1 © 2001, Cisco Systems, Inc. MPLS Architecture & Operation Peter Tomsu Senior Consultant EMEA Peter Tomsu Senior Consultant EMEA

1© 2001, Cisco Systems, Inc.

MPLSArchitecture & Operation

Peter TomsuSenior Consultant EMEA

[email protected]

2© 2001, Cisco Systems, Inc. 2© 2000, Cisco Systems, Inc.

• Scalability of IP Based Forwarding• MPLS Architecture - A New Paradigm• MPLS Building Blocks• MPLS Packet Forwarding & Label Switched Paths• Frame-mode MPLS - Label Assignment & Distribution• Cell-mode MPLS - Label Assignment & Distribution• Configuring MPLS

Agenda


Scalability of IP Based Forwarding


4MPLS Arch_Oper_0701 © 2001, Cisco Systems, Inc. www.cisco.com

London London POP routerPOP router

IGP

IGP

IGP

IGP

IGP

Paris Paris Peering PointPeering Point

•Independent hop by hop forwarding

•decision based on layer-3 header of the incoming packet and

•routing algorithm output

Packet Forwarding - IP Networks

•Forwarding decision based solely on IP destination addressHop-by-hop connectionless destination-only paradigmNo end-to-end pathsNo QoS or Optimization


• Layer-3 overlay scheme across layer-2Usually point-to-point links (VCs)

connection orientedFull mesh for optimal routing

Packet Forwarding – Layer 3 Overlay

London London Core routerCore router

Paris Paris Core routerCore router

Munich Munich Core routerCore router

ATM BackboneATM Backbone

ATM Switch ATM Switch LondonLondon

ATM Switch ATM Switch ParisParis

ATM Switch ATM Switch MunichMunich

ATM PVCATM PVC


MPLS Architecture A New

Paradigm


MPLS – Multi Protocol Label Switching

“

”draft-ietf-mpls-framework

“The primary goal of the MPLS working group is to standardise a base technology that integrates the label swapping forwarding paradigm with network layer routing.

Label Swapping is expected to improve •price/performance of network layer routing

•scalability of the network layer

•provide greater flexibility in the delivery of (new) routing services

•new routing services can be added without changing the forwarding paradigm


MPLS Peer Model

OSPF, BGP

PNNIOSPF, BGP

Overlay Modeleg Classical IP, MPOA, NHRPRouters and Switches totally isolatedRouters have no idea of ATM TopoIP features must be approximately mapped into ATM

Peer Modeleg MPLSRouters and Switches totally integratedRouters & Switches share topologyIP features directly supported by ATM



IGP

IGP

IGP

IGP

IGP


Packet forwarding based on labels - no reclassification of packets

Label Swapping

Multi-protocol Label Switching

MPLS Concepts

Packets are classified on entry into the MPLS

domain

Classification can be based on IGP but could be

QOS, TE, etc..


MPLS Building Blocks


MPLS - IP Switching Service 1a. The Routed protocol (OSPF, IGRP,...) computes 1a. The Routed protocol (OSPF, IGRP,...) computes

the shortest path to destination within the corethe shortest path to destination within the core

3.LSR: Label Switch Router• switches packet based on

label - Label Swapping

4.The last MPLS router removes label

2. ELSR (Edge LSR):• Inbound router receives

packets• runs usual L3 services• adds labels to packets

1b. The LDP (Label Distribution Protocol) binds a label to every destination address


Forwarding ComponentForwarding Component

also referred to as the data planedata planeresponsible for forwarding packets/cells based on labels

uses a label forwarding database maintained by the label switch

MPLS Forwarding Component

Simple Label SwappingSimple Label Swapping


MPLS Control Component

Control ComponentControl Component

also referred to as the control planecontrol planeresponsible for creating and maintaining label forwarding

information (known as label bindingslabel bindings)forwarding information taken from the FIB label mappings distributed via

LabelLabel Distribution ProtocolDistribution Protocol

The IntelligenceThe Intelligence


Cisco Express Forwarding

• Each LSR must have CEFCEF enabled globallyFIBFIB = Forwarding Information Baseonly IOS layer-3 switching method that uses the FIB tablewhen FIB entry created, label information is taken from the LIB

• Any interface that will perform label imposition must also have CEF enabled

as label imposition references the FIB


MPLS Specific Tables

• Each LSR will use a LIBLIB Label Information BaseLabel Information Base

Contains all label/prefix mappings from all TDP/LDP neighbours

• Each LSR will also use a LFIBLFIBLabel Forwarding Information Base

Contains only label/prefix mappings that are currently in use for label forwarding


