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2011 Juniper Networks, Inc. All rights reserved. | www.juniper.net

Day 5

MPLS Foundamentals

Johnson Liu

[email protected] Nov. 10, 2011

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MPLS Fundamentals

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2011 Juniper Networks, Inc. All rights reserved. www.juniper.net | 3

IGP Metric-Based Forwarding

IGP ForwardingTraffic is routed based on the IGPs best path selection

Traffic that is destined for networks attached to R6 and R7

uses the same path

R1 R2

R3

R4

R5

R6

1

1

1

21

1

R7

1

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Downsides of IGP Metric-Based Forwarding

Redirecting traffic from R1, destined for R7, totraverse R4 causes traffic destined to R6 to use R4

also

This redirecting of traffic causes some of your links to be

underutilized, while others are overutilized

R1 R2

R3

R4

R5

R6

1

1

3

21

1

R7

1

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Additional Drawbacks of IGP Metrics

Adjusting the IGP metric might destabilize the networkMoves the problem to another section of the network

Some of the links will be underutilized

Some of the links will be congested and overutilized

Lacks control All traffic flows over the IGP shortest path

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Internet Growth Required Changes

ATM switched networks also known as an OverlayNetwork

Benefits of using ATM

ATM switches offered performance and predictable behavior

Virtual circuits (VCs) could be reengineered without physicalnetwork changes.

Traffic statistics on a per-VC basis

Downsides of ATM

Maintain separate infrastructure

ATM cell overhead (1 cell = 5 bytes

Header + 48 bytes payload)

Scalability issues

Not well integrated

ATM

IP

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Additional Options for Connecting Sites

Frame Relay networksBenefits of using Frame Relay

Uses virtual circuits (VCs) to move traffic to its destination

Uses Data Link Connection Identifier (DLCI) number to separate

VCs

Built in Congestion Control (DE, FECN, BECN)

Downsides of Frame Relay

Maintain separate infrastructure

FR

IP

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Traditional IP Routing

Destination-based routing lookup is needed on every hop

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MPLS Example

MPLS core routers swap labels and forward packets based on simple

label lookups

MPLS edge routers also perform a routing table lookup, and add or

remove labels

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MPLS Benefits

MPLS support multiple applications

Fixed label lookup faster than IP routing

MPLS support forwarding non-IP protocols

MPLS VPN solve overlapping intranet IP problem

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Benefits of MPLS (1 of 2)

Some benefits of MPLS include:Improved route lookup time by using labels to forward traffic

Increased scalability

Additional control over how traffic moves through the

network usingtraffic engineering (TE)

R1R2

R3

R4

R5

R6

1

1

3

21

1

R7

1

LSP A

LSP B

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Benefits of MPLS (2 of 2)

Service Providers can offer different technologies likeATM, Frame Relay, Ethernet, and IPsec over the same

infrastructure

R1

R2

R3

R4

R5

R6

ATM-LSP

Site 1

Site 2

CoreInfrastructure

ATM

ATM

Site 3Site 4

Frame Relay LSP

Site 5

Site 6

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Labeled Packets

MPLS is often referred to as a "Layer 2.5" protocol.

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MPLS Header information

The MPLS packet headerMPLS header is prepended to packet with a pushoperation

at ingress node

Label is added immediately after Layer 2 encapsulation

header

Packet is restored at the end of the LSP with a pop

operation

Normally the label stack is popped at the penultimate()router

32-Bit

MPLS shim Header

DataL2 Header MPLS Header

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MPLS Shim Header Structure

MPLS shim header consist of four fields

Labelused to associate packet with an LSP (Label SwitchedPath)

Experimental(EXP) bitscarry packet queuing priority (Class of

Service, CoS)

Bottom ofStacking bit

Time to live (TTL)limits packet lifetime within LSP In most cases, the IP TTL is copied into the MPLS TTL

TTLLabel (20 bits) CoS S

Data

32 bits

L2 Header MPLS Header

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MPLS Label Stack

The PID(0x8847=MPLS unicast packets) in a layer 2 headerspecifies that the payload starts with a label followed by an IPheader

The bottom-of-stack bit(S bit) indicates whether the label isthe last label in the stack

The receiving router uses the top label only

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MPLS Labels

Key things to remember about labels:Labels can be assigned manually or by a signaling protocol

in each LSR during path setup

Label values will change at each segment in the path

The LSR(Label Switch Router) will swap incoming label withnew unique outgoing label

