A Mpls Basics

8/8/2019 A Mpls Basics

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1 2006, Cisco Systems, Inc. All rights reserved.AS Training MPLS

Introduction to Multi-protocol LabelSwitchingMPLS Architecture and

Configuration

Mak Mahalingam


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222 2006, Cisco Systems, Inc. All rights reserved.MPLS VPN

Agenda

MPLS Concepts

LSRs and labels

Label assignment and distribution Label Switch Paths

Loop detection/prevention

TDP/LDP overview Configuring MPLS on Cisco IOSFrame-Mode

Interfaces

Monitoring MPLS on Cisco IOSFrame-Mode

Interfaces


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MPLS Concepts


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MPLS concepts

MPLS: Multi Protocol Label Switching

MPLS architecture is defined in:draft-ietf-mpls-arch/RFC3031

Packet forwarding is done based on Labels

Labels are assigned when the packets enter intothe network

Labels are on top of the packets

MPLS nodes forward packets/cells based on thelabel value (not on the IP information)


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MPLS Overview

Label Edge Routers

Label Distribution Protocol

(LDP/TDP)

Label Switch Routers


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MPLS Operation


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Packet forwarding: IP networks

IP forwarding

IP forwarding is done independently at every hop

IP forwarding decision is made on:

Packet header

Routing algorithm output (routing table)

Each IP hop runs its own instance of the routingalgorithm

Each IP hop makes its own forwarding decisions


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Packet forwarding: FEC and Next-Hop

Packets are classified into FECs (ForwardingEquivalency Class) and next-hop is determinedfor each FEC.

FEC is a group of IP packets which are forwarded inthe same manner

Over the same path

With the same forwarding treatment

In IP routing each hop:

re-classify the packet into one FEC

recalculate the next-hop of the FEC


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AddressAddress

Prefix and maskPrefix and mask

171.68.10/24171.68.10/24

......

NextNext--HopHop

171.68.9.1171.68.9.1

......

IP Packet forwarding: FEC and Next-Hop

171.68.10/240 1

InterfaceInterface

Serial1Serial1

......

IP packetD=171.68.10.12IP packet

D=171.68.10.23

Router-A forwards packets with differentdestination addresses using the same route,same next-hop and same interface

Rtr-A


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MPLS make use of FECs

MPLS nodes assign a label to each FEC

Packet classification (into a FEC) is done wherethe packet enters the network

The packet classification is encoded as a label

In the core, packets are forwarded withouthaving to re-classify them in the MPLS network

No further packet analysis

Label swapping


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NextNext--HopHop


171.68.10/240 1

Router-A forwards labelled packets bylooking at the label value against thelabel table. No packet classification intoFEC is done.

Rtr-A

InIn

LabLab

55

......

AddressAddress

PrefixPrefix

171.68.10171.68.10

......

OutOut

I/FI/F

11

......

OutOut

LabLab

33

......

InIn

I/FI/F

00

......

IP packetD=171.68.10.12

Label = 5

NextNext--HopHop

InIn

LabLab

xx

......

AddressAddress

PrefixPrefix

171.68.10171.68.10

......

OutOut

I/FI/F

33

......

OutOut

LabLab

55

......

InIn

I/FI/F

44

......

34

IP packetD=171.68.10.12

Rtr-B

Router-B classify the IP packet into a FEC andassign the corresponding label.

IP packetD=171.68.10.12

Label = 3


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MPLS forwarding

As packet enters into MPLS network,classification mechanism may be complex

IGP, Multicast, Traffic Engineering, VPN. QoS ...

Packet forwarding in the core is not affected bythe complexity of packet classification at theedge

Information needed to do packet classificationmay not be present in the core

All classifications made are identified by thelabel and/or by Exp bits of the label header


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LSRs and labels


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LSR: Label Switch Router

Can be an ATM switch or a router

Edge-LSRs do label imposition and label removal

Label imposition (PUSH) where the packet enters theMPLS network

Label removal (POP) where the packet leaves the MPLSnetwork

All LSRs use existing IP routing protocols toexchange routing information

All LSRs use a label distribution protocol

Not necessarily the same in all LSRs


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MPLS forwarding

MPLS forwarding is performed in the same wayin 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 different protocols

LDP, RSVP, PIM, BGP, ...


