mpls architecture overview v1.1. course objectives mpls overview mpls concepts lsrs and labels label...

MPLS Architecture Overview

V1.1

Course Objectives

MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

MPLS Concepts

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

MPLS Origin

MPLS is developed by integrating IP switching protocols of different manufactures IP Switching (Ipsilon) Tag Switching (Cisco) Aggregate RouteBased IP Switching (IBM) Cell Switch Router (Toshiba) IP Navigator (Ascend)

MPLS Origin

MPLS is developed by the combination of ATM and IP at 1990s. In 1996 , Ipsilon proposed IP switching protocol which solve the problem of better supporting IP on ATM switch, this makes an ATM switch a router and at the same time having the high performance of an ATM switch， breaking the performance bottleneck on the router (can not achieve wire speed on POS3,POS12)

MPLS History

When OC48,OC192 interface achieves wire speed, MPLS research switched to these application：TE (Traffic engineering), VPN.

MPLS TE becomes an important method to manage network traffic、 reduce congestion and ensure QOS on IP network.

VPN is regarded as an important method to provide value-added service and explore new service by carriers.

Disadvantages： Change the current network topology, increase the network complexity, VPN security factors.

MPLS Overview

Multi-protocol label switch Label is at the layer 2.5, between link layer and network l

ayer. This technology have the advantage of layer 2 switching and layer 3 routing.

The successful point of MPLS technology is that it introduced connection-oriented mechanism into IP network;

Core idea of MPLS is： routing on the edge and switching on the core part

Establishing of LSP（ label switch path）： LSP is established by topology not by data. The establishing of LSP can be done by LDP， CR-LDP， RSVP protocols.

MPLS Developing Motion

Current core network is made up of router and ATM switch

Core network based on MPLS is superior than core network based on router and ATM

This is the developing motion of MPLS.

Comparison between MPLS and routing

MPLS simplifies the process of packet forwarding

MPLS supports valid explicit routing MPLS can implement TE best MPLS can support QoS routing Mapping from IP packet to FEC MPLS support dividing network by function MPLS implements single forwarding rule no matt

er how many levels of Qos are there

Advantage of MPLS

Comparison of label switching and hop-by-hop forwarding based on destination IP address： MPLS is implemented on switch, thus taking full advan

tage of high speed of switch. Traditional switch can search label and replace them, but can not analyze network header or at least not fast enough to analyze it, so it is difficult to implement forwarding based on network header information on traditional switch

When packet comes into MPLS network, ingress router will decide which FEC it belongs to, it can use any information about the packet not only the network layer information.（ for example the port information）

Advantage of MPLS (Continued.)

MPLS can assign different labels to packet coming from different routers, thus making it easy to implement routing based on ingress routers. Because ingress router id can not be sent with the packet , so traditional routing patterns do not support this function.

Sometimes it is reasonable to designate a predefined route for the packets than hop-by-hop routing.

Course Objectives


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.

Key words in MPLS

Label FEC (Forwarding equivalence class) LSR (Label switching router) LER (Label edge router) LSP (Label switching path) LDP (Label distribution path)

Label

Label is a key point in MPLS, it is a short fixed-length and locally used identifier which is used to identify FEC

Label processing is done by high speed ASIC chip thus making the delay of packet processing and queuing decrease to a great extent

Why fixed-length label? Balancing between forwarding efficiency and switching

performance. Although fixed-length making the forwarding efficiency low but it can highly improve switching performance.

Label is locally used identifier. It is only meaningful between the sending out port of upstream router and receiving port of downstream router.

Label Format

MPLS label is an integer ranged between 0 to 1048575,it is used to identify specific FEC.

This label is encapsulated in layer 2 header.

Label stack

Two or more MPLS labels, encapsulated after link layer header and before network header.

Top label in the stack appear earliest. Network layer header follow the last label in the

stack

Label stack

Forwarding of packets is based on the top label in the stack， when LSR receives a packet， checks the top label and decides the next hop

The operation about the label stack： replace： Use a new label to replace the top label in t

he stack Pop (delete)： pop the top label in the label stack and

delete it push (add)： replace the top label and add a new lab

el into the stack

FEC

Forwarding Equivalence Class (FEC)： A group of packets that have common attributes. These packets will be forwarded in the same way by LSR in the MPLS network， just because they are forwarded in the same way they are equivalence.

FEC

FEC – packets forwarded in the same way Same destination prefix unicast packet Same destination address multicast packet Same Qos packets

MPLSDomain

LERa

LSRy

LERb

LERc

LERd

LERe

LERf

LSRx LSRz

LSP

Ingress

Egress

FECp

FECq

FECp+q

MPLS key points

LSR –responsible for establishing LSP for FEC LER –responsible for FEC classification, TE， begin the

process of establishing LSP, IP packet forwarding LSP –used for IP packet forwarding LDP –responsible for assigning labels

MPLSDomain

LERa

LSRy

LERb

LERc

LERd

LERe

LERf

LSRx LSRz

LSP

Ingress

Egress

FECp

FECq

FECp+q

MPLS key points

Label switch router (LSR)： one node in MPLS network, it is located in the middle of MPLS network, it runs MPLS control protocol and layer 3 routing protocol, its responsibility lies in: exchanging routing information with other LSR to form

route table, implementing the mapping from FEC to IP packets，

redistributing label binding information, establishing label forwarding table and maintaining it.

