Mpls Day 1 - Introduction to Mpls

8/21/2019 Mpls Day 1 - Introduction to Mpls

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COPYRIGHT 2013 ALCATEL-LUCENT. ALL RIGHTS RESERVED.

APRICOT 2014 MPLS WORKSHOPINTRODUCTION TO MPLS

Speaker NameDate


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AGENDA

1. Introduction to MPLS

2. MPLS Terminology

3. Fundamentals of MPLS

4. Label Distribution Protocol (LDP)


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INTRODUCTION TO MPLS

MPLS IS A LABEL SWITCHING TECHNOLOGY THAT COMBINES THE TRAFFIC ENGINECAPABIL IT IES OF ATM WITH THE FLEXIB IL ITY AND SCALABIL ITY OF IP

MPLS PROVIDES THE AB IL ITY TO ESTABL ISH CONNECTION-ORIENTED PATHS OVERCONNECTIONLESS IP NETWORK, AND FACIL ITATES A MECHANISM TO ENGINEER NTRAFFIC PATTERNS INDEPENDENTLY OF SHORTEST PATH ROUTING TABLES

MPLS TECHNOLOGY OFFERS MANY SERVICES , INCLUDING LAYER 2 AND LAYER 3 VTRAFFIC ENGINEERING, AND RES IL IENCY


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MULTI PROTOCOL LABEL SWITCHINGINTRODUCTION

RFC 3031 describes the Multiprotocol Label Switching (MPLS) architecture

The term Multiprotocol indicates that an MPLS architecture can transport paylmany different protocols (IPv4, IPv6, Ethernet, ATM, Frame Relay, etc.)

Label Switching describes that an MPLS domain switches, rather than routes, p

Service Provider Core

MPLS routers forward packets using pre-determined labels


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MPLSIMPROVING PACKET FORWARDING PERFORMANCE

Label switching was initially considered an improvement over IP packet routing a

simpler lookup

- 20-bit fixed label size versus 32/128-bit longest match IP destination lookup

Advances in network processors lead to MPLS for Layer 3 packet forwarding perfo

an obsolete use-case

IP SOURCE

IP DESTINATION

OPTIONS

PAYLOAD

IP SOURCE

IP DESTINATION

OPTIONS

PAYLOAD

MPLS LABEL


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MPLSIP ROUTING REVIEW

IP routing follows a simple process at each router:

1. Check and remove the L2 encapsulation header of the incoming packet

2. Examine the L3 (IP) header and perform a longest match lookup on the destination IP a

forwarding table

3. Determine the next-hop interface

4. Build a new L2 encapsulation header and forward the packet to the next-hop router, de

IP TTL/hop count

IP

L2

IP

L2

IP

L2

Longest match lookup ondestination address

Prefix Next-hop Metric

10.1.1.0/24 R5 50

10.1.1.0/24 R6 20

10.1.0.0/16 R7 10

R1 R2 R3 R4 R6

R5

R7


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MPLSLIMITATIONS OF IP FORWARDING PARADIGM

Destination based routing lookup only

- No ability to look at the source address of packets without complicated configuration such as

next-hop- No ability to steer different types of traffic over different next-hops without PBR

IP over underlay technologies (e.g. ATM)

- Was used extensively in the past to abstract the L2 and L3 topologies of networks by using undsuch as ATM or Frame Relay to build any-to-any paths in large networks

- This approach could build a pseudo-full-mesh in a large scale network to optimize traffic flow

- Has been repeated in the MPLS world with IP-over-MPLS overlays

- Early form of IP TE

Traffic Engineering limitations- In many networks it is desirable to send different traffic types via different paths (e.g. lowest

bandwidth, etc)

- This sort of TE is difficult to represent in IP routing without complicated PBR configurations

- MPLS-TE allows for this information to be reflected in the network, and to map traffic to diffeselected by the source node

- In many networks, not all links were utilized to get traffic across networks, leading to congestand under-utilization on others


