MPLS Architecture Overview

1© 2001, Cisco Systems.

MPLS Architecture Overview

MPLS Architecture Overview

Jay [email protected] [email protected]

Adopted from Stefano Previdi’s presentation

22001 Cisco Systems, Inc.

www.cisco.com

AgendaAgenda

• 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 ConceptsMPLS 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 LabelsLSRs 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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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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• 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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PPP 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



www.cisco.com

Label Switch Path (LSP)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 PoppingLabel 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 PoppingLabel 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


www.cisco.com

ATM LSRsATM 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 Demand

ATM LSRs Non-Merging

Downstream on Demand

In 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 Demand

ATM LSRs VC-Merging Downstream

on Demand

In 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


www.cisco.com

AgendaAgenda

• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview

• Summary


www.cisco.com

Loops and TTLLoops 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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• 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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• 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


www.cisco.com

AgendaAgenda

• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview



www.cisco.com

LDP ConceptsLDP 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 MessagesLDP 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 MessagesLDP Messages

• Session messages

• Establish, maintain and terminate LDP sessions

• Advertisement messages

• Create, modify, delete label mappings

• Notification messages

• Error signalling


www.cisco.com

• MPLS Concepts

• LSRs and labels



• ATM LSRs

• Loops and TTL

• LDP overview


AgendaAgenda


www.cisco.com

Day in the life of a PacketDay 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

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

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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Day in the life of a PacketDay in the life of a Packet

IP

M PLS

In-boundinterface driver

checksencaps,invokes

handler thruIDB vectors

checkearly

features

- bgp-based policy m apping (QPPB) and set qos-group ID- authentication proxy- check for input ACLs- check for crypto- check for setting input QoS m arking- Input police/rate-input- check for NAT (outside -> inside- check policy routing- check for WEB cache redirection

FIBlookup

adja--cencylookup

- check for NAT (inside -> outside)- perform QoS classification- check for crypto- output ACL check- check for setting output QoS m arking- QoS - WFQ- output police/rate-lim iting

IP

M PLS

postlookup

features

labelim position

send IPpacket

IPfragm ent

?

- com pute length of tag(s)to be copied & check iffrag. is required.- copy tos field- set ttl- copy the label(s)- if (features) { - perform output QoS classification - check for setting output QoS m arking - do output rate-lim iting - check for m ulti-vc }- fragm ent, if necessary- send labelled packet

IP

M PLS

processlabelledpacket


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Day in the life of a PacketDay in the life of a PacketIn-bound interface

driv er checksencaps, inv okeshandler thru IDB

v ectors

checkearly

features

IP

M PLS

processlabelledpacket

checkearly

features

- Input QoS Classification- Setting input QoS m arking- Do input rate-lim it/policing

tagsw itchinline

- get label header- from tfib get the tag entry- get rew rite info from the tag entry

tagforw ard

inline

- update TT L value- update EXP value- SW AP/POP label, process inner label..- handle m ulti-vc CoS

checkpost

features

norew riteinfo, doLayer-3lookup

- Output QoS Classification- Setting output QoS m arking- Do output rate-lim it/policing

transm itpacket

- Platform specific W RED done in the final transm it path


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SummarySummary

• 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


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SummarySummary

• MPLS allows flexible packet classification and network resources optimisation

• Labels are distributed by different protocols

•LDP, RSVP, BGP, PIM

• Different distribution protocols may co-exist in the same LSR

• Label have local (LSR) significanceNo need for global (domain) wide label allocation/numbering


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Thank You!Thank You!

MPLS Architecture Overview

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