MPLS Labels only have local significance

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Reserved Labels

Label values 0 through 15 are currently reserved0 = IPv4Explicit NULL

1 = Router Alert Label

2 = IPv6 Explicit NULL

3 = Implicit NULL (default)

4 through 15 = for future use

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user@R3> show route table mpls.0

mpls.0: 4 destinations, 4 routes (4 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

0 *[MPLS/0] 01:13:17, metric 1

Receive

1 *[MPLS/0] 01:13:17, metric 1

Receive

2 *[MPLS/0] 01:13:17, metric 1

Receive

1000050 *[MPLS/6] 01:13:16, metric 1

> to 172.20.100.14 via ge-1/0/6.0, Swap 1000515

Label Information Base (LIB)

Label Information BaseThe LIB is stored in the mpls.0 table

The mpls.0 table is automatically created, with label values for 0,

1, and 2, when you configure the MPLS protocol

This table is used by transit routers to make forwarding decisions

The mpls.0 table maps the incoming labels with the outgoing

label and next hop to forward the packets

Incoming Label

Outgoing Label

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Label-Switching Routers

Label-switching router (LSR) performs:- MPLS packet forwarding

- LSP setup

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Label-Switching Routers (LSR)

The LSR performs:MPLS packet forwardingLSP setup

All M Series Routers, T Series Routers, and MX Series

Ethernet Services Routers support LSR capabilities

Simply called routers in this material

R1 R2

R3

R4

R5

R6

R7

LSRLSP A

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Label-Switched Path (LSP)

LSPUnidirectional path through network

Generally within a single MPLS domain

LSP

R1 R2

R3

R4

R5

R6

R7

LSP A

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1000050 COS TTLS

MPLS Router Functions: Ingress

Ingress router

Packets enter LSP at ingress

Also called a head-end router

Upstream from other routers

Performs label push operation

Ingress

R1 R2

R3

R4

R5

R6

R7

Layer 2

LSP A

Data FCS

label push

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1000515 COS TTLS

MPLS Router Functions: Transit

Transit routerThere can be zero or more transit routers

Perform label swap operations

Forward traffic to next hop in LSP

Transit

R1

R3

R4

R5

R6

R7

LSP A

label swap

Layer 2 Data FCS

R2

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MPLS Router Functions: Penultimate

Penultimate router

Second-to-last router

Normally pops the label stack

Unlabeled packets sent to egress

Penultimate

R1 R2

R3

R4

R5

R6

R7

LSP A

Layer 2 Data FCS

label pop

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MPLS Router Functions: Egress

Egress router

Packets exit LSP at egress

Also called tail-end router

Downstream from other routers

Forwards packets based on IP address Egress

R1 R2

R3

R4

R5

R6

R7

LSP A

Layer 2 FCSData

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Label Stacking

PE 2

PE 1

X

P P

Inner Tunnel(LDP-signaled LSP)

Outer Tunnel(RSVP-signaled LSP)

PPY

1) Packet enters

LDP tunnel with

LDP label push

4) Packet

restored with LDP

label pop

2) Packet enters

RSVP engineered

core with RSVP label

push

3) Packet leaves

outer tunnel with

RSVP label pop

5) Packet leavesMPLS domain

Label stacking improves scalability- Similar to ATMs VP and VC hierarchy
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MPLS SIGNALING(LDP vs RSVP)

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Label Distribution Protocol(LDP)

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LDP Signaling Overview

LDP messages types

Discovery: Locate potential LDP peers Session: Manage peer-to-peer TCP sessions

Advertisement: Create, change, or delete label mappings

Notification: Provide advisory information

Upstream

LDP peer

Downstream

LDP peer

TCP Session Establishment

Initialization Messages

Label Request Messages

Hello messages

Label Mapping Messages

Session

Advertisement

Discovery

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Label Assignment

LSR FEC: 10.0.0.1/32

Label: 52

FEC: 10.0.0.1/32

Label: 29

MPLS Table

In Out

(so-0/0/1, 17)

Receive

Outgoing

Label

FEC: 10.0.0.1/32

Label: 17

Advertise

Incoming

Label

(fe-0/0/2, 35)

LDP label mapping:

Downstream peer assigns labels

Benefits:

Traffic engineering information is not piggybacked on routing protocols

Limitations: LSPs follow the conventional IGP path

Does not support explicit routing

fe-0/0/2 so-0/0/1 so-0/0/1 so-0/0/3 so-0/0/3 at-0/2/0

MPLS Table

In Out

(so-0/0/3, 52)(so-0/0/1, 17)

