SMUCSE 8344 Lecture 9 Mark E. Allen SMU 8344. SMUCSE 8344 Agenda Summarize MPLS –Discussion from Cisco Presentation Discuss QoS in MPLS –Chapter 6 in

SMU CSE 8344

Lecture 9

Mark E. AllenSMU 8344

SMU CSE 8344

Agenda

• Summarize MPLS– Discussion from Cisco Presentation

• Discuss QoS in MPLS– Chapter 6 in MPLS Book

• Traffic Engineering in MPLS– Chapter 7 MPLS Book

• Virtual Private Networks– Chapter 8 MPLS Book

• Introduction to Optical Networking

CSE 8344SMU

MPLS Architecture Overview

Adapted from Stefano Previdi’s and Jay Kumarasamy presentation

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Agenda• MPLS Concepts

• LSRs and labels

• Label assignment and distribution

• Label Switch Paths

• ATM LSRs

• Loops and TTL

• LDP overview

• Day in the Life of a Packet

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MPLS Concepts

• MPLS: Multi Protocol Label Switching• MPLS is a layer 2+ switching• Developed to integrate IP and ATM• MPLS forwarding is done in the same

way as in ATM switches• Packet forwarding is done based on

Labels

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MPLS ConceptsMPLS Concepts• Unlike IP, classification/label can be based

on:

Destination Unicast address

Traffic Engineering

VPN

QoS

• FEC: Forwarding Equivalence Class

• A FEC can represent a: Destination address prefix, VPN, Traffic Engineering tunnel, Class of Service.

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Agenda

• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview

• Summary

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LSRs and Labels

• LSR: Label Switch Router• Edge-LSR: LSRs that do label

imposition and disposition• ATM-LSR: An ATM switch with

Label Switch Controller

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LSRs and Labels

IGP domain with a label distribution protocol

• 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 mappings between adjacent neighbors

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

• Core LSRs switch packets/cells based on the label value

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

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LSRs and Labels

• Uses new Ethertypes/PPP PIDs/SNAP values/etc

• More than one Label is allowed -> Label Stack• MPLS LSRs always forward packets based on the

value of the label at the top of the stack

Label = 20 bits Exp = Experimental, 3 bits S = Bottom of stack, 1bitTTL = 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

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LSRs and LabelsPPP HeaderPPP Header Layer 3 HeaderLayer 3 HeaderShim HeaderPPP Header(Packet over

SONET/SDH)

Ethernet HdrEthernet Hdr Layer 3 HeaderLayer 3 HeaderShim HeaderEthernet

FR HdrFR Hdr Layer 3 HeaderLayer 3 HeaderShim HeaderFrame Relay

ATM Cell Header HECHEC DATADATACLPCLPPTIPTIVCIVCIGFCGFC VPIVPI

Label

HECHEC DATADATACLPCLPPTIPTIVCIVCIGFCGFC VPIVPI

Label

Subsequent cells

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AgendaAgenda

• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview


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Label Assignment and Distribution

• Labels have link-local significanceEach LSR binds his own label mappings

• Each LSR assign labels to his FECs

• Labels are assigned and exchanged between adjacent neighboring LSR

• Applications may require non-adjacent neighbors

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• Rtr-C is the downstream neighbor of Rtr-B for destination 171.68.10/24

• Rtr-B is the downstream neighbor of Rtr-A for destination 171.68.10/24

• LSRs know their downstream neighbors through the IP routing protocol– Next-hop address is the downstream

neighbor

171.68.10/24

Rtr-BRtr-A Rtr-C

171.68.40/24

Upstream and Downstream LSRs

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•LSRs distribute labels to the upstream neighbors

171.68.10/24

Rtr-BRtr-A Rtr-C

171.68.40/24

Next-HopNext-Hop

In In LabLab

--

......

Address Address PrefixPrefix

171.68.10171.68.10

......

OutOutI/FI/F

11

......

Out Out LabLab

3030......

In In I/FI/F

00

...... Next-HopNext-Hop

In In LabLab

3030

......


171.68.10171.68.10

......

OutOutI/FI/F

11

......

Out Out LabLab

4040......

In In I/FI/F

00

......

Next-HopNext-Hop

In In LabLab

4040

......


171.68.10171.68.10

......

OutOutI/FI/F

11

......

Out Out LabLab

--......

In In I/FI/F

00

......

