3.2.7 Feature Description - MPLS

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HUAWEI CX600 Metro Services Platform

V600R003C00

Feature Description - MPLS

Issue 02

Date 2011-09-10

HUAWEI TECHNOLOGIES CO., LTD.


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Copyright Huawei Technologies Co., Ltd. 2011. All rights reserved.

No part of this document may be reproduced or transmitted in any form or by any means without prior written

consent of Huawei Technologies Co., Ltd.

Trademarks and Permissions

and other Huawei trademarks are trademarks of Huawei Technologies Co., Ltd.

All other trademarks and trade names mentioned in this document are the property of their respective holders.

Notice

The purchased products, services and features are stipulated by the contract made between Huawei and the

customer. All or part of the products, services and features described in this document may not be within the

purchase scope or the usage scope. Unless otherwise specified in the contract, all statements, information,and recommendations in this document are provided "AS IS" without warranties, guarantees or representations

of any kind, either express or implied.

The information in this document is subject to change without notice. Every effort has been made in the

preparation of this document to ensure accuracy of the contents, but all statements, information, and

recommendations in this document do not constitute the warranty of any kind, express or implied.

Huawei Technologies Co., Ltd.

Address: Huawei Industrial Base

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Website: http://www.huawei.com

Email: [email protected]

Issue 02 (2011-09-10) Huawei Proprietary and Confidential

Copyright Huawei Technologies Co., Ltd.

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About This Document

Purpose

This document describes the MPLS feature in terms of its overview, principle, and applications.

This document together with other types of document helps intended readers get a deep

understanding of the MPLS feature.

Related Versions

The following table lists the product versions related to this document.

Product Name Version

HUAWEI CX600 Metro

Services Platform

V600R003C00SPC300

Intended Audience

This document is intended for:

l Network planning engineers

l Commissioning engineers

l Data configuration engineers

l System maintenance engineers

Symbol Conventions

The symbols that may be found in this document are defined as follows.

Symbol Description

DANGER

Indicates a hazard with a high level of risk, which if not

avoided, will result in death or serious injury.


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Symbol Description

WARNING

Indicates a hazard with a medium or low level of risk, which

if not avoided, could result in minor or moderate injury.

CAUTION

Indicates a potentially hazardous situation, which if not

avoided, could result in equipment damage, data loss,

performance degradation, or unexpected results.

TIP Indicates a tip that may help you solve a problem or save

time.

NOTE Provides additional information to emphasize or supplement

important points of the main text.

Change History

Updates between document issues are cumulative. Therefore, the latest document issue contains

all updates made in previous issues.

Changes in Issue 02 (2011-09-10)

Second commercial release.

Changes in Issue 01 (2011-06-30)Initial field trial release.


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Contents

About This Document.....................................................................................................................ii

1 MPLS Overview.............................................................................................................................1

1.1 Introduction to MPLS.........................................................................................................................................2

1.2 References..........................................................................................................................................................21.3 Principles............................................................................................................................................................3

1.3.1 Concepts....................................................................................................................................................3

1.3.2 Establishing LSPs....................................................................................................................................10

1.3.3 MPLS Forwarding...................................................................................................................................12

1.3.4 MPLS Ping/Traceroute............................................................................................................................17

1.4 Applications......................................................................................................................................................19

1.4.1 MPLS-based VPN...................................................................................................................................19

1.4.2 PBRto an LSP.........................................................................................................................................20

1.5 Terms andAbbreviations..................................................................................................................................21

2 MPLS LDP.....................................................................................................................................24

2.1 Introduction to LDP..........................................................................................................................................25

2.2 References........................................................................................................................................................25

2.3 Principles..........................................................................................................................................................25

2.3.1 Concepts..................................................................................................................................................26

2.3.2 LDPSessions...........................................................................................................................................27

2.3.3 Advertising and Managing Labels...........................................................................................................28