MPLS key points

Label edge router (LER)： Responsible for connecting MPLS domain and non-MPLS domain. Implementing the function of classifying service、assigning labels、 taking off labels etc. LER is the key device in implementing MPLS.

MPLS Key Points

Label switch path (LSP)： The forwarding path made up of the LSRs along the way to forward packets which belong to the same FEC.

Label distributing protocol (LDP)： Responsible for controlling the label binding information exchanging process between LSRs， LSR establish and maintain the LIB (Label information base) according to the binding information between label and FEC.

MPLS Key points

Upstream router Downstream router

Course Objectives


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

LSRs and Labels

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

IGP domain with a label distribution protocol

LSRs and Labels

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

Course Objectives


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


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


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


Upstream LSRs request labels to downstream neighbors Downstream LSRs distribute labels upon request

171.68.10/24

Rtr-BRtr-A Rtr-C

171.68.40/24



Request label for destination 171.68.10/24

Request label for destination 171.68.10/24

On-Demand Downstream Distribution


Label Retention Modes Liberal retention mode

LSR retains labels from all neighbors– Improve 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 neighbors

– LSR discards all labels for FECs without next-hop

– Free memory and label space


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


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

Course Objectives


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



LSP establishing process

The establishing of LSP in MPLS network including these three steps: Every node run routing protocols such as BGP、 OS

PF、 IS-IS to form its own route table According to the route table, every node establish labe

l information base under the control of LDP From the ingress LSR 、middle LSR and egress LSR,

the ingress lable and outgress lable together form a LSP.

1st step： form of route table

Dynamic routing protocols help each router form route table.

47.1

47.247.3

Dest Out

47.1 147.2 2

47.3 3

1

23

Dest Out

47.1 147.2 2

47.3 3

Dest Out

47.1 147.2 2

47.3 3

1

23

1

2

3

Traditional ‘hop by hop’ forwarding

47.1

47.247.3

IP 47.1.1.1

Dest Out

47.1 147.2 2

47.3 3

1

23

Dest Out

47.1 147.2 2

47.3 3

1

2

1

2

3

IP 47.1.1.1

IP 47.1.1.1IP 47.1.1.1

Dest Out

47.1 147.2 2

47.3 3

2nd step： form of LIB

IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

47.1

47.247.3

12

3

1

2

1

2

3

3IntfIn

Dest IntfOut

LabelOut

3 47.1 1 0.50 Mapping: 0.40

Request: 47.1

Mapping: 0.50

Request: 47.1

3rd step： form of LSP

IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

47.1

47.247.3

1

2

31

2

1

2

3

3IntfIn

Dest IntfOut

LabelOut

3 47.1 1 0.50

IP 47.1.1.1

IP 47.1.1.1

Routing method in MPLS

hop-by-hop routing This method allows each node to select the next hop

for each FEC independently This kind of routing method is commonly used in IP

network

Routing method in MPLS

Explicit Routing In this kind of routing method, each LSR can not selec

t next hop independently， on the contrary， path selection is done under the network management policy, for example, the ingress or egress LSR define the way of the LSP.

When the ingress of egress LSR define every hop along the LSP , we call it “strict explicit routing”， if it only define part of the nodes along the way， we call it “loose explicit routing”

“strict explicit routing” is also called “source routing” in IP network， but compared to IP source routing, strict explicit routing has higher efficiency.

47.1

47.247.3

IP 47.1.1.1

Dest Out

47.1 147.2 2

47.3 3

1

23

Dest Out

47.1 147.2 2

47.3 3

1

2

1

2

3

IP 47.1.1.1

IP 47.1.1.1IP 47.1.1.1

Dest Out

47.1 147.2 2

47.3 3

hop-by-hop routing

#216

#14

#462

#972

#14 #972

A

B

C

Route={A,B,C}

Explicit Routing

IntfIn

LabelIn

Dest IntfOut

3 0.40 47.1 1

IntfIn

LabelIn

Dest IntfOut

LabelOut

3 0.50 47.1 1 0.40

47.1

47.247.3

1

2

3

1

2

1

2

3

3

IntfIn

Dest IntfOut

LabelOut

3 47.1.1 2 1.333 47.1 1 0.50

IP 47.1.1.1

IP 47.1.1.1

ER-LSP

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

Label Switch Path (LSP) Penultimate Hop Popping

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

Egress LSR need NOT receive a labelled packet

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

Course Objectives


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

Loops and TTL

LSRs using ATM do not have TTL capability Some suggested options:

hop-count object in LDP Path Vector object in LDP

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

Course Objectives


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

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

LDP Messages

Session messages Establish, maintain and terminate LDP sessions

Advertisement messages Create, modify, delete label mappings

Notification messages Error signalling

Course Objectives


Day in the life of a Packet

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

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



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

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

Day in the life of a Packet Database LayoutOS 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

Summary

MPLS basic conception MPLS working process LSP FEC label distribution LDP

mpls architecture overview v1.1. course objectives mpls overview mpls concepts lsrs and labels label...

Documents

mpls network

network layer

ip packet

mpls research

network complexity

atm switchcore network

developing motion of

motioncurrent core network