Use label 40 for destination 171.68.10/24


IGP derived routes

Unsolicited Downstream Distribution

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• Upstream LSRs request labels to downstream neighbors

• Downstream LSRs distribute labels upon request

171.68.10/24

Rtr-BRtr-A Rtr-C171.68.40/24



Request label for destination 171.68.10/24

Request label for destination 171.68.10/24

On-Demand Downstream Distribution

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• Liberal retention mode

• LSR retains labels from all neighborsImprove convergence time, when next-hop is again available after IP convergence

Require more memory and label space

• Conservative retention mode

• LSR retains labels only from next-hops neighborsLSR discards all labels for FECs without next-hop

Free memory and label space

Label Retention Modes

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Label Distribution Modes

• Independent LSP control

LSR binds a Label to a FEC independently, whether or not the LSR has received a Label the next-hop for the FEC

The LSR then advertises the Label to its neighbor

• 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

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Several protocols for label exchange• LDP

Maps unicast IP destinations into labels

• RSVP, CR-LDP

Used in traffic engineering

• BGP

External labels (VPN)

• PIM

For multicast states label mapping

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AgendaAgenda

• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview


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Label Switch Path (LSP)

• LSPs are derived from IGP routing information

• LSPs may diverge from IGP shortest path

LSP tunnels (explicit routing) with TE

• LSPs are unidirectional

Return traffic takes another LSP

LSP follows IGP shortest path LSP diverges from IGP shortest path



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Label Switch Path (LSP) Penultimate Hop Popping

• The label at the top of the stack is removed (popped) by the upstream neighbor of the egress LSR

• The egress LSR requests the “popping” through the label distribution protocol

•Egress LSR advertises implicit-null label

• The egress LSR will not have to do a lookup and remove itself the label

•One lookup is saved in the egress LSR

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Label Switch Path (LSP) Penultimate Hop Popping

0 1

Summary route for 171.68/16

01

171.68.10/24

Next-HopNext-Hop

In In LabLab


OutOutI/FI/F

Out Out LabLab

In In I/FI/F

44 171.68/16171.68/16 22 poppop00

...... ...... ...... ............Next-HopNext-Hop

In In LabLab


OutOutI/FI/F

Out Out LabLab

In In I/FI/F

-- 171.68/16171.68/16 11 4400

...... ...... ...... ............

Egress LSR summarises morespecific routes and advertises a label for the new FEC

Summary route is propagate through the IGP and label is assigned by each LSR

Use label “implicit-null” for FEC 171.68/16


Use label 4 for FEC 171.68/16

171.68.44/24

Address Address Prefix and maskPrefix and mask

171.68.10/24171.68.10/24

Next-HopNext-Hop

171.68.9.1171.68.9.1

InterfaceInterface

Serial1Serial1

171.68.44/24171.68.44/24 171.68.12.1171.68.12.1 Serial2Serial2

171.68/16171.68/16 ...... NullNull

Egress LSR needs to do an IP lookup for finding more specific route

Egress LSR need NOT receive a labelled packet

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AgendaAgenda

• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview

• Summary

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ATM LSRs• ATM switches forward cells, not packets

• Label Dist is Downstream on-demand, Ordered

• IGP label is carried in the VPI/VCI field

• Merging LSR: Ability to use the same label for different FECs if outgoing interface is the same

Save label space on ATM-LSRs

Cell interleave problem

• Non Merging LSR: ATM-LSR requests one label per FEC and per incoming interface (upstream neighbors)

Downstream LSR may request itself new label to its downstream neighbors

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ATM LSRs Non-Merging

Downstream on DemandIn In

LabLab

55

88

......


171.68171.68

171.68171.68

......

OutOutI/FI/F

00

00

......

Out Out LabLab

33

44

......

In In I/FI/F

11

22

......

171.68

IPPacket

IPPacket

ATMcell

5

ATMcell

8

ATMcell

8

ATMcell

8

ATMcell

5

ATMcell

3

ATMcell

4

ATMcell

4

ATMcell

4

ATMcell

3

ATM-LSR requested additional label for same FEC in order to distinguish between incoming interfaces (Downstream on Demand)

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ATM LSRs VC-Merging

Downstream on DemandIn In

LabLab

55

88

......


171.68171.68

171.68171.68

......

OutOutI/FI/F

00

00

......

Out Out LabLab

33

33

......

