0 - MPLS_Basic_Training_-_Intro (Cisco Training)

8/10/2019 0 - MPLS_Basic_Training_-_Intro (Cisco Training)

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2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 1

MPLS Basic Training


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MPLS Introduction


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MPLS Introduction What is MPLS?

!

Multi Protocol Label Switching

!MPLSis an efficient encapsulation mechanism

!Uses Labelsappended to packets (IP packets, AAL5 frames) fortransport of data

!

MPLS packets can run on many layer 2 technologiessuch as ATM, FR, PPP, POS, Ethernet

!Other layer 2 technologies can be run over an MPLS network

- Pseudowires

!

Labels can be used as designators

For exampleIP prefixes, ATM VC, or a bandwidth guaranteed path

!MPLS is a technology for delivery of IP Services


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Original Motivation for MPLS

!

Allow core routers/networking devices to switch packetsbased on some simplified, fixed length header

!Provide a highly scalable mechanism that was topologydriven rather than flow driven

!Leverage hardware so that simple forwarding paradigmcan be used

!It has evolved a long way from the original goal

Hardware became better and looking up the longest bestmatch was no longer an issue

By associating labels with prefixes, groups of sites orbandwidth paths or light paths with new services such as MPLSVPNs and Traffic engineering, GMPLS were now possible


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MPLS Concepts

!Few components play a role in creating an MPLSnetwork

IGP: Core Routing Protocol

MPLS Label (or shim, think OSI layer 2.5)

Encapsulation of MPLS label (push/pop/swap)

Forwarding Equivalence Class (FEC)

Label Distribution Protocol (LDP)

MPLS Applications related protocols: MP-BGP, RSVP, etc.


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MPLS Terminology

!Acronyms

PE- Provider edge router (or Label Edge Router (LER))

P- Provider core router (or Label Switch Router (LSR))

CE- Customer Edge router (also referred to as CPE)

ASBR - Autonomous System Boundary Router

RR - Route Reflector

!TE -Traffic Engineering

TE Head end - Router that initiates a TE tunnel

TE Midpoint - Router where the TE Tunnel transits

!VPN- Collection of sites that share common policies

!

AToM- Any Transport over MPLSCisco scheme for building layer 2 circuits over MPLS

Attachment Circuit- Layer 2 circuit between PE and CE

Emulated circuit - Pseudowirebetween PEs


7/58 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 7

MPLS Concepts

! Create new services via flexible classification

! Provides the ability to setup bandwidth guaranteed paths (TE)

At Edge:

Classify packets

Label them

Label Imposition

In Core:

Forward using labels (asopposed to IP addr)

Label indicates service classand destination

Label Swapping or Switching

P or Label Switch Router(LSR)

Router

ATM switch + Label

Switch ControllerLabel Distribution Protocol

PE or LabelSwitch Router

(LER)

(ATM Switch or

IP Router)

At Edge:Remove Labels and

forward packets

Label Disposition



MPLS Operation

1a. Existing routing protocols (e.g. OSPF, IS-IS)establish reachability to destination networks

1b. Label Distribution Protocol (LDP)establishes label to destinationnetwork mappings

2. Ingress LER receives packet,performs Layer 3 value-added services,and labelspackets

3. LSR switchespackets using labelswapping

4. LER at egressremoves label anddelivers packet



MPLS Labels



Data Encapsulation Review

3k of Data to be sent

Segment

Packet

Frame

BitsDATATCP

HeaderIP

HeaderFrameHeader FCS011010101

DATATCP

HeaderIP

HeaderFrameHeader FCS

DATATCP

HeaderIP

Header

DATATCP

Header

MTU = 1500 Bytes (Data segment is MSS)MSS = MTU Encapsulation Headers

TCP Header = 20 bytesIP Header = 20 bytesMSS = 1500 40 = 1460 bytes of

data sent

Frame Header = 14 bytesFCS = 4 bytes

L2 Frame = 1518 bytes



Label Header for Packet Media

! Can be used over Ethernet, 802.3, or PPP links

! Fixed length label 4 bytes per label (32 bits)

!

