MPLS Architecture Overview Jay Kumarasamy [email protected]

Adopted from Stefano Previdi’s presentation

© 2001, Cisco Systems.

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Agenda •

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

LDP overview

•

Day in the Life of a Packet

2001 Cisco Systems, Inc.

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2

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 2001 Cisco Systems, Inc.

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MPLS 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. 2001 Cisco Systems, Inc.

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Agenda

•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

LDP overview

•

Summary

2001 Cisco Systems, Inc.

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5

LSRs and Labels

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

2001 Cisco Systems, Inc.

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6

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

2001 Cisco Systems, Inc.

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7

LSRs and Labels 0 1 2 3 01234567890123456789012345678901 Label

| Exp|S|

TTL

Label = 20 bits Exp = Experimental, 3 bits S = Bottom of stack, 1bit TTL = Time to live, 8 bits

• 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

2001 Cisco Systems, Inc.

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LSRs and Labels PPP Header(Packet over SONET/SDH) Ethernet Frame Relay

ATM Cell Header GFC

PPP Header

Shim Header

Layer 3 Header

Ethernet Hdr

Shim Header

Layer 3 Header

FR Hdr

Shim Header

Layer 3 Header

VPI

VCI

PTI CLP HEC

DATA

VCI

PTI CLP HEC

DATA

Label Subsequent cells GFC

VPI

Label 2001 Cisco Systems, Inc.

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Agenda

2001 Cisco Systems, Inc.

•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

LDP overview

•

Day in the Life of a Packet

www.cisco.com

10

Label Assignment and Distribution • Labels have link-local significance Each 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

2001 Cisco Systems, Inc.

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11

Label Assignment and Distribution Upstream and Downstream LSRs 171.68.40/24

171.68.10/24 Rtr-A

Rtr-B

Rtr-C

• 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 2001 Cisco Systems, Inc.

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Label Assignment and Distribution Unsolicited Downstream Distribution Use label 30 for destination 171.68.10/24

Use label 40 for destination 171.68.10/24

171.68.40/24

171.68.10/24

Rtr-A

Rtr-B

Rtr-C

In I/F

In Lab

Address Prefix

Out I/F

Out Lab

In I/F

In Lab

0

-

171.68.10

1

30 ...

0

30 171.68.10

... ...

NextNext-Hop... ...

... ...

Address Prefix

Out I/F

Out Lab

1

40 ...

NextNext-Hop... ...

In I/F

In Lab

Address Prefix

0

40 171.68.10

Out I/F

Out Lab

1

...

NextNext-Hop... ...

... ... IGP derived routes

• LSRs distribute labels to the upstream neighbors 2001 Cisco Systems, Inc.

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Label Assignment and Distribution On-Demand Downstream Distribution

Use label 40 for destination 171.68.10/24

Use label 30 for destination 171.68.10/24

171.68.10/24 171.68.40/24 Rtr-A

Rtr-B Request label for destination 171.68.10/24

Rtr-C Request label for destination 171.68.10/24

• Upstream LSRs request labels to downstream neighbors • Downstream LSRs distribute labels upon request 2001 Cisco Systems, Inc.

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Label Assignment and Distribution 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 2001 Cisco Systems, Inc.

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15

Agenda

2001 Cisco Systems, Inc.

•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

LDP overview

•

Day in the Life of a Packet

www.cisco.com

16

Label Switch Path (LSP)

IGP domain with a label distribution protocol

IGP domain with a label distribution protocol

LSP follows IGP shortest path

LSP diverges from IGP shortest path

• 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 2001 Cisco Systems, Inc.

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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 2001 Cisco Systems, Inc.

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Label Switch Path (LSP) Penultimate Hop Popping In I/F 0

In Lab -

...

...

Address Prefix 171.68/16

Out Out I/F Lab 1 4

NextNext ... -Hop...

...

In I/F 0

In Lab 4

...

...

Address Prefix 171.68/16

Out I/F 2

NextNext ... -Hop...

...

Summary route for 171.68/16 0

1

Address NextNext-Hop Prefix and mask

Out Lab pop

1

Interface

171.68.10/24

171.68.9.1

Serial1

171.68.44/24

171.68.12.1

Serial2

171.68/16

...

Null

Summary route for 171.68/16

0 171.68.44/24

Use label 4 for FEC 171.68/16

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

171.68.10/24

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

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

Egress LSR needs to do an IP lookup for finding more specific route Egress LSR need NOT receive a labelled packet 2001 Cisco Systems, Inc.

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19

Agenda

2001 Cisco Systems, Inc.

•

MPLS Concepts

•

LSRs and labels

•

Label assignment and distribution

•

Label Switch Paths

•

LDP overview

•

Day in the Life of a Packet

www.cisco.com

20

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

2001 Cisco Systems, Inc.

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21

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

2001 Cisco Systems, Inc.

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22

LDP Messages • Session messages • Establish, maintain and terminate LDP sessions

• Advertisement messages • Create, modify, delete label mappings

• Notification messages • Error signalling

2001 Cisco Systems, Inc.

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23

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

2001 Cisco Systems, Inc.

www.cisco.com

24

Day in the life of a Packet In I/F 0 ...

In Lab ...

Address Prefix 171.68/16

Out Out I/F Lab 1 4

NextNext ... -Hop...

...

In I/F 0

In Lab 4

...

Address Prefix 171.68/16

Out Out I/F Lab 1 7

NextNext ... -Hop...

...

...

In I/F 0 ...

P1

1

PE Use label 4 for FEC 171.68/16

Address Prefix 171.68/16

Out I/F 2

NextNext ... -Hop...

Out Lab pop ...

Address NextNext-Hop Prefix and mask

Interface

171.68.10/24

171.68.9.1

Serial1

171.68.44/24

171.68.12.1

Serial2

171.68/16

...

Null

2

PE

0

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

Use label 7 for FEC 171.68/16

171.68.44/24

Summary route for 171.68/16

Summary route for 171.68/16

CE

...

P

0 0

In Lab 7

171.68.10/24

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

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

Egress LSR needs to do an IP lookup for finding more specific route 2001 Cisco Systems, Inc.

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Summary • LSRs forward packet based on label information • IP header and forwarding decision have been de-coupled for better flexibility • Label information can derive from different sources IP routing protocols (destination based unicast routing) Multicast Traffic Engineering QoS VPN 2001 Cisco Systems, Inc.

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Summary • MPLS allows flexible packet classification and network resources optimisation • Labels are distributed by different protocols •LDP, RSVP, BGP, PIM

• Different distribution protocols may coexist in the same LSR • Label have local (LSR) significance No need for global (domain) wide label allocation/numbering 2001 Cisco Systems, Inc.

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Thank You!

2001 Cisco Systems, Inc.

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