MPLS Table

In Out

(at-0/2/0, 29)(so-0/0/3, 52)

FEC: 10.0.0.1/32

Label: 35

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Hello-Based Neighbor Discovery

Neighbor discovery is asymmetric process

Respond only if LDP session is desired

Active node has the higher IP address

Transport address takes precedence over source address

Basic Discovery

224.0.0.2, UDP port 646Router A Router B

Extended Discovery

Specific Address, UDP port 646

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LDP Session Establishment

Active Node initiates TCP session

LDP Session initiated after TCP session established

Router A

(Passive)

Router B

(Active)TCP 3-way Handshake

10.0.1.210.0.1.1

Session Initialization

(Version, Label modes, Timer Values)Session Initialization

(Version, Label modes, Timer Values)

Keepalives

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LDP Session Maintenance

LDP session requires at least 1 hello adjacency

Hello interval: 5-second default

Hold timer: 15-second default

If hold timer expires, LSR deletes hello adjacency

Can be asymmetric

Transport address selection:

Interface address

Router ID

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[edit]

lab@r7# show protocols mpls

label-switched-path test {

to 10.0.6.1;

ldp-tunneling;

no-cspf;

}

interface all;

LDP Tunneling Through RSVP-TE LSP

Router A Router B

RSVP

LDP LDP

R7 R8

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MPLS Terminology

Label Swapping Connection table maintains mappings

Exact match lookup

Input (port, label) determines:

Label operation

Output (port, label)

Port 1

Port 3

Port 2

Port 4

Connection TableIn

(port, label)Out

(port, label)(1, 22)(1, 24)(1, 25)(2, 23)

(2, 17)(3, 17)(4, 19)(3, 12)

LabelOperation

SwapSwapSwapSwap

25P

19P

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MPLS Forwarding Example

134.5.1.5

200.3.2.7

1 2

200.3.2.1

134.5.6.1

Ingress Routing TableDestination Next Hop134.5/16

200.3.2/24(2, 84)(3, 99)

MPLS TableIn Out

(1, 99) (2, 56)

MPLS TableIn Out

(3, 56) (5, 0)

DestinationEgress Routing Table

Next Hop134.5/16

200.3.2/24134.5.6.1200.3.2.1

200.3.2.7

MPLS TableIn Out(2, 84) (6, 0)

200.3.2.73 5

23

2 6

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LDP over RSVP LSP LDP neighbor relationships between:

A and B via the physical interface

B and E via bi-directional RSVP LSPs

E and F via the physical interface RTR-B performs a swap and push operation

Swap label 583 for label 101 (advertised by E)

Push label 106 (advertised by C)

A B C D E FLDP LDPSVP

PE1 P1 P2 P3 P4 PE2

LDP over RSVP(T-LDP)

IP583

IP101106

IP101150

IP101

IP

*33500683

101

*Note: Label 3 = implicit null

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Resource Reservation Protocol(RSVP)

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Basic RSVP Path/Resv signaling

UnidirectionalflowsIngress router initiates connectionSoft state

Path and resources are maintained dynamicallyCan change during the life of the RSVP sessionPath message (containing label-request object) sent downstreamResv message (containing label object) sent upstream

PATH PATH PATH PATH PATH

Label requestobject

Label requestobject

Label requestobject

Label requestobject

Label requestobject

R2 R1R3R4 R5 R6RESV RESV RESV RESV RESV

1004

Labelobject

5341

Labelobject

4711

Labelobject

2477

Labelobject

0

Labelobject

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RSVP Signaling Example: Path

Miami

Seattle

San

Francisco

(Ingress)

New York(Egress)

RSVP sets up path from San Francisco to New York

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RSVP Signaling Example: Reservation

San

Francisco

(Ingress)

New York

(Egress)

Miami

Seattle

3

LSP Established!

The RESV message visits each router on the path in reverseorder labels assigned hop to hop in the upstream direction

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Traffic Engineering(TE)

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What is traffic engineering?

A major goal of Internet Traffic Engineering is to facilitateefficient and reliable network operations while

simultaneously optimizing network resource utilization

and performance

RFC 2702, Requirements for Traffic Engineering over MPLS

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IGP Metric-Based Forwarding

1 1

1 2

A B

C

Traffic sent to A or B follows path with lowest metrics

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Drawbacks of IGP Metric Forwarding

1 4

1 2

A B

C

Redirecting traffic flow to A via C causes traffic for B to move

also!