Uses two new Ethertypes/PPP PIDs

! Contains everything needed at forwarding time

! One word per label

Label = 20 bits

COS/EXP = Class of Service, 3 bits

S = Bottom of Stack, 1 bit

TTL = Time to Live, 8 bits

0 1 2 30 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



MPLS Labels increase MTU!!!

3k of Data to be sent

Segment

Packet

Frame

BitsDATATCP

HeaderIP

HeaderFrameHeader FCS011010101

DATATCP

HeaderIP

HeaderFrameHeader FCS

DATATCP

HeaderIP

Header

DATATCP

Header

MTU = 1500 Bytes (Data segment is MSS)MSS = MTU Encapsulation Headers

TCP Header = 20 bytesIP Header = 20 bytesMSS = 1500 40 = 1460 bytes of

data sent

Labels imposedon the IP packet

(4 bytes each)

MPLSLabel

MPLSLabel



MPLS MTU

!

You need to account for the increased packet size

Change the physical MTU of the interface

Or

Use the mpls mtu command

!

FE interfaces cannot increase MTU, have to use mplsmtucommand

!GE can go up to 9000 bytes

!Switches can change system mtu

set CE facing interfaces back to 1500

!Choose MTU large enough to account for all labels

min of 2 labels



Forward Equivalence Class (FEC)

! Describes a set of packets with similar and / or identicalcharacteristics which may be forwarded the same way

I.E. bound to the same MPLS label

!An FEC is a set of packets that a single router:

(1) Forwards to the same next hop;

(2) Out the same interface; and

(3) With the same treatment (such as queuing).

!Determines how packets are mapped to LSPs

IP Prefix/host address

Layer 2 Circuits (ATM, FR, PPP, HDLC, Ethernet)

Groups of addresses/sites - VPN x

A VPLS instance - VFI

Tunnel Interface - Traffic Engineering



MPLS in hardware



MPLS Control and Forwarding Planes

! Control plane used to distribute labels - LDP, RSVP or MP-BGP

! Forwarding plane consists of label imposition, swapping anddisposition

!

Key: There is a separation of Control Plane and Forwarding Plane

Basic MPLS: destination-based unicast

Labels divorce forwarding from IP address *

Many additional options for assigning labels

Labels define destination and service

* BGP Free core



Control and Forward Plane Separation

LFIB

RoutingProcess

MPLSProcess

RIB

LIB

FIB

Route

Updates/

Adjacency

Label Bind

Updates/

Adjacency

IP TrafficMPLS Traffic



Routing Basic

s



Router Example: Forwarding Packets

0

1

1

128.89

171.69

0

128.89.25.4 Data 128.89.25.4 Data

128.89.25.4 Data128.89.25.4 Data

Packets Forwarded Based

on IP Address

...

128.89

171.69

addressprefix

I/F

1

1

...

128.89

171.69

addressprefix

I/F

0

1 ...

128.89

address

prefix I/F

0



MPLS Example: Routing Information

128.89

171.69

1

01

Inlabel

AddressPrefix

128.89

171.69

...

OutI

face

1

1

...

Outlabel

Inlabel

AddressPrefix

128.89

171.69

...

OutI

face

0

1

...

Outlabel

Inlabel

AddressPrefix

128.89

...

OutI

face

0

...

Outlabel

0

You can reach 171.69 thru me

You can reach 128.89 and

171.69 thru me

Routing Updates(OSPF, EIGRP,!)

You can reach 128.89 thru me



MPLS Example: Assigning Labels

128.89

171.69

1

01

Inlabel

-

-

...

AddressPrefix

128.89

171.69

...

OutI

face

1

1

...

Outlabel

4

5

...

Inlabel

4

5

...

AddressPrefix

128.89

171.69

...

OutI

face

0

1

...

Outlabel

9

7

...

Inlabel

9

...

AddressPrefix

128.89

...

OutI

face

0

...

Outlabel

-

...