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An IP routing protocol is used within the routing domain (e.g.:OSPF, i-ISIS)

A label distribution protocol is used to distribute address/label mappingsbetween adjacent neighbors

The ingress LSR receives IP packets, performs packet classification, assigna label, and forward the labelled packet into the MPLS network

Core LSRs switch packets/cells based on the label value (no packetclassification in the core)

The egress LSR removes the label before forwarding theIP packet outside the MPLS network


IGP domain with a label

distribution protocol


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Labels

Label

Label is a short fixed length, locally significant used toidentify a FEC

Label is assigned on the network layer address but it isnot encoding of the network layer address

Label format and length depends onencapsulation

More than one label is allowed

Label stack: ordered set of labels

MPLS LSRs always forward packets based on thevalue of the label at the top of the stack


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Labels

Generic: can be used over Ethernet, 802.3, PPPlinks, Frame Relay, ATM PVCs, etc.

Uses new Ethertypes/PPP PIDs/SNAPvalues/etc.

Different Ethertypes for unicast and multicast

4 octets (per tag level)

Label = 20 bits

Exp = Experimental, 3 bitsS = Bottom of stack, 1bit

TTL = Time to live, 8 bits

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

Label | Exp|S| TTL


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Labels

PPP Header

(Packet over SONET/SDH)

ATM Cell Header

LAN MAC Label Header

HECHEC DATADATACLPCLPPTIPTIVCIVCIGFCGFC VPIVPI

Label

PPP HeaderPPP Header Layer 3 Header Layer 3 HeaderLabel

Label Layer 3 HeaderLayer 3 HeaderMAC HeaderMAC Header

Shim header


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MPLS Label Assignment andDistribution


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Label assignment and distribution

Labels have local significance

Each LSR binds his own label mappings

Each LSR assign labels to each of their FECs

Labels are assigned and exchanged between adjacentLSRs

Label distribution may be upstream or downstream driven

Most implementations use downstream with two variants

Unsolicited Downstream Downstream on demand

no need for upstream allocation

Applications may require non-adjacent neighbors

Traffic Engineering or VPN


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Upstream and Downstream LSRs

Rtr-C is the downstream neighbor of Rtr-Bfor destination 171.68.10/24

Rtr-B is the downstream neighbor of Rtr-Afor destination 171.68.10/24

LSRs know their downstream neighborsthrough the IP routing protocol

Next-hop address is the downstream neighbor

171.68.10/24

Rtr-BRtr-A Rtr-

C

171.68.40/24


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Downstream on demanddistribution

LSRs assign a label to each FEC

Upstream LSRs request labels to downstreamneighbors

Downstream LSRs distribute labels upon request

171.68.10/24

Rtr-BRtr-A Rtr-

C

171.68.40/24

Use label 7 for destination171.68.10/24

Use label 5 for destination171.68.10/24

Request label fordestination 171.68.10/24

Request label fordestination 171.68.10/24


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Label retention modes

Liberal retention mode

LSR retains labels from all neighbors

Improve convergence time, when next-hop is againavailable after IP convergence

Require more memory and label space

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

Conservative retention mode

LSR retains labels only from next-hops neighbors

LSR discards all labels for FECs without next-hop

Free memory and label space


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Label DistributionOrdered vs. Independent Control

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 ithas to receive 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

Each packet/cell may have more than one label

Label stack is the ordered list of labels ATM cells?

Label format is negotiated between peers

VPI/VCI fields may contain different labels

Label stack is copied in the payload of the FIRST cell ofthe packet

LSR nodes label switch packets based ONLY onthe label at the top of the stack


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The Label Stack

171.68.10/24

Rtr-A

NextNext--HopHop

InIn

LabLab

55

......

AddressAddress

PrefixPrefix

171.68.10171.68.10

......

OutOut

I/FI/F

11

......

OutOut

LabLab

77

......

InIn

I/FI/F

00

......