In In I/FI/F

11

22

......

171.68

IPPacket

IPPacket

ATMcell

5

ATMcell

8

ATMcell

8

ATMcell

8

ATMcell

5

ATMcell

3

ATMcell

3

ATMcell

3

ATMcell

3

ATMcell

3

ATM-LSR transmitted cells in sequence in order for the downstream LSR to re-assembling correctly the cells into packets

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AgendaAgenda

• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview

• Summary

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Loops and TTL

• In IP networks TTL is used to prevent packets to travel indefinitely in the network

• MPLS may use same mechanism as IP, but not on all encapsulations

• TTL 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

• LSRs using ATM do not have TTL capability

• Some suggested options:

- hop-count object in LDP

- Path Vector object in LDP

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Loops and TTL

• TTL is decremented prior to enter the non-TTL capable LSP

If TTL is 0 the packet is discarded at the ingress point

• TTL is examined at the LSP exit


LSR-1

LSR-2

LSR-4 LSR-5

LSR-3

LSR-6

Egress

IP packetTTL = 6

Label = 25

IP packetTTL = 6

IP packetTTL = 10

LSR-6 --> 25Hops=4

IP packetTTL = 6

Label = 39

IP packetTTL = 6

Label = 21

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Agenda• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview


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LDP Concepts

• Label Distribution Protocol

• Labels map to FECs for Unicast Destination Prefix

• LDP works between adjacent/non-adjacent peers

• LDP sessions are established between peers

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LDP Messages

• Discovery messages

• Used to discover and maintain the presence of new peers

• Hello packets (UDP) sent to all-routers multicast address

• Once neighbor is discovered, the LDP session is established over TCP

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LDP Messages

• Session messages

• Establish, maintain and terminate LDP sessions

• Advertisement messages

• Create, modify, delete label mappings

• Notification messages

• Error signalling

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• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview


Agenda

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Day in the life of a Packet

0


01

171.68.10/24

Next-HopNext-Hop

In In LabLab


OutOutI/FI/F

Out Out LabLab

In In I/FI/F

77 171.68/16171.68/16 22 poppop00

...... ...... ...... ............Next-HopNext-Hop

In In LabLab


OutOutI/FI/F

Out Out LabLab

In In I/FI/F

-- 171.68/16171.68/16 11 4400

...... ...... ...... ............

Egress LSR summarises morespecific routes and advertises a label for the new FEC

Summary route is propagate through the IGP and label is assigned by each LSR

Use label “implicit-null” for FEC 171.68/16



Egress LSR needs to do an IP lookup for finding more specific route

171.68.44/24

Address Address Prefix and maskPrefix and mask

171.68.10/24171.68.10/24

Next-HopNext-Hop

171.68.9.1171.68.9.1

InterfaceInterface

Serial1Serial1

171.68.44/24171.68.44/24 171.68.12.1171.68.12.1 Serial2Serial2

171.68/16171.68/16 ...... NullNull


Next-HopNext-Hop

In In LabLab


OutOutI/FI/F

Out Out LabLab

In In I/FI/F

44 171.68/16171.68/16 11 7700

...... ...... ...... ............

CE

PEP P PE

0

0

21

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Day in the life of a Packet Basic Layout

IP Routing Protocols

Label Distribution Protocol

IP Routing Table

Forward Information Block (FIB)

Label Forward Information Block(LFIB)

Control Plane

Forwarding Plane

Incoming IP PacketsOutgoing IP Packets

Outgoing Labelled PacketsIncoming LabelledPackets

Routing Exchange

Label Binding Exchange

Label RemovedL3 lookup

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Day in the life of a Packet Database Layout

OS P F IS IS BGP

fas t-ad jacency

fas ttag-rew ritetag_info

FIB

TFIB

tfib_entrytag_rew rite

Routing Table

LDP

TIB

tfib_entry

tfib_entryloadinfotag_info

output-ifencaps

incom ing-tag

outgoing-tag

tfib_entry

tag_rew rite

Incom ing tag

Dest. IP address

incom ing-tag

tag_rew rite [ ]

tag_hash

IDB v ectors

ip_turbo_fs

tag_optim um _fsip2_tag_optim um _fs

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DISCUSSION OF QoS and Constraint Based Routing

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Key Questions

• How does MPLS Support QoS?• What is the difference between

Integrated Services (INT-SERV) Differentiated Services (DIFF-SERV)?– Integrated services

• T-Spec and R-Spec• Much of this is similar to ATM

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Integrated Services

• An attempt to bring the ATM capabilities to IP– T-Spec: Max burst size, token rate, committed rate,

etc.– R-Spec: Effective bandwidth or amount of resource

required within the network.