0

Use label 7 for 171.69

Use label 4 for 128.89 and


Label DistributionProtocol (LDP)

(Downstream Unsolicited Label Allocation)



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Unicast Routing Protocols

!OSPF, IS-IS, BGP are needed in the network

They still provide reachability between devices in the network

!Label distribution protocols distribute labels for prefixesadvertised by unicast routing protocols using

Label Distribution Protocol (LDP,TDP)

RSVP-TE for Traffic Engineering

Extending existing protocols like BGP (MP-BGP address-families)to distribute Labels (inter-AS) *

* Not covering inter-AS or CSC in this class


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Label Distribution Protocol

!

Defined in RFC 3035 and 3036

! Used to distribute Labels in an MPLS network

! Forwarding Equivalence Class (FEC)

How packets are mapped to LSPs (Label Switched Paths)

VPN_ID

VC_ID

Class of Service

!Advertise Labels per FEC

Reach destination a.b.c.d with label x


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Label Distribution: UnsolicitedDownstream

Label for a prefix is allocated and advertised to allneighbor LSRs, regardless of whether the neighborsare upstream or downstream LSRs for the destination

A B C D

E

Network X


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Label Distribution: UnsolicitedDownstream

Label for a prefix is allocated and advertised to allneighbor LSRs, regardless of whether the neighborsare upstream or downstream LSRs for the destination

A B C D

E

Network X

Network LSR label

X local 25

LIB on B

X = 25X = 25


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Label Distribution: Downstream onDemand

An LSR will only assign a label to a prefix when askedfor a label by an upstream LSR

Label distribution is a hop-by-hop parameter different label distribution mechanisms can coexist inan MPLS network

A

E

Network X

C

D

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


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LDP on Cisco platforms

!

LDP and TDP are both supported

! The protocol to use is configurable

Per-interface basis for directly connected peers

Per-target basis for non-directly connected peers

Default is TDP

!A platform may support LDP and TDP sessionssimultaneously but not using the same label space tothe same LSR

!

An LSP may be signaled by LDP on some segmentsand by TDP on other segments

! Why???


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Other Label Distribution Protocols RSVP-TE

!

Used in MPLS Traffic Engineering

!Additions to RSVP signaling protocol

! Leverage the admission control mechanism of RSVP to

create an LSP with bandwidth requirements

! Label requests are sent in PATH messages and bindingis done with RESV messages

! EXPLICT-ROUTE object defines the path over whichsetup messages should be routed

! Using RSVP has several advantages

Fixed path through network

Bandwidth guarantee


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Other Label Distribution Protocols - MP-BGP

!

Used in the context of MPLS VPNs

Inter-AS VPNs

Carrier Supporting Carrier

!

Need multi-protocol extensions to BGP!Routers need to be BGP peers

!Label mapping info carried as part of NLRI (NetworkLayer Reacheability Information)

!Lots of neat tricks can be done!


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LDP Peer Discovery


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LDP Peer Discovery Mechanism

!

LSRs discover LDP peers by exchanging LDP Hellomessages (dynamic neighbor discovery)

Basic Neighbor Discovery

Discover directly attached neighborspt-to-pt links

(including Ethernet)

LDP link Hellos are sent periodically using UDP port 646

(multicast to 224.0.0.2)

Establish a TCP session and Exchange prefix/FEC and

label information


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LDP Peer Discovery Mechanism (cont)

!

LSRs discover LDP peers by exchanging LDP Hellomessages (dynamic neighbor discovery)

Extended neighbor discovery

Establish peer relationship with a non-directly connected

router

LDP Targeted Hellos are sent using UDP port 646 (unicast)

Establish a TCP session and Exchange prefix/FEC andlabel information

* Used in Traffic Engineering


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Normally Routed Path

Traffic Engineering Route

NormallyRouted Path

TrafficEngineering

Route

R9

R8

R7

R6

R5

R1

R4

R3

R2

LDP Discovery - Non-adjacent Neighbors


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TE

LDP

Packet

LDP

Packet

R9R8

R7

R6 R5

R1

R4R3R2

TE

LDP

Packet

TE

LDP

Packet

LDP Discovery - Non-adjacent Neighbors


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LDP Session Establishment

!