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 Distribution Protocols

Several protocols for label exchange

LDP/TDP

Maps unicast IP destinations into labels

RSVP, CR-LDP

Used for traffic engineering and resource reservation

PIM

For multicast states label mapping

BGP

External labels (VPN)


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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 through

which the labelled packets have to gothrough in order to reach the egress LSR

LSP egress node may be the aggregationpoint of prefixes


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LSR-ingress to LSR-egress path is the

same for packets of the same FEC LSPs are unidirectional

Return traffic takes another LSP

IGP domain with a label

distribution protocol

Ingress-

LSREgress-LSR


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FEC is determined in LSR-ingress

LSPs derive from IGP routing information LSPs may diverge from IGP shortest path

LSP tunnels (explicit routing) with Traffic Engineering

LSP follows IGP shortest path LSP diverges from IGP shortest path

IGP domain with a labeldistribution protocol

IGP domain with a labeldistribution protocol


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Penultimate Hop Popping

The label at the top of the stack isremoved (popped) by the upstream

neighbor of the egress LSR

The egress LSR requests the poppingthrough the label distribution protocol

Egress LSR advertises implicit-nulllabel

One lookup is saved in the egress LSR


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Aggregation and layer 3summarisation

The LSR which does summarisation willbe the end node LSR of all LSPs related tothe summary address

Aggregation point

The LSR will have to examine the secondlevel label of each packet

If no second label, the LSR has to examinethe IP header


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MPLS and routing to BGP next-hopaddresses

Labels are assigned to FECs

FECs are derived from IP routing protocols (IGP)

Labels are NOT assigned to BGP routes

BGP routes use recursive routing to find next-hop reachability

Labels are assigned to BGP next-hops

This saves CPU/Memory, label space andstability on core LSRs

Core LSRs are preserved from BGP instability


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MPLS and BGP

128.14.10/24

0 1

AddressAddress

Prefix and maskPrefix and mask

BGP: 128.14.10/24BGP: 128.14.10/24

OSPF: 171.68.9.1/32OSPF: 171.68.9.1/32

NextNext--HopHop

171.68.9.1171.68.9.1

171.68.14.13171.68.14.13

IntInt

--

Serial1Serial1

IP packetD=128.14.10.1

InIn

LabLab

--

......

AddressAddress

PrefixPrefix

171.68.9.1171.68.9.1

......

OutOut

I/FI/F

11

......

OutOut

LabLab

33

......

InIn

I/FI/F

00

......

Label table

Routing table

IP packetD=128.14.10.1

Label = 3

171.68.9.1

Ingress LSR receives IP packetDestination is given by BGP

BGP has next-hop known in the IGP

Label is available for BGP next-hop, through IGP route

Packet will traverse the core using IGP (BGP next-hop) label


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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 hopmakes its own 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 byconfiguration

The LSP can be set dynamically usinglink-state topology information

Traffic Engineering makes use of explicitrouted LSPs


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Loop Detection and prevention


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Loops and TTL

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

TTL is decremented at each router-hop

If TTL = 0 packet is discarded

MPLS mayuse same mechanism but not

on all encapsulationsTTL is present in the label header for PPP and

LAN headers (shim headers)

ATM cell header does not have TTL


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Loops and TTL

For PPP/LAN encapsulation TTL is used in thesame way it is used in IP networks

When a packet enters into the MPLS network the

LSRs may be configured so thatThe Ingress LSR does a copy of the layer 3 TTL into the

Label TTL

MPLS TTL propagation

Each hop decrements TTL

If TTL = 0 packet is discarded

The Egress LSR does a copy of the label TTL into thelayer 3 TTL when the packet leaves the MPLS network


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Loops and TTL in ATM

LSRs using ATM or FR do not have TTLcapability

Still when a packet emerges from an LSP it hasto have a TTL reflecting the number of nodes ittraversed

Control plane loop detection uses a TLV Pathvector attribute (similar to BGP AS loop

detection) For details see Chapter 5 in Cisco Press book

MPLS and VPN Architectures.


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Label Distribution Protocol

(LDP)


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MPLS Control Plane


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TIB and TFIB/LFIB


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Tag Distribution Protocol

Runs in parallel with routing protocols

Distributes bindings

Incremental updates over TCP

Other tag distribution mechanisms can

run in parallel with it


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TDP Transport

Uses TCP for reliable transport

Well-known contact port (711)

Design Choice:

One TDP session per TCP connection


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TDP/LDP Identifier

Identifies tag space

6 bytes

Cisco convention


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LDP operationsIdentifiers

LDP Identifier

6 bytes identifying the LSR and the label space

4 bytes: IP address of one of the platform interfaces

2 bytes: Label space identifier:

LSR using different label spaces will useseparate LDP sessions

TCP port 646


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LDP OperationsSessions

R1

R2

R3

R4

L1 L3

L2 L4

R1 R2L1

L2

L3

R1 R2L1

Session for L1, L2 and L3

R1

R2

R3

R4

Session for L1 Session for L3

Session for L2 Session for L4

R1 R2L1

L2 (ATM)