Control-Plane to Data-Plane

Data plane in a nodeData plane in a node

IP Routing ProtocolIP Routing Protocol

IP Routing TableIP Routing TableC

o ntr o

l pl a

n e i n

a n

ode

Co n

tr ol p

l an e

i n a

nod

e

RouterRouter

IGPIGPRouting information

exchange with other routers

IP IP FIBFIBIncoming IP packets

Outgoing IP packets


Control-Plane to Data-Plane MPLS / E-LSR



IP Routing TableIP Routing Table

MPLS LIBMPLS LIB

Co n

tr ol p

l an e

i n a

nod

eC

o ntr o

l pl a

n e i n

a n

ode

MPLS LFIBMPLS LFIB

E-LSRE-LSREdgeEdgeLabelLabel

SwitchSwitchRouterRouter



(Link-state recommended)

Label Distribution Label Distribution ProtocolProtocol

Label binding exchange with other

routers

Incoming labelled packets

Outgoing IP packets

Outgoing labelled packets

Incoming IP packets IP IP FIBFIB


Control-Plane to Data-Plane MPLS / LSR



IP Routing TableIP Routing TableC

o ntr o

l pl a

n e i n

a n

ode

Co n

tr ol p

l an e

i n a

nod

eLSRLSRLabelLabel

SwitchSwitchRouterRouter

Incoming labelled packets

Outgoing labelled packets

MPLS LFIBMPLS LFIB

MPLS LIBMPLS LIB



(Link-state recommended)

Label Distribution Label Distribution ProtocolProtocol

Label binding exchange with other

routers


MPLS Packet Forwarding &

Label Switched Paths


MPLS Example:Routing Information

128.89

171.69

1

01

0

You can reach 171.69 through me

You can reach 128.89 and 171.69 through me

Routing Updates (OSPF, EIGRP, …)

You can reach 128.89 through me

Address Address PrefixPrefix

128.89128.89

171.69171.69

......

OutOutI’faceI’face

00

11

......


128.89128.89

171.69171.69

......


11

11

......


128.89128.89

......


00

......


MPLS Example:Assigning Labels

1

01

0

Use label 7 for 171.69

Use label 4 for 128.89 andUse label 5 for 171.69

Label Distribution

Use label 9 for 128.89

128.89

171.69

In In LblLbl


128.89128.89

171.69171.69

......


00

11

......

Out Out LblLbl

In In LblLbl


128.89128.89

171.69171.69

......


11

11

......

Out Out LblLbl

In In LblLbl


128.89128.89

......


00

......

Out Out LblLbl

--

--

44

55

44

55

99

77

99 --


MPLS Example:Forwarding Packets

128.89

171.69

1

01

128.89.25.4 Data4128.89.25.4 Data

128.89.25.4 Data

128.89.25.4 Data9

0

Label Switch Forwards Based on Label

In In LblLbl


128.89128.89

171.69171.69

......


00

11

......

Out Out LblLbl

In In LblLbl


171.69171.69

......


11

11

......

Out Out LblLbl

In In LblLbl


128.89128.89

......


00

......

--

--

44

55 55 77

--4 9128.89

Out Out LblLbl

99


Label Encapsulation

• Label header is equal to 4 octets– Label value is 20 bits– Experimental is 3 bits– S (bottom of stack) is 1 bit– TTL (Time to live) is 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 | CoS|S| TTL


Label Encapsulation

ATM label switching

Packet-over-SONET/SDH

Ethernet: similar

Frame Relay PVCs: similar

Label over ATM PVCs

PPP Header LabelLabel Layer 3 Header Data

Ethernet Hdr LabelLabel Layer 3 Header Data

Frame Rly Hdr LabelLabel Layer 3 Header Data

ATM Header LabelLabel Layer 3 Header Data

ATM Header Data(subsequent cells)

GFC VPI VCI

LabelLabel

PTI CLP HEC Layer 3 Header Data

GFC VPI VCI PTI CLP HEC Data(subsequent cells)

LabelLabel

LabelLabel


The Label StackMPLS is recursive

171.68.10/24

Rtr-A

Next-HopNext-Hop

In In LabLab55

......


171.68.10171.68.10

......

OutOutI/FI/F11

......