TCP connection establishment

Once neighbors are discovered, TCP session established

Open a TCP connection to be used to distribute label bindings

Use Router-id or the IP source address of its Hello

messages to avoid two concurrent TCP connections

!Session Initialization

LDP peer exchange and negotiate Session parameters

Protocol version, label distribution method, timer values,

label ranges, etc.

Active LSR will track this parameter and reply withinitialization message; and keepalives are exchanged


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LDP Session

R2#sh mpls ldp neighbor

Peer LDP Ident: 192.168.0.3:0; Local LDP Ident 192.168.0.2:0

TCP connection: 192.168.0.3.11000 - 192.168.0.2.646

State: Oper; Msgs sent/rcvd: 1658/1659; Downstream

Up time: 23:58:08

LDP discovery sources:

Serial2/0, Src IP addr: 10.0.1.10

Addresses bound to peer LDP Ident:

10.0.1.10 10.0.1.13 192.168.0.3

R2s loopback or Router-Id is 192.168.0.2

R3s loopback or Router-Id is 192.168.0.3


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Loop Detection

!LDP relies on loop-detection mechanisms built into IGPs that are usedto determine the path

!If, however, a loop is generated (that is, mis-configuration with staticroutes), the TTL field in the label header is used to prevent indefinitelooping of data packets

!TTL functionality in the label header is equivalent to TTL in the IPheaders

!TTL is usually copied from the IP headers to the label headers (TTLpropagation)

0 1 2 30 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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Label Stacking

! There may be more than one label in an MPLS packet

Need to account for 4 bytes in MTU per label

!As we know labels correspond to forwarding equivalence classes (FECs)

Examplethere can be one label for routing the packet to an egress point andanother that separates a customer A packet from customer B

Inner labels can be used to designate services/FECs, etc.

ie: VPNs, fast reroute

! Outer label used to route/switch the MPLS packets in the network

Called the Transport Label

! Last label in the stack is marked with End of Stack (EOS) bit

0 1 2 30 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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MPLS Configuration


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Lab Topology

192.168.1.4/3

0

10.0.48.0/30

10.0.78.0/30 10.0.27.0/30

192.168.1.0/3

0

172.16.0.0/30

S0/0

10.0.34.0/30

10

.0.2

3.0

/30

172.16.0.4/30

PE

R5

CE

R9

CE

R6

CE

R1

CE

R4

PE

R2

PE

R8

P

R7

P

R3

P

S0/0

S3/0

S2/0 E0/0

E1/0

E0/0

E1/0 E1/0 E0/0E0/0

E0/0

E1/0

S2/0

S3/0

E1/0

S0/0

S0/0

IGP=OSPF


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MPLS Configuration Steps

1.

Insure CEF is running on the Router2.

Insure MPLS is running on the router MPLS is enabled by default

3.

Determine which Label Distribution Method youwill use (TDP or LDP)

This can be done globally setting the default or setting it on a per interface basis

4.

Establish the Router-ID for the LDP5.

Configure all Provider Interfaces to run LDP6.

Verify the configuration


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MPLS Configuration

1.

Enable CEFRouter(config)# ip cef [distributed]

2. Enable MPLS (default is on)Router(config)# mplsip

3.

Enable LDP Protocol globally (optional)Router(config)# mpls label protocol ldp

4. Establish LDP router ID (optional/recommended)Router(config)#mpls ldp router-idinterface [force]

5.

Enable LDP per interfaceRouter(config-if)# mpls ip

6.

Enable LDP Protocol per interface (optional)Router(config-if)# mpls label protocol ldpNOTE: CAN ALSO BE DONE IN GLOBAL CONFIGURATION

7. Verify


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Verification Commands

Which interfaces are running MPLS?R2#sh mpls interfacesInterface IP Tunnel BGP Static OperationalSerial2/0 Yes (ldp) No No No YesSerial5/0 Yes (ldp) No No No YesR2#

Which neighbors have been discovered?