L3

R1 R2Session for L2

Session for L1, L3


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Configuring MPLS on CiscoIOSFrame-Mode

Interfaces


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MPLS Configuration Tasks

Mandatory:

Enable CEF switching

Configure label pool (mandatory in some IOS software

releases)Configure TDP or LDP on every label-enabled interface

Optional:

Configure MTU size for labeled packets

Configure IP TTL propagation

Configure conditional label advertising

MPLS C fi ti


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MPLS ConfigurationCommands

Base MPLS functionality is configured using tag-switching configuration commands until IOSrelease 12.1(3)T and 12.0(x)ST

IOS 12.1(3)T/12.0(x)ST introduces MPLSconfiguration commands that are usuallyequivalent to tag-switching configurationcommands

tag-switching version of configurationcommands appear in saved configuration forbackward compatibility

C fi i L b l P l


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Configuring Label Pooltag-switching Syntax

Defines label pools to be used by all downstream

label distribution protocols (LDP/TDP as well asothers).

Usually not needed, but required on some IOSreleases to start label switching.

Parameters Minimum minimum label value (default = 10)

Maximum maximum label value (default = 16777215)

Reserved number of reserved labels (default = 16)

tag tag-range downstream minimum maximum [reserved]

Router(config)#

C fi i L b l P l


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Configuring Label Poolmpls Syntax

Defines label pools to be used by all downstreamlabel distribution protocols (LDP/TDP as well as

others).

Usually not needed, but required on some IOSreleases to start label switching.

Parameters

Minimum minimum label value (default = 16)

Maximum maximum label value (default = 1048575)

Labels 0 through 15 are reserved and cannot beallocated

mpls label range minimum maximum

Router(config)#


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MPLS Configuration Example

Mi d TDP/LDP E i t


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Mixed TDP/LDP EnvironmentConfiguration Example

C fi i L b l S it hi


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Configuring Label SwitchingMTU

Label switching increases the maximum MTUrequirements on an interface, due to additional labelheader.

Interface MTU is automatically increased on WAN

interfaces; IP MTU is automatically decreased on LA

Ninterfaces.

Label switching MTU can be increased on LANinterfaces (resulting in jumbo frames) to prevent IPfragmentation.

The jumbo frames are not supported by all LAN switches.

(3)T

tag-switching mtu mtu-size

mpls mtu mtu-size 12.1(3)T

Router(config-if)#

MPLS on LAN


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MPLS on LANConfiguration Example

Configuring IP TTL


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Configuring IP TTLPropagation

By default, IP TTL is copied into label header atlabel imposition and label TTL is copied into IPTTL at label removal.

This command disables IP TTL and label TTL

propagation. TTL value of 255 is inserted in the label header.

The TTL propagation has to be disabled oningress and egress edge LSR.

3)T

no tag-switching ip propagate-ttl

no mpls ip propagate-ttl 12.1(3)T

Router(config)#


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Disabling IP TTL Propagation


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Disabling IP TTL PropagationConfiguration Example

C fi i IP TTL P ti


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Configuring IP TTL Propagation Extended Options

Selectively disables IP TTL propagation for:

Forwarded traffic (traceroute does not work for transittraffic labeled by this router).

Local traffic (traceroute does not work from the routerbut works for transit traffic labeled by this router).

(xno mpls ip propagate-ttl [ forwarded | local ] 12.1(5)T/12.0(x

Router(config)#

Disabling IP TTL Propagation


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Disabling IP TTL Propagationfor Customer Traffic

Configuring Conditional Label


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Configuring Conditional LabelDistribution

By default, labels for all destinations are announced to all

LDP/TDP neighbors.

This command enables you to selectively advertise somelabels to some LDP/TDP neighbors.

Conditional label advertisement only works over frame-mode interfaces.

Parameters:

Net-ACL the IP ACL that selects the destinations for whichthe labels will be generated.

TDP-ACL the IP ACL that selects the TDP neighbors that will

receive the labels.

tag-switching advertise-tags fornet-acl[ to tdp-acl]

Router(config)#

Conditional Label Distribution


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Conditional Label DistributionExample

The customer is already running IPinfrastructure.

MPLS is only needed to support MPLS/VPNservices.

Labels should only be generated for loopbackinterfaces (BGP next-hops) of all routers.