2.3.4 Establishment of LDP LSP......................................................................................................................31

2.3.5 LDP Extension for Inter-Area LSP.........................................................................................................31

2.3.6 Outbound and Inbound LDP Policies......................................................................................................33

2.3.7 LDP-IGP Synchronization.......................................................................................................................34

2.3.8 Synchronization Between LDP and Static Routes..................................................................................36

2.3.9 LDPGR...................................................................................................................................................37

2.3.10 LDP NSR...............................................................................................................................................38

2.3.11 LDP FRR...............................................................................................................................................38

2.3.12 LDP MTU..............................................................................................................................................40

2.3.13 LDP MD5..............................................................................................................................................41

2.3.14 LDP Authentication...............................................................................................................................41

2.3.15 Distributing Labels for BGP by LDP....................................................................................................42


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2.3.16 LDP over TE..........................................................................................................................................45

2.3.17 LDP GTSM............................................................................................................................................46

2.3.18 Coexistence of the Local and Remote LDP Sessions............................................................................47

2.3.19 Distributing Labels for All Peers by LDP.............................................................................................48

2.4 Terms and Abbreviations..................................................................................................................................49

3 MPLS TE........................................................................................................................................51

3.1 Introduction to MPLS TE.................................................................................................................................52

3.2 References........................................................................................................................................................56

3.3 Principles..........................................................................................................................................................57

3.3.1 RSVP-TE.................................................................................................................................................58

3.3.2 Make-Before-Break.................................................................................................................................59

3.3.3 P2MP RSVP-TE......................................................................................................................................60

3.3.4 Automatic Bandwidth Adjustment..........................................................................................................63

3.3.5 Re-optimization.......................................................................................................................................64

3.3.6 TE FRR....................................................................................................................................................65

3.3.7 SRLG.......................................................................................................................................................69

3.3.8 CR-LSP Backup......................................................................................................................................70

3.3.9 DS-TE......................................................................................................................................................75

3.3.10 Static Bidirectional Co-routed LSP.......................................................................................................87

3.3.11 TE Tunnel Protection Group.................................................................................................................88

3.3.12 BFD for TE CR-LSP.............................................................................................................................90

3.3.13 BFD for TE Tunnel................................................................................................................................92

3.3.14 RSVP Authentication............................................................................................................................93

3.3.15 RSVP GR...............................................................................................................................................94

3.3.16 RSVP Summary Refresh.......................................................................................................................95

3.3.17 RSVP Hello...........................................................................................................................................96

3.3.18 BFD for RSVP.......................................................................................................................................97

3.3.19 TE LSP Configuration Template...........................................................................................................97

3.3.20 Multi-Area Advertisement of an MPLS LSR-ID..................................................................................99

3.4 Terms and Abbreviations................................................................................................................................100

4 MPLS OAM................................................................................................................................102

4.1 Introduction to MPLS OAM...........................................................................................................................103

4.2 References......................................................................................................................................................103

4.3 Principles........................................................................................................................................................104

4.3.1 MPLS OAM Detection..........................................................................................................................104

4.3.2 Reverse Tunnel......................................................................................................................................106

4.3.3 MPLS OAM Auto Protocol...................................................................................................................106

4.3.4 Protection Switching..............................................................................................................................107

4.4 Terms andAbbreviations................................................................................................................................108


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1MPLS OverviewAbout This Chapter

1.1 Introduction to MPLS

1.2 References

1.3 Principles

1.4 Applications

1.5 Terms and Abbreviations


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1.1 Introduction to MPLS

Background of MPLS

The Internet based on the IP technology prevailed in the middle 1990s. The IP technology,

however, performs poorly in forwarding packets because of inevitable software dependence on

searching routes through the longest match algorithm. As a result, the forwarding capability of

IP technology becomes a bottleneck to the network development.