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MPLSHOW IT HELPS

MPLS is a label based forwarding paradigm

- Separating IP destination from the forwarding lookup

Labels usually correspond to IP destination networks

- Similar to traditional IP routing

- Labels can also correspond to other parameters such as service IDs for pseudowire service

parameters, traffic engineering tunnel, etc

Separating packet forwarding from destination lookup gives operators flexibility t

the problems discussed on the previous slide


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MPLSHOW IT HELPS DESTINATION BASED FORWARDING LOOKUP

MPLS changes the forwarding paradigm by using labels and switching between thecenter of the network

Only edge routers need to be IP aware, core routers are label-aware

Label tables contain ingress label, egress label, next-hop, and action information

swap)

Label based lookup

FEC In-Label Out-label Next-Hop

X 1000 2000 R2

Y 1234 4567 R2

R1 R2 R3 R4

IP

L2

1000

IP

L2

2000

IP

L2

3000


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MPLSHOW IT HELPS TRAFFIC ENGINEERING

MPLS gives operators flexibility for traffic engineering (TE) in their networks, incability to use multiple links

Typically, IP will pick the best path (commonly, lowest metric). In topologies suc

here, ECMP is not possible to implement due to multiple links and metrics

MPLS-TE can force traffic at the ingress node (R1) to pick specific links, or share

multiple paths

R1

R2 R3

R4

R5 R6


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MPLSHOW IT HELPS TRAFFIC ENGINEERING

In a complex topology, sometimes bandwidth is a key driver

- It might be required to use a higher-bandwidth, but less-metric-preferred path, to carry

across the network

Sometimes latency is a key driver

- Low-latency paths might be preferred for voice traffic

Put other traffic on remaining path(s)

R1

R2 R3

R4

R5 R6

1G

1G

10G

10G

10G

1G

1G

1G


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MPLSHOW IT HELPS

Service flexibility and convergence

- Since MPLS matured and the primary goal of IP performance improvements became less r

has been optimised for service flexibility and delivery

- Delivering Layer 2 services such as point-to-point pseudowires or multipoint services like

- Delivering Layer 3 services such as L3VPNs (aka VRF, VPRN, IP-VPN, RFC2547bis, RFC4364

- This allows a single IP core to provide many service types across the network optimizing

for service providers

Network resiliency

-As part of signalling paths through an IP network, MPLS can signal for backup / alternate both the originating router and mid-point routers perform protection of paths should a lin

- Typically this is seen as IP networks competing with traditional optical protection mecha

DWDM platforms

BGP free core operations

- As traffic can be tunneled across the core nodes, BGP can be removed from these nodes

BGP/L3 only to exist on the IP edge


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MPLS TERMINOLOGY

MPLS Terminology

- iLER: Ingress Label Edge Router

- eLER: Egress Label Edge Router

- LSR: Label Switch Router

- LSP: Label Switched Path

- FEC: Forwarding Equivalence Class

Service Provider Terminology

- CE: Customer Edge router

- PE: Provider Edge router

- P: Provider (Core) Router

CE PE P P PE

iLER eLER


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MPLS TERMINOLOGYLABEL SWITCHED PATHS

Labels are pushedonto packets when they enter the service provider network

Labels are swappedacross the network as they transit a core/transit router

Labels are poppedwhen they reach the egress edge of the network

LSPs refer to the end-to-end unidirectional tunnel across the network, regardleslabels it consists of

LSP is a logical entity that reflects the connection between routers

CE PE P P PE

PACKET

PACKET

LABEL 1

LabelPush

PACKET

LABEL 2

LabelSwap

LabelSwap

PACKET

LABEL 3 PACKET

LabelPop

LSP


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MPLS TERMINOLOGYFORWARDING EQUIVALENCE CLASS

The Forwarding Equivalence Class (FEC) refers to packets that are forwarded in t

manner over the same path with the same forwarding treatement

In IP-only networks, FECs usually correspond to an IP prefix in the routing table

- 10.1.1.0/24 with next-hop 192.0.2.1

- Packets destined for 10.1.1.1 and 10.1.1.2 will both be forwarded to 192.0.2.1; meaning

same FEC

- IP-only FEC lookups are performed at each hop

In MPLS networks, FECs can be defined based on destination IP prefixes and othe

criteria

- MPLS based FEC lookups are performed onlyat the ingress LER on incoming packets