• This is very different than “best-effort” and requires sophisticated queuing mechanisms

• Many in the industry saw this as a “reinvention” of ATM

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Integrated Services• architecture for providing QOS guarantees in IP

networks for individual application sessions• resource reservation: routers maintain state

info of allocated resources• admit/deny new call setup requests:

Question: can newly arriving flow be admitted with performance guarantees while not violated QoS guarantees made to already admitted flows?

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Intserv: QoS guarantee scenario

• Resource reservation– call setup, signaling (RSVP)– traffic, QoS declaration– per-element admission control

– QoS-sensitive scheduling (e.g., WFQ)

request/reply

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Call Admission

Arriving session must :• declare its QOS requirement

– R-spec: defines the QOS being requested• characterize traffic it will send into network

– T-spec: defines traffic characteristics• signaling protocol: needed to carry R-spec and T-

spec to routers (where reservation is required)– RSVP

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Intserv QoS: Service models [rfc2211, rfc2212]

Guaranteed service:• worst case traffic arrival:

leaky-bucket-policed source

Controlled load service:• "a quality of service

closely approximating the QoS that same flow would receive from an unloaded network element."

WFQ

token rate, r

bucket size, b

per-flowrate, R

D = b/Rmax

arrivingtraffic

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IETF Differentiated ServicesConcerns with Intserv:• Scalability: signaling, maintaining per-flow router

state difficult with large number of flows • Flexible Service Models: Intserv has only two

classes. Also want “qualitative” service classes– “behaves like a wire”– relative service distinction: Platinum, Gold, Silver

Diffserv approach: • simple functions in network core, relatively

complex functions at edge routers (or hosts)• Don’t define service classes, provide functional

components to build service classes

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Diffserv Architecture

Edge router:- per-flow traffic management

- marks packets as in-profile and out-profile

Core router:

- per class traffic management

- buffering and scheduling

based on marking at edge

- preference given to in-profile packets- Assured Forwarding

scheduling

...

r

b

marking

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Edge-router Packet Marking

• class-based marking: packets of different classes marked differently

• intra-class marking: conforming portion of flow marked differently than non-conforming one

• profile: pre-negotiated rate A, bucket size B• packet marking at edge based on per-flow profile

Possible usage of marking:

User packets

Rate A

B

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Classification and Conditioning

• Packet is marked in the Type of Service (TOS) in IPv4, and Traffic Class in IPv6

• 6 bits used for Differentiated Service Code Point (DSCP) and determine PHB that the packet will receive

• 2 bits are currently unused

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Classification and Conditioning

may be desirable to limit traffic injection rate of some class:

• user declares traffic profile (egs., rate, burst size)

• traffic metered, shaped if non-conforming

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Forwarding (PHB)

• Per Hop Behavior (PHB)• PHB result in a different observable

(measurable) forwarding performance behavior

• PHB does not specify what mechanisms to use to ensure required PHB performance behavior

• Examples: – Class A gets x% of outgoing link bandwidth over time

intervals of a specified length– Class A packets leave first before packets from class

B

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Forwarding (PHB)

PHBs being developed:• Expedited Forwarding: pkt departure

rate of a class equals or exceeds specified rate – logical link with a minimum guaranteed rate

• Assured Forwarding: 4 classes of traffic– each guaranteed minimum amount of

bandwidth– each with three drop preference partitions

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Summary

• REFER TO MPLS 8 LECTUREFOR More Detail on these QoS and CBR (Constraint Based Routing)

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Virtual Private Networks (VPNs)

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When VPN?