With the evolvement of network technologies, the Asynchronous Transfer Mode (ATM)

technology comes out. It uses labels (namely, cells) of fixed length and maintains a label table

that is much smaller than a routing table. Therefore, compared with the IP technology, the ATM

technology performs much better in forwarding packets. The ATM technology, however, is

difficult to popularize because of its complex protocol and high cost in deployment.

The traditional IP technology is simple and costs little in deployment. People then are eager to

making a technical breakthrough to combine advantages of IP and ATM technologies. Thus, the

MPLS technology comes forth.

Initially, MPLS emerges to increase the forwarding rate of devices. Different from IP routing

in forwarding packets, MPLS analyzes a packet header only on the network edge but not at each

hop. In this manner, the time to process packets is shortened.

The application specific integrated circuit (ASIC) technology is developed and the routing rate

is no longer a bottleneck of the network development. As a result, MPLS does not have

advantages in high-speed forwarding any more. MPLS supports multi-layer labels, and its

forwarding plane is connection-oriented. Thus, MPLS is widely used in Virtual Private Network

(VPN), traffic engineering (TE), and Quality of Service (QoS).

Introduction to MPLS

MPLS works between the data link layer and the network layer in the TCP/IP protocol stack. It

provides the IP layer with connection services and obtains services from the data link layer.

MPLS replaces IP forwarding with label switching. A label is a short connection identifier of

fixed length that is meaningful for the local end. The label is similar to the ATM virtual path

identifier (VPI)/virtual channel identifier (VCI) and the Frame Relay data link connection

identifier (DLCI). The label is encapsulated between the data link layer and the network layer.

MPLS is not limited by any specific protocol of the data link layer and is enabled to use any

Layer 2 media to transfer packets.

The origin of MPLS is the Internet Protocol version 4 (IPv4). The core MPLS technology can

be extended to multiple network protocols, such as the Internet Protocol version 6 (IPv6), Internet

Packet Exchange (IPX), Appletalk, DECnet, and Connectionless Network Protocol (CLNP).

Multiprotocol in MPLS means that the protocol supports multiple network protocols.

In fact, the MPLS technology is a tunneling technology rather than a service or an application.

It supports multiple protocols and services. Moreover, it can ensure the security for data

transmission.

1.2 References





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The following table lists the references of this document:

Document No. Description

RFC 3031 Multiprotocol Label Switching Architecture

RFC 3036 LDP Specification

RFC 3032 MPLS Label Stack Encoding

RFC 3443 Time To Live (TTL) Processing in Multi-Protocol Label

Switching (MPLS) Networks

RFC 3034 Use of Label Switching on Frame Relay Networks

Specification

RFC 2702 Requirements for Traffic Engineering Over MPLS

RFC 3209 RSVP-TE: Extensions to RSVP for LSP Tunnels

RFC 2547 BGP/MPLS VPNs

1.3 Principles

1.3.1 Concepts

MPLS Network Structure

Figure 1-1shows the typical structure of an MPLS network. The fundamental element of an

MPLS network is Label Switching Router (LSR). Many LSRs on a network form an MPLS

domain. LSRs that reside at the edge of an MPLS domain and connect to other networks are

Label Edge Routers (LERs). LSRs within an MPLS domain are core LSRs. If an LSR connects

to one or more adjacent nodes that do not run MPLS, the LSR is the LER. If all the adjacent

nodes of an LSR run MPLS, the LSR is the core LSR.





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Figure 1-1MPLS network structure

Core LSR

LER

MPLS networkLER

LER

Core LSR

Core LSRCore LSR

LER

LER

Non- MPLS

network

Non- MPLS

network

Non- MPLS

network

Non- MPLS

network

The transfer of packets in the MPLS domain is based on labels. When IP packets enter an MPLS

network, the LER at the entrance analyzes IP packets and then adds proper labels to them. All

nodes on the MPLS network forward data according to labels. When IP packets leave the MPLS

network, the labels are deleted on the LER that is the exit.