Out Out LabLab77

......

In In I/FI/F00

......

IP packetD=171.68.10.12

Label = 5

Label = 21

IP packetD=171.68.10.12

Label = 7

Label = 21

• Rtr-A forwards the labeled packet based on the label at the top of the label stack


Forwarding Equivalent Class

• IP Packets are classified into FECsFECs

Forwarding Equivalent ClassForwarding Equivalent Classgroup of packets forwarded in the same manner, over the same path, with the same forwarding treatment

determined (by default) through the output of the IGP (or static routing)

each FEC corresponds to an IP destination prefixdestination-based unicast routing (default)could be QOS, all BGP prefixes reachable via a particular exit point etc...


Label Switch Path (LSP)

• FEC is determined in LSR-ingress• LSR-ingress to LSR-egress path is the same for packets of the same

FEC• LSPs are derived from IGP routing information

• LSPs may diverge from IGP shortest pathLSP tunnels (explicit routing) with Traffic

Engineering

LSP follows IGP shortest path

IGP domain with a label distribution protocol

LSP diverges from IGP shortest path

IGP domain with a label distribution protocol


Frame-mode MPLSLabel Assignment

& Distribution



• Each LSR assigns a local label for each FEC

label to IP prefix mapping held within the LIB (Label Information Base)

label has local significance

Frame-mode Label Assignment

London# show tag-switching tdp bindings 197.26.15.0 tib entry: 197.26.15.0/24, rev 7

local binding: tag: 30remote binding: tsr: 172.16.1.1:0, tag: 33remote binding: tsr: 172.16.1.2:0, tag: 35remote binding: tsr: 172.16.1.3:0, tag: 23remote binding: tsr: 172.16.2.1:0, tag: 59remote binding: tsr: 172.16.3.1:0, tag: 28


• Frame-mode LSRs use independent controlindependent control label assignment

as soon as a FEC appears within the IP routing table, a label is assigned and held within the LIB

LSR may label forward packets to a next-hop that does not yet have label information for the FEC

labels are not assigned to BGP routeslabels are not assigned to BGP routes

Frame-mode Label Assignment


Frame-mode Label Distribution

• Labels are exchanged between adjacent LSRs and a binding is agreed

binding will be downstream to upstream in respect of traffic flow

• Applications may require non-adjacent neighbours

Traffic EngineeringVirtual Private NetworksLDP/TDP Directed Peers


London London Brussels Brussels Paris Paris 197.26.15.0/24

Downstream/Upstream Neighbours

146.15.27.0/24

• Paris router is downstream neighbour of Brussels for FEC 197.26.15.0/24

• London is downstream neighbour of Brussels for FEC 146.15.27.0/24


Label Distribution Protocols

• Cisco IOS implements two label binding protocols for the purpose of destination-based unicast routing

TDPTDP (Tag Distribution Protocol)proprietary protocol - TCP port 711

LDPLDP (Label Distribution Protocol)IETF standards based protocol - TCP port 646

• Both protocols are functionally equivalent and can be used concurrently


London London Brussels Brussels Paris Paris 197.26.15.0/24

In Label FEC Out Label

- 197.26.15.0/24 28


28 197.26.15.0/24 41


41 197.26.15.0/24 -

Use label 28 for destination 197.26.15.0/24


Frame-mode Label Distribution


Label Retention

• Frame-mode LSR uses liberalliberal retention modeLSR keeps labels from neighbours that are not the best next-hop for the FECrequires more memory and label space

problem for ATM-LSR as label is a VC

improves convergence if layer-3 routing information changes, next-hop label forwarding information is already available


P-1P-2

Liberal Label Retention

LIB contains mappings from ALL neighbours

P-3

Use label 41 for destination 197.26.15.1/32 Use label 21 for destination

197.26.15.1/32



P-4

P-5



24 197.26.15.1/32 23

local binding: tag: 24 remote binding: tsr: 172.16.1.1:0, tag: 23 remote binding: tsr: 172.16.1.2:0, tag: 41