- Some links become underutilized or overutilized

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Additional Drawbacks of IGP Metrics

Adjusting an IGP metric might destabilize the network- Moves problem around

Some links underutilized

Some links overutilized

- Lacks control

All traffic flows via the IGP shortest path

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Constraint-Based Path Computation

Constraint-basedrouting is demand-driven. Resource-reservation-aware routing paradigm:

- Based on criteria including, but not limited to, network topology- Calculated at the edge of a network:

Modified Dijkstras algorithm at tunnel headend (CSPF [Constraint-based SPF]).

Output is a sequence of IP interface addresses (next-hop routers)between tunnel endpoints.

Constraint-based routing takes into account:- Using the least-cost path computed by OSPF or IS-IS- Policy constraints associated with the tunnel and physical links- Physical resource availability- Network topology state

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IGP-Based Traffic Engineering

San

Francisco

New York

192.168.1/24

134.112/16

Standard IGP routing IP prefixes are bound to a physical next hop

- Typically based on IGP view of shortest path

30M

50M

30M

30M

50M30M

30M

30M

30M

IGP Shortest Path

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MPLS-Based Traffic Engineering

San

Francisco

IGP Shortest Path

RSVP Traffic Engineered Path

(Need 20M)

New York

Engineer unidirectional paths through your networkwithout using the IGPs shortest path calculation

30M(5M available)

50M

(10M available)

30M

(20M available)30M

(20M available)

30M

(25M available)

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Fast Reroute schemes supported in JUNOS

1. One-to-one backupSeparate backup LSP (called Detour LSP) for each LSP thatrequires protection

2. Facility backupBypass Tunnel created to protect a given facility (a link or anode). Multiple LSPs can share the same bypass tunnel.Two variants:

Link protection Node protection

The schemes are described in RFC 4090 Fast Reroute Extensions to RSVP-TE for LSPTunnels http://www.ietf.org/rfc/rfc4090.txt
http://www.ietf.org/rfc/rfc4090.txthttp://www.ietf.org/rfc/rfc4090.txt

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Comparison of FRR schemes

R11

R13

R1 R2

LSP X

R3 R4 R5 R6

R7

R8 R9

LSP Y

R10

LSP Z

R14

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Facility protection, Link protectionvariant

R1 R2

LSP X

R3 R4 R5 R6

R7

R8 R9

LSP Y

Shared bypass

tunnel

R10

LSP Z

R11

R13

R14

R1 computes and signals a single bypass tunnel to the next-hop node, R2, that avoids the

R1-R2 link. The bypass tunnel can be shared between all LSPs using R1->R2 link, if

desired. Merge point is R2.

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Facility protection, Node protectionvariant

R1 R2

LSP X

R3 R4 R5 R6

R7

R8 R9

LSP Y

Shared bypass

tunnel

R10

LSP Z

R11

R13

R14

R1 computes and signals bypass tunnel(s) to the next-next-hop node(s) which avoids R1-R2

link and R2 itself. Same bypasstunnel can be shared between all LSPs that pass from R1

to R2 and have the same next-next-hop. In the example, the 3 LSPs have the same next-

next-hop so only one bypass was needed to protect R2.

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FRR 1:1 protection

R1 R2

LSP X

R3 R4 R5 R6

R7

R8 R9

LSP Y

Detour LSP for

LSP Y

Detour LSP for

LSP X

R10

LSP Z

R11

R13

R14

Detour LSP for

LSP Z

Separate detour LSP for each protected LSP.

1:1 protection always protects downstream link and downstream node.

Case study 1 Facility protection:

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BR

BR

CR

CR

CR

CR

CR CR

BR

BRMPLS Core

Link Protection

When one backbone link disconnect, it will compute and signal a single bypass tunnel tothe next-hop node. The bypass tunnel can be shared between all LSPs. One of the CR will

need to carry all traffic of all LSP when failure occurs.

BR: Border Router

CR: Core Router

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BR

BR

CR

CR

CR

CR

CR CR

BR

BRMPLS Core

Case study 2 1:1 FRR

Separate detour LSP for each protected LSP. 1:1 protection can help network admin to load sharebetween different path and node

resource when detouring LSP generated.

BR: Border Router

CR: Core Router

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20111110_CHT_TL_教育訓練 Day5

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