- The FEC lookup determines the next-hop LSR and the label to be pushed onto the packet


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MPLS TERMINOLOGYFORWARDING EQUIVALENCE CLASS

FEC lookup is performed at the ingress LER (R1 in this example), and the label is on the FEC lookup

LSRs R2 and R3 perform label swap operations

FEC lookup


X 1000 2000 R2

Y 1234 4567 R2

R1 R2 R3 R4

IP

L2

1000

IP

L2

2000

IP

L2

3000

Label swap


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MPLS TERMINOLOGYPLANES

The Control Plane in a router refers to the proc

control protocols, such as routing information a

-OSPF, IS-IS, LDP, BGP are control plane processes

The Routing Information Base (RIB) receives all

information from routing protocols, and comput

path information for the Forwarding Informatio

MPLS protocols exchange label bindings for thei

build the Label Information Base (LIB), which is

the LFIB

The FIB is a data plane (or forwarding plane) fu

refers to how the line cards will actually forwar

Control Plane

Data Plane

RIB OSPF

BGP IS-IS

FIB

LIB

LFIB


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MPLS ARCHITECTUREMPLS LABELS

MPLS labels use a 32-bit field thats inserted between the L2 and L3 headers, oft

as a shim header

-Known as frame mode

Header format:

- 20 bit Label

- 3 bit Traffic Class field (aka EXPerimental, or Class of Service)

- 1 bit Bottom of Stack field

- 8 bit Time To Live (TTL) field

Label TC S TTL

3 bits 120 bits 8 bits

Ethernet

MAC

MPLSShim

(0x8847 or0x8848)

Packet

Payload


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MPLS ARCHITECTURESPECIAL LABEL VALUES

Some special MPLS label values have been reserved:

- 0: IPv4 Explicit Null

-1: Router Alert

- 2: IPv6 Explicit Null

- 3: Implicit Null

- Used for Penultimate Hop Popping (PHP)

- 7: Entropy Label Indicator (ELI)

- 13: GAL Label

-14: OAM Alert Label

Reserved labels are documented by IANA:

- http://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml
http://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtmlhttp://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml


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MPLS ARCHITECTUREMPLS LABEL STACK

MPLS allows multiple labels to be inserted to a packet, referred to as stacking

These labels are typically used to provide different functions in the network

- A service labelmight identify a customer specific VPN

- A transport labelmight identify the LSP between two routers

- Other labels might be added depending on network complexity and topology, such as Fas

Label stacks impact your MTU, as each time one

label is added the packet size grows by 32 bits

-

Its not unusual to have 5-6 labels in a packet in thecore of a network

- Check your routing platforms for limitations around

pushing/popping labels

- Ensure your MTU is engineered correctly!

The bottom label has the S bit set to 1

- Indicating bottom of stack


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MPLSLABEL ASSIGNMENT AND DISTRIBUTION

Labels are locally significant

- Meaning that they are only relevant to an LSR

LSR assigns labels to prefixes learnt in the routing table

Label bindings are exchanged by adjacent LSRs

-

When LSRs are exchanging label binding information, two approaches can be take

- Unsolicited modewhere the downstream LSR advertises label bindings to all adjacent LS

whether the adjacent LSR demands the label or not (MP-iBGP, LDP)- On-demand mode where the downstream LSR advertises label bindings to adjacent LSRs

the label binding (RSVP-TE)

MPLS


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MPLSCOMPARISON OF UNSOLICITED AND ON-DEMAND

Unsolicited mode: Downstream-on-demand mode:


R1

10.1.2.0/24 - 20 R2

10.1.3.0/24 - 40 R2

R2

10.1.2.0/24 20 - Loopback

10.1.3.0/24 40 30 R3

R3

10.1.3.0/24 30 - Loopback

30|10.1.3.0/2420|10.1.2.0/24

40|10.1.3.0/24

Request labelfor 10.1.3.0/24

30|10.1.3.0/24


R1

10.1.3.0/24 - 40 R2

R2

10.1.2.0/24 20 - Loopback

10.1.3.0/24 40 30 R3

R3

10.1.3.0/24 30 - Loopback

MPLS


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MPLSCONTROL MODES

Ordered Control

- LSRs will advertise FECs upstream only when downstream routers have advertised a label

Independent Control

- LSRs will advertise FECs upstream regardless of whether a downstream router has advert

--|10.1.3.0/24

I know where10.1.3.0/24 is, butI will not advertise

it yet

--|10

I know where10.1.3.0/24 is and Iwill advertise it

now!