Penultimate Hop Popping

• Frame-mode LSR may use Penultimate Hop Popping procedures during label forwarding

label at top of stack is removed (popped) by the upstream neighbour of the egress-LSR

requested by egress-LSR using TDP/LDPuses implicit-null label

one lookup saved at egress-LSR


Penultimate Hop Popping

London London Brussels Brussels Paris Paris

197.26.15.1/32


- 197.26.15.1/32 28


28 197.26.15.1/32 POP


- 197.26.15.1/32 -


Use label implicit-null for destination 197.26.15.1/32

Paris# show tag-switching tdp binding 197.26.15.1 tib entry: 197.26.15.1/32, rev 10

local binding: tag: imp-null(1)remote binding: tsr: 172.16.3.1:0, tag: 28

Brussels# show tag-switching tdp binding 197.26.15.1 tib entry: 197.26.15.1/32, rev 10

local binding: tag: 28 remote binding: tsr: 172.16.3.2:0, tag: imp-null(1)

Brussels# show tag-switching forwarding Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 28 Pop tag 197.26.15.1/32 0 Se0/0/2 point2point


Cell-mode MPLSLabel Assignment

& Distribution



Cell-Mode MPLS

• ATM switches cannot perform MPLS label or IP lookup

Packets exchanged as cells at layer-2VPI/VCI pair used as label

• ATM switches cannot directly exchange IP traffic

VC required for MPLS data planeControl VC used for MPLS control plane


Label Switch Router

IP Routing Protocols

IP Routing Table

MPLS Signalling Protocol

Label Forwarding Table

Con

trol P

lan e

Data Plane

Label Switch Router

IP Routing Protocols

IP Routing Table

MPLS Signalling Protocol

Label Forwarding Table

Con

trol P

lan e

Data Plane

Routing Information Exchange

Label Binding Exchange

Labelled Packets

IP & MPLS AdjacenciesControl VC


MPLS Control VC

ATM Edge LSR ATM Edge LSR

MPLS Control Plane MPLS Control Plane

ATM LSR ATM LSR

MPLS Control Plane MPLS Control Plane

ATM Data Plane ATM Data Plane

Switching Matrix Switching Matrix

ATM Control VC (0/32) - aal5snap


55

8 8

In In TagTag

240/5240/5

240/8240/8

......


128.89128.89

128.89128.89

......

OutOutI/FI/F

00

00

......

Out Out TagTag

240/3240/3

240/3240/3

......

In In I/FI/F

11

22

......

2

5

8 33 33 33Packet

Cells

128.89

?

1 0

Packet 5

8

5 5 5

38

38 8 8 3 3 3 3

Cell-mode Label Distribution

Unsolicited Downstream label distribution breaks ATM !

(because use of same label for same FEC)


55

8 8

In In TagTag

240/5240/5

240/8240/8

......


128.89128.89

128.89128.89

......

OutOutI/FI/F

00

00

......

Out Out TagTag

240/3240/3

240/7240/7

......

In In I/FI/F

11

22

......

2

5

8 73 73 73Packet

Cells

128.89

Now I am happy :)

1 0

Packet 5

8

5 5 5

78

7

8 8 8 3 3 7 3

Cell Flow with Multiple Labels

• Cell-mode uses Downstream on Demand label distribution with Ordered label allocation

unique label mapping for each upstream neighbour

multiple labels for the same FEC from the same next-hop neighbour


Cell-mode Label Distribution



Step #1: Label request for 146.27.15.0/24 sent to next-hop neighbour

146.27.15.0/24

Step #2: Paris allocates label and responds with

label mappingStep #3: Label mapping is propagated back to the source and labels are assigned by intermediate ATM LSRs

RQ for 146.27.15.0/24

RQ for 146.27.15.0/24

RQ for 146.27.15.0/24

146.27.15.0/24 = 1/241

146.27.15.0/24 = 1/239

146.27.15.0/24 = 1/244


Cell-mode Label Retention

• Cell-mode uses Conservative label retention mode

keep only labels that are relevant to forwarding

achieved by requesting label mapping ONLY from next-hop downstream neighbour

146.27.52.0/24

Label request to next-hop neighbour

onlyTwo paths available to 146.27.52.0/24


VC Merge

• Further technique to reduce label space is vc-merge

55

8 8

In In TagTag

240/5240/5

240/8240/8

......


128.89128.89

128.89128.89

......

OutOutI/FI/F

00

00

......

Out Out TagTag

240/3240/3

240/3240/3

......

In In I/FI/F

11

22

......