The path that IP packets pass through on an MPLS network is called the LSP. An LSP is a

unidirectional path in the same direction with the data flow.

Figure 1-2MPLS LSP

LSP

MPLS network

Transit

LER

TransitIngress Egress

Core LSR Core LSRLER

Non-MPLS

network

Non-MPLS

network

The beginning node of an LSP is called the ingress. The end node of the LSP is called the egress.

The nodes between both ends along the LSP are transits. An LSP may have none, one, or severaltransit(s), but only one ingress and one egress.





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Forwarding Equivalence Class

The forwarding equivalence class (FEC) is a set of data flows with the same attributes. These

data flows are processed in the same way by LSRs during transmission.

FECs can be identified by the address, service type, and QoS. For example, during IP forwardingaccording to the longest match algorithm, packets with the same destination belong to an FEC.

Label

A label is a short identifier of a fixed length that is only meaningful for the local end. It is used

to uniquely identify an FEC to which a packet belongs. In some cases such as load balancing,

an FEC can be mapped to multiple incoming labels, but one label only represents one FEC on

a device. The label is a connection identifier, similar to the ATM VPI/VCI and the Frame Relay

DLCI.

A label is 4 bytes long. Figure 1-3shows the encapsulation structure of the label.

Figure 1-3Structure of the MPLS packet header

A label contains the following fields:

l Label: indicates the value field of a label. The length is 20 bits.

l Exp: indicates the bits used for extension. The length is 3 bits. Generally, this field is used

for the Class of Service (CoS) that serves similarly to Ethernet 802.1p.

l S: identifies the bottom of a label stack. The length is 1 bit. MPLS supports multiple labels,

namely, the label nesting. When the S field is 1, it means that the label is at the bottom of

the label stack.

l TTL: indicates the time to live. The length is 8 bits. This field is the same as the TTL in IP

packets.

Labels are encapsulated between the data link layer and the network layer. Thus, labels can be

supported by all protocols of the data link layer.

Figure 1-4shows the position of the label in a packet.

Figure 1-4Position of the label in a packet

Label Space

Label space means the range of label values. In the CX600 implementation, the label space is

classified as follows:

l 0 to 15: indicates special labels. For details about special labels, see Table 1-1.





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l 16 to 1023: indicates the label space shared by static LSPs and static CR-LSPs.

l 1024 or above: indicates the label space for dynamic signaling protocols, such as LDP,

RSVP-TE, and MP-BGP.

The label space for dynamic signaling protocols is independent and successive but cannot

be shared or affect each other. Label space for dynamic signaling protocols is defined bythe license file on the device. To change the label space, you need to modify the license

file.

Table 1-1Special labels

Label Value Label Description

0 IPv4 Explicit

NULL Label

Indicates that the label must be popped out, and the packets

must be forwarded on the basis of IPv4. If the egress

allocates a label whose value is 0 to the penultimate hop,

the penultimate hop LSR needs to push label 0 to the top

of a label stack and forward the packet to the egress. When

the egress finds that the value of the label carried in the

packet is 0, the egress pops out the label. The label 0 is

valid only at the bottom of the label stack.

1 Router Alert

Label

Indicates a label that is only valid when it is not at the

bottom of a label stack. The label is similar to the Router

Alert Option field in IP packets. When the node receives

such a label, the label is sent to a local software module

for further processing. The packet forwarding is

determined by the next layer label. If the packet needs to

be forwarded continuously, the node needs to push the

Router Alert Label to the top of the label stack again.

2 IPv6 Explicit

NULL Label

Indicates that the label must be popped out, and the packets

must be forwarded on the basis of IPv6. If the egress

allocates a label whose value is 2 to the penultimate hop,

the penultimate hop LSR needs to push label 2 to the top

of a label stack and forward the packet to the egress. When

the egress finds that the value of the label carried in the

packet is 2, the egress pops out the label directly. The label

2 is valid only at the bottom of the label stack.