MPLS


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MPLSLABEL RETENTION MODES

LSRs maintain received label bindings through two approaches

Conservative Retention mode

- A router that does not need a specific label binding will not hold the binding

Liberal Retention mode

- A router that does not need a specific label binding willhold the binding

30|10.1.3.0/24

I dont need thebinding for10.1.3.0/24 now, I

will delete it

I dont need thebinding for10.1.3.0/24 now, I

but I will keep it

MPLS


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MPLSPENULTIMATE HOP POPPING

Penultimate Hop Popping (PHP) is when the label at the top of the stack is poppe

upstream LSR of the egress LER

The egress LER requests the popping through the label distribution protocol

- Egress LER advertises the implicit-nulllabel

This saves a lookup in the egress LER, optimizing performance

MPLS


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MPLSIMPLICIT AND EXPLICIT NULLS

When a downstream router advertises a FEC with the Implicit Null label, it is req

upstream router perform Penultimate Hop Popping

-This removes the transport tunnel MPLS header and leaves the router with only the paylotunnel header), and can result in a loss of QoS information as the MPLS Transport Class fi

present

- Note: Implicit Null label is a signaling-only label, and not visible in the forwarding plane

A downstream router may also advertise a FEC with the Explicit Null label, which

used for PHP

-The upstream LSR will send the packet to the LER with the Explicit Null label set, which Transport Class field

- The LER will not use the MPLS header for anything other than the QoS information

- Thus, the Explicit Null header is visible in the forwarding plane

MPLS


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MPLSTRANSPORT TUNNEL SIGNALING PROTOCOLS

Label Distribution Protocol (LDP)

Defined in RFC5036

IGP-based tunnels only

Simple configuration

Automatic tunnel creation

No Traffic Engineering support

IGP dependant convergence time

Also called Link or Interface LDP

Downstream Unsolicited mode

Liberal retention

Resource Reservation ProtocoEngineering (RSVP-

Defined in RFC3209

Fully customizable tunnel paths

Ability to run more complex pa

with administrative constraints

Decoupled from IGP

Traffic protection mechanisms

Higher administrative overhead

Downstream on Demand mode

Conservative retention

MPLS


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MPLSSERVICE TUNNEL SIGNALING PROTOCOLS

Targeted LDP (T-LDP)

Defined in RFC4447

Used for L2VPNs

Creates an end-to-end session between two PE

routers

Not to be confused with LDP

Multi-Protocol Border GatewayBGP)

Based on RFC4364 and many ex

Used for L3VPN services, L2VPN

many extensions

Multi-Protocol due to its suppor

address families in addition to

MPLS


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MPLSLABEL DISTRIBUTION PROTOCOL (LDP) INTRODUCTION

RFC 3036, later updated by RFC 5036, defines LDP as a label distribution protoco

Routers configured for LDP establish an LDP session between them and become p

The LDP sessions enable the exchange of label/FEC binding (mapping) informatio

LDP operates in two distinct modes:

- Link (or interface) LDP - Establishing Transport Tunnels

- Targeted LDP - Establishing Service Tunnels between PE routers

LDP is a TLV based messaging protocol

MPLS


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Service Tunnel 1

Service Tunnel 2

MPLSLDP: TRANSPORT AND SERVICE TUNNELS

Link LDP is used to establish transport tunnels

-iLER uses the transport tunnel to reach the eLER

Targeted LDP is used to establish L2VPN service tunnels

- eLER uses the service tunnel for service de-multiplexing

Transport Tunnel

Service 1

Service 2

Service 1

Service 2

MPLS


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MPLSLDP: LINK LDP

Link LDP sessions are established between all directly adjacent LDP routers

Routers exchange label bindings with each other over LDP sessions

This creates a full-mesh of transport tunnels in the network

LDP relies on IGP for operation and convergence

MPLS


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MPLSLDP: LINK LDP OPERATION OVERVIEW