• Internet as your own private network– Communicate securely between

various corporate sites (Intranet) – Communicate securely between

partner sites (Extranet)– Connect remote dial-up users

securely to corporate networks

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Advantages

• Flexible and cost effective• Better business-to-business

connectivity– business partners, service providers,

contractors, and customers• Advances in security

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Layer2 vs. Layer3 VPNs

Layer 3 VPNs Layer 2 VPNs

•Provider devices forward customer packets based on Layer 3 information (e.g., IP)

•MPLS/BGP VPNs (RFC 2547), GRE, virtual router approaches

•SP involvement in routing

•Provider devices forward customer packets based on Layer 2 information

•“pseudo-wire” concept

•Tunnels, circuits, LSPs, MAC address

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Layer2 Example

IP or MPLSCore

IP Core

R2R1

L2TPv3 Tunnel

Ethernet EthernetIP L2TP Ethernet

Server BWorkstation A

Step #2 R1 takes Ethernet frame and

encapsulates it in L2TP and routes it to tunnel

destination

Step #3 R2 receives IP/L2TP/Ethernet

Packet and removes the IP/L2TPv3 headers. The remaining Ethernet frame is forwarded to

Server B.

Step #1 Workstation A sends packet destined for

Server B

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Overlay Model

• Each site has a router connected via P-T-P links to routers on other sites– Leased lines– Frame relay– ATM circuit

• Connectivity– Fully connected– Hub-and-spoke

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Limitations of Overlay

• Customers need to manage the back-bones

• Mapping between Layer2 Qos and IP QoS

• Scaling problems– Cannot support large number of

customers– (n-1) peering requirement

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The Peer Model

• Aims to support large-scale VPN service

• Key technologies– Constrained distribution of routing

info.– Multiple forwarding tables– VPN-IP addresses– MPLS switching

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Terminology

• CE router• Customer Edge router

• PE router– Provider Edge router. Part of the P-

Network and interfaces to CE routers • P router

– Provider (core) router, without knowledge of VPN

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Terminology (cont’d)• Route Distinguisher

• Attributes of each route used to uniquely identify prefixes among VPNs (64 bits)

• VPN-IPv4 addresses• Address including the 64 bits Route

Distinguisher and the 32 bits IP address

• VRF– VPN Routing and Forwarding Instance– Routing table and FIB table

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Connection Model

• The VPN backbone is composed by MPLS LSRs• PE routers (edge LSRs)• P routers (core LSRs)

• PE routers are faced to CE routers and distribute VPN information through BGP to other PE routers

• P routers do not run BGP and do not have any VPN knowledge

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Model (cont’d)

• P and PE routers share a common IGP

• PE and CE routers exchange routing information through:• EBGP, OSPF, RIP, Static routing

• CE router run standard routing software

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Routing

• The routes the PE receives from CE routers are installed in the appropriate VRF

• The routes the PE receives through the backbone IGP are installed in the global routing table

• By using separate VRFs, addresses need NOT to be unique among VPNs

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Forwarding

• PE and P routers have BGP next-hop reachability through the backbone IGP

• Labels are distributed through LDP (hop-by-hop) corresponding to BGP Next-Hops

• Label Stack is used for packet forwarding• Top label indicates Next-Hop (interior

label)• Second level label indicates outgoing

interface or VRF (exterior label)

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Forwarding (cont’d)

• The upstream LDP peer of the BGP next-hop (PE router) will pop the first level label

• The egress PE router will forward the packet based on the second level label which gives the outgoing interface (and VPN)

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Forwarding Example

PE2

PE1

CE1

CE2

P1 P2

IGP Label(PE2)VPN LabelIPpacket

PE1 receives IP packet

Lookup is done on site VRF

BGP route with Next-Hop and Label is found

BGP next-hop (PE2) is reachable through IGP route with associated label

IGP Label(PE2)VPN LabelIP

packet

P routers switch the packets based on the IGP label (label on top of the stack)

VPN Label

IPpacket

Penultimate Hop Popping

P2 is the penultimate hop for the BGP next-hop

P2 remove the top label

This has been requested through LDP by PE2

IPpacket

PE2 receives the packets with the label corresponding to the outgoing interface (VRF)

One single lookup

Label is popped and packet sent to IP neighbourIP

packet

CE3

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Scalability

• Existing BGP techniques can be used to scale the route distribution

• Each edge router needs only the information for the VPNs it supports

• Directly connected VPNs

• Easy to add new sites– configure the site on the PE connected to it,

the network automatically does the rest

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QoS Support

• Pipe model– Similar to int-serv

• Hose Model– Similar to diff-serv

Documents

SMUCSE 8344 Lecture 9 Mark E. Allen SMU 8344. SMUCSE 8344 Agenda Summarize MPLS –Discussion from Cisco Presentation Discuss QoS in MPLS –Chapter 6 in