS will notshow any private addresses).

Common Design and


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Common Design andConfiguration Errors

BGP next-hop addresses should not besummarized by the IGP used in the AS.

Summarization of next-hop addresses

causes LSP tunnels to break into twoshorter LSP tunnels.

The summarizing routers would have torun BGP to overcome the summarization

problem.The recommendation is to have all BGPnext hops reachable as host routes ororiginal subnets throughout the AS (nosummarization .

S


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Summary

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

Describe label allocation procedures for

external IP routes

Explain label sharing between externalroutes and BGP next hops

Describe traditional BGP core design

requirements

Explain the relaxation of core designrequirements made possible by MPLS

List BGP design rules applicable in MPLS-based networks


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Documents

MPLS10S04-Advanced MPLS Technology