The following four processes create and maintain a Link LDP session:

- Peer Discovery Routers use LDP Hello messages to automatically discover other LDP pe

-Session Establishment and Management LDP sessions are built between LDP peering rouare maintained via keepalive messages

- Label Management After sessions are established, LDP distributes label bindings, and w

necessary

- Notification LDP uses notification messages to alert LDP peering routers about errors

LDP uses both UDP and TCP for transport services

-UDP based messages (port 646)- Discovery messages periodically announce and maintain an LDP router in a network

- TCP based messages (port 646)

- Session messages establish, maintain, and terminate sessions between LDP peers

- Advertisement messages create, change, and delete label mappings for FECs

- Notification messages signal errors and other events

MPLS


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MPLSLDP: PDU STRUCTURE

LDP ID ( 4 bytes LSR ID + 2 bytes label space)

PDU Length ( 26 )Version ( 1 )

Message ID

Message Length ( 16 )Message Type ( 0x4001)U

U Length ( 8 )FEC TLV ( 0x100 )F

Prefix ( 100.0.0.2 )

FEC Element Length ( 32 )FEC Element Address Type ( ipv4 == 1 )FEC Type ( prefix FEC == 2 )

MPLS


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MPLSLDP: PEER DISCOVERY PROCESS (HELLO)

MPLS


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MPLSLDP: HELLO PARAMETERS

LDP-ID (LDP Identifier): 6-byte field that identifies an LSR uniquely along with its

Used in all the LDP messages

-The LSR ID is typically a loopback/system address

- The Label Space ID identify label space within the LSR. For platform-wide label spaces, i

zero

Transport Address: A necessary parameter to establish the subsequent LDP sessio

neighbor

Hello Timeout: Routers continue exchanging LDP Hellos after a successful discove

is declared down if no hello messages are received from that neighbor within the

LSR ID

(32-bit router ID)

Label Space ID

(16 bits)

MPLS


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LDP: LDP SESSION ESTABLISHMENT

MPLS


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LDP: MESSAGE TYPES

TYPE NAME FUNCTION

0x0001 Notification Signals errors and other events

0x0100 Hello Announces the presence of an LSR

0x0200 Initialization Starts the session establishment process

0x0201 KeepAlive Monitors the integrity of the LDP session transport connection

0x0300 Address Advertises the interface addresses to an LDP peer

0x0301 Address Withdraw Withdraws a previously advertised interface address

0x0400 Label Mapping Advertises a FEC-label binding to an LDP peer

0x0401 Label Request Requests a FEC-label binding from an LDP peer

0x0402 Label Withdraw Requests the peer remove from its LIB a previously signaled labe

0x0403 Label Release Signals the peer the LSR no longer needs specific FEC-label mapprequested of and/or advertised by the peer

0x404 Label Abort Request Aborts an outstanding Label Request message

0x3E00 0x3EFF Vendor Private Conveys vendor-private information between LSRs

0x3F00 0x3FF Experimental LDP experimental extensionsundefined use

MPLS


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LDP: LABEL ADVERTISEMENT

Link LDP sessions are established between all adjacent routers

Label bindings are generated for Loopback/System Ips R6 generates a label binding and advertises it for 203.0.113.0/24

- This is advertised (flooded) throughout the network (Downstream Unsolicited mode)

- Routers may receive the advertisement multiple times in a highly meshed network

203.0.113.0/24

Version =

LDP-Id =

Message-TMapping

FEC = 203

Label = 1

MPLS


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LDP: TARGETED LDP

Used to exchange service labels for Layer 2 Services (VLL, VPLS)

Used independently from its Link LDP counterpart

Peers do not have to be directly connected (typically established between 2 PE ro

have services configured)

Also used in LDP over RSVP environments

T-LDP

MPLS


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LDP: TARGETED LDP OPERATION

Operation is very similar to Link LDP

T-LDP sends hellos via Unicast


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Mpls Day 1 - Introduction to Mpls