All loopback interfaces are in one contiguous addressblock (192.168.254.0/24).


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Conditional Label Distribution


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Conditional Label DistributionRouter Configuration

Step #2 Enable conditional labeladvertisment

no tag-switching advertise-tags

!

! Configure conditional advertisments

!

tag-switching advertise-tags for 90 to 91

!

access-list 90 permit ip 192.168.254.0 0.0.0.255

access-list 91 permit ip any


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Monitoring MPLS on CiscoIOSFrame-Mode

Interfaces

Basic MPLS Monitoring


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Basic MPLS MonitoringCommands

Displays TDP parameters on the localrouter.

show tag-switching tdp parameters

Router(config)#

show tag-switching interface

show mpls interface 12.1(3)T

Router(config)#

show tag-switching tdp discovery

Router(config)#

Displays MPLS status on individual interfaces.

Displays all discovered TDP neighbors.

show tag-switching tdp


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show tag-switching tdpparameters

Router#show tag-switching tdp parameters

Protocol version: 1

No tag pool for downstream tag distribution

Session hold time: 180 sec; keep alive interval: 60

sec

Discovery hello: holdtime: 15 sec; interval: 5 sec

Discovery directed hello: holdtime: 180 sec;

interval: 5 sec


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show tag-switching tdp


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show tag-switching tdpdiscovery

Router#show tag-switching tdp discovery

Local TDP Identifier:

192.168.3.102:0TDP Discovery Sources:

Interfaces:

Serial1/0.1: xmit/recv

TDP Id: 192.168.3.101:0

Serial1/0.2: xmit/recv

TDP Id: 192.168.3.100:0

More TDP Monitoring


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More TDP MonitoringCommands

Displays individual TDP neighbors.

show tag-switching tdp neighbor

Router(config)#

show tag-switching tdp neighbor detail

Router(config)#

show tag-switching tdp bindings

Router(config)#

Displays more details about TDP neighbors.

Displays Tag Information Base (TIB).


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show tag tdp neighbor

Router#show tag-switching tdp neighbors

Peer TDP Ident: 192.168.3.100:0; Local TDP Ident

192.168.3.102:0

TCP connection: 192.168.3.100.711 - 192.168.3.102.11000

State: Oper; PIEs sent/rcvd: 55/53; ; Downstream

Up time: 00:43:26

TDP discovery sources:

Serial1/0.2

Addresses bound to peer TDP Ident:

192.168.3.10 192.168.3.14 192.168.3.100


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show tag tdp bindings

Router#show tag tdp bindings

tib entry: 192.168.3.1/32, rev 9

local binding: tag: 28

remote binding: tsr: 19.16.3.3:0, tag: 28

tib entry: 192.168.3.2/32, rev 8



tib entry: 192.168.3.3/32, rev 7


remote binding: tsr: 19.16.3.3:0, tag: imp-null(1)

tib entry: 192.168.3.10/32, rev 6

local binding: tag: imp-null(1)



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Monitoring Label Switching

Displays contents of Label Forwarding InformationBase.

show tag-switching forwarding-table

show mpls forwarding-table

Router(config)#

show ip cef detail

Router(config)#

Displays label(s) attached to a packet during labelimposition on edge LSR.


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show tag-switching


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show tag switchingforwarding-table


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show ip cef detail

Router#show ip cef 192.168.20.0 detail

192.168.20.0/24, version 23, cached adjacency to Serial1/0.2

0 packets, 0 bytes

tag information set

local tag: 33

fast tag rewrite with Se1/0.2, point2point, tags imposed: {32}

via 192.168.3.10, Serial1/0.2, 0 dependencies

next hop 192.168.3.10, Serial1/0.2

valid cached adjacency

tag rewrite with Se1/0.2, point2point, tags imposed: {32}

Debugging Label Switching


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ebugg g abe S tc gand TDP

Debugs TDP adjacencies, session establishment, and label bindingsexchange.

debug tag-switching tdp ...

Router(config)#

debug tag-switching tfib ...

debug mpls lfib 12.1(3)T/12.0(x)ST

Router(config)#

debug tag-switching packets [interface ]

debug mpls packets [interface ] 12.1(3)T/12.0(x)ST

Router(config)#

Debugs Tag Forwarding Information Base events: label creations,removals, rewrites.

Debugs labeled packets switched by therouter.


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