5

8Packet

Cells

128.89

ATM LSR transmits cells in sequence so that the downstream LSR is able to correctlyreassemble the cells into packets

1

Packet 5

8

5 5 5

8 8 8 8 3 3 33 3 3 333


Configuring MPLS


Configure MPLS

• Global: tag-switchingtag-switching ipip is by default

• On all interfaces in the backbone : tag-switching iptag-switching ip (or) mpls ipmpls ip

• Label distribution :– TDP– LDP

• 12.0(10)ST• 12.1(1)T


Debug

• sh tag tdp discov -> Broadcast Discovery neighbors protocol

• sh tag tdp nei -> TDP (TCP session 711)

• sh tag tdp bind-> Full label exchange with all neighbors

• sh ip cef -> Select next-hop

• sh tag forwarding -> View selected Tag for destination

• sh ip cef IP@ det -> View all forwarding infos (& multi-labels)

• traceroute IP@ -> View LSP (path & tags & multi-tags)

• debug tag pac -> View LSR (P router) switching


LDP - Discovery

sh tag tdp discoveryLocal TDP Identifier: 10.21.2.2:0TDP Discovery Sources: Interfaces: Serial0/0: xmit/recv TDP Id: 10.1.2.2:0 Serial0/1: xmit/recv TDP Id: 10.21.4.4:0 Serial1/0: xmit/recv TDP Id: 10.21.3.3:0


Adjacency TDP / LDPsh tag tdp neighborPeer TDP Ident: 10.21.4.4:0; Local TDP Ident 10.21.2.2:0 TCP connection: 10.21.4.4.11001 - 10.21.2.2.711 State: Oper; PIEs sent/rcvd: 54/50; ; Downstream Up time: 00:37:57 TDP discovery sources: Serial0/1 Addresses bound to peer TDP Ident: 10.21.140.4 10.21.4.4 10.21.46.4 10.21.24.4

Peer TDP Ident: 10.21.3.3:0; Local TDP Ident 10.21.2.2:0 TCP connection: 10.21.3.3.11266 - 10.21.2.2.711 State: Oper; PIEs sent/rcvd: 24/25; ; Downstream Up time: 00:17:45 TDP discovery sources: Serial1/0 Addresses bound to peer TDP Ident: 10.21.130.3 10.21.3.3 10.21.13.3 10.21.35.3 10.21.23.3


View the LIB

sh tag tdp binding

tib entry: 10.21.5.5/32, rev 34 local binding: tag: 34 remote binding: tsr: 10.21.4.4:0, tag: 35 remote binding: tsr: 10.21.3.3:0, tag: 28 remote binding: tsr: 10.1.2.2:0, tag: 38

tib entry: 10.21.13.0/24, rev 16 local binding: tag: 28 remote binding: tsr: 10.21.4.4:0, tag: 31 remote binding: tsr: 10.21.3.3:0, tag: imp-null remote binding: tsr: 10.1.2.2:0, tag: imp-null


View the LFIB

3640-L21-R2#sh tag forwarding-tableLocal Outgoing Prefix Bytes tag Outgoing Next Hoptag tag or VC or Tunnel Id switched interface17 Aggregate 100.10.57.0/24[V] 021 20 10.10.4.4/32 0 Se0/1 point2point22 17 10.10.12.0/24 0 Se0/1 point2point26 Untagged 10.21.24.4/32 0 Se0/1 point2point27 Untagged 10.21.23.3/32 0 Se1/0 point2point28 Pop tag 10.21.13.0/24 0 Se0/0 point2point Pop tag 10.21.13.0/24 0 Se1/0 point2point29 Pop tag 10.21.35.0/24 0 Se1/0 point2point30 Pop tag 10.21.3.3/32 0 Se1/0 point2point32 Pop tag 10.21.4.4/32 0 Se0/1 point2point33 27 10.21.57.0/24 0 Se1/0 point2point34 28 10.21.5.5/32 0 Se1/0 point2point35 Untagged 10.21.12.1/32 0 Se0/0 point2point36 Pop tag 10.1.2.2/32 0 Se0/0 point2point


View the LFIB

L10-R5# sh tag forwarding tag 34 detailLocal Outgoing Prefix Bytes tag Outgoing Next Hoptag tag or VC or Tunnel Id switched interface 34 16 100.20.57.0/24[V] 0 Se0/1 point2point

MAC/Encaps=4/12, MTU=1496, Tag Stack{16 60} 0F008847 000100000003C000 VPN route: RED No output feature configured Per-packet load-sharing


Documents

1 © 2001, Cisco Systems, Inc. MPLS Architecture & Operation Peter Tomsu Senior Consultant EMEA Peter Tomsu Senior Consultant EMEA