R2#sh mpls ldp discoveryLocal LDP Identifier:

10.0.0.2:0Discovery Sources:Interfaces:

Serial2/0 (ldp): xmit/recvLDP Id: 10.0.0.3:0


R2#


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Verification Commands (cont)

R2#sh mpls forwarding-table 10.0.0.8 detail

Local Outgoing Prefix Bytes Label Outgoing Next Hop

Label Label or VC or Tunnel Id Switched interface

19 19 10.0.0.8/32 0 Se5/0 point2point

MAC/Encaps=4/8, MRU=1500, Label Stack{19}

0F008847 00013000

No output feature configured

R2#


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Verification Commands (cont)

! Verifying Labels Are Set traceroute is MPLS aware

R2#traceroute 10.0.0.8

Type escape sequence to abort.

Tracing the route to 10.0.0.8

1 10.0.1.18 [MPLS: Label 19 Exp 0] 52 msec 40 msec 52 msec

2 10.0.1.22 44 msec * 32 msec

R2#


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Stupid Router Tricks (but useful)

mpls ip propagate-ttl

! To control the generation of the time-to-live (TTL) field in theMultiprotocol Label Switching (MPLS) header when labels are first addedto an IP packet, use the mpls ip propagate-ttlcommand in globalconfiguration mode. To use a fixed TTL value (255) for the first label ofthe IP packet, use the noform of this command.

mpls ip propagate-ttl

nompls ip propagate-ttl [forwarded | local]

forwarded (Optional) Prevents the traceroutecommand from

showing the hops for forwarded packets. (customercan not see)

local (Optional) Prevents the traceroutecommand fromshowing the hops only for local packets. (you cannot see)


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Troubleshooting MPLS Configuration

Which interfaces are running MPLS?R2#sh mpls interfacesInterface IP Tunnel BGP Static OperationalSerial2/0 Yes (ldp) No No No YesSerial5/0 Yes (ldp) No No No YesR2#

Which neighbors have been discovered?

R2#sh mpls ldp discoveryLocal LDP Identifier:

10.0.0.2:0Discovery Sources:Interfaces:



R2#


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Troubleshooting MPLS Configuration (cont)

Which labels are being used for a subnet?

R2#sh mpls ldp binding 10.0.0.8 255.255.255.25510.0.0.8/32

in label: 19out label: 19 lsr: 10.0.0.3:0out label: 19 lsr: 10.0.0.7:0 inuse

R2#

Summary of labels learned and assigned!

R2#sh mpls ip binding summaryTotal number of prefixes: 12

Generic label bindingsassigned learned

prefixes in labels out labels12 12 20

R2#


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Troubleshooting MPLS Configuration (cont)

R2#debug mpls ldp messages received

LDP received messages, excluding periodic Keep Alives debugging is on

R2#debug mpls ldp messages sent

LDP sent PDUs, excluding periodic Keep Alives debugging is on

R2#clear mpls ldp neighbor 10.0.0.3

00:49:46: %LDP-5-CLEAR_NBRS: Clear LDP neighbors (10.0.0.3) by console

00:49:46: %LDP-5-NBRCHG: LDP Neighbor 10.0.0.3:0 is DOWN

00:49:53: ldp: Rcvd init msg from 10.0.0.3 (pp 0x0)

00:49:53: ldp: Sent init msg to 10.0.0.3:0 (pp 0x0)

00:49:53: ldp: Sent keepalive msg to 10.0.0.3:0 (pp 0x0)

00:49:53: ldp: Rcvd keepalive msg from 10.0.0.3:0 (pp 0x0)

00:49:53: %LDP-5-NBRCHG: LDP Neighbor 10.0.0.3:0 is UP

00:49:53: ldp: Sent address msg to 10.0.0.3:0 (pp 0x63C3B360)

00:49:53: ldp: Sent label mapping msg to 10.0.0.3:0 (pp 0x63C3B360)< cut for brevity>


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