3 Implicit

NULL Label

When the label whose value is 3 is swapped on the

penultimate hop LSR, the penultimate hop LSR pops outthe label and forwards the packet to the egress. After the

egress receives the packet, the egress forwards the IP

packet or the VPN packet.

4 to 13 Reserved None.

14 OAM Router

Alert Label

Indicates a label for the packets of Operation

Administration and Maintenance (OAM) over an MPLS

network. MPLS OAM sends OAM packets to detect LSPs

and notify faults. OAM packets are transparent on transits

and the penultimate LSR.

15 Reserved None.





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Label Stack

A label stack is a set of arranged labels. An MPLS packet can carry multiple labels at the sametime. The label next to the Layer 2 header is called the top label or the outer label. The label next

to the Layer 3 header is called the bottom label or inner label. Theoretically, MPLS labels can

be nested limitlessly.

Figure 1-5Label stack

The label stack organizes labels according to the rule of Last-in, First-Out. The labels are

processed from the top of the stack.

Label Operations

Information about the basic label operations is a part of the label forwarding table. The operations

are described as follows:

l Push: When an IP packet enters an MPLS domain, the ingress adds a new label to the packetbetween the Layer 2 header and the IP header; or, an LSR adds a new label to the top of

the label stack, namely, the label nesting.

l Swap: When a packet is transferred within an MPLS domain, the local node swaps the label

at the top of the label stack in the MPLS packet for the label allocated by the next hop

according to the label forwarding table.

l Pop: When a packet leaves an MPLS domain, the label is popped out from the MPLS packet;

or, the top label of the label stack is popped out at the penultimate hop on an MPLS network

to decrease the number of labels in the stack.

Penultimate Hop PoppingIn fact, the label is useless at the last hop of an MPLS domain. In this case, the feature of

penultimate hop popping (PHP) is applied. On the penultimate node, the label is popped out of

the packet to reduce the size of the packet that is forwarded to the last hop. Then, the last hop

directly forwards the IP packet or forwards the packet by using the second label.

PHP is configured on the egress. In addition, the egress only allocates the following label to the

PHP:

Label 3: indicates the implicit-null label. This label is not listed in the label stack. When an LSR

receives an implicit-null label, the LSR pops out the label in the packet rather than uses this

implicit-null label to replace the label at the top of the label stack. The egress directly forwards

the packet through an IP link or according to the next layer label.





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Label Switching Router

A Label Switching Router (LSR) refers to devices that can swap labels and forward MPLS

packets. It is also called the MPLS node. The LSR is a fundamental element on an MPLS

network. All LSRs support the MPLS protocol.

LER

An LER is the LSR that resides on the edge of an MPLS domain. When an LSR connects to one

node that does not run MPLS, the LSR acts as the LER.

The LER classifies the packets entering an MPLS domain by FECs and pushes labels into FECs.

Then, the LER forwards MPLS packets based on labels. When packets leave the MPLS domain,

the labels are popped out. The packets become IP packets again and are forwarded continuously.

Label Switched PathThe path that an FEC passes through in the MPLS network is called the LSP.

An LSP functions similarly to virtual circuits of ATM and Frame Relay. The LSP is a

unidirectional path from the ingress to the egress.

Ingress, Transit, and Egress LSRs

The LSP is a unidirectional path. LSRs along an LSP can be classified as follows:

l Ingress LSR: indicates the beginning of an LSP. Only one ingress exists on an LSP.

The ingress pushes a new label to the packet and encapsulates the IP packet as an MPLSpacket to forward.

l Transit LSR: indicates the middle node of an LSP. Multiple transit LSRs may exist on an

LSP.

The transit LSR mainly searches routes in the label forwarding table. Then, it swaps labels

to complete the forwarding of MPLS packets.

l Egress LSR: indicates the end node of an LSP. Only one egress exists on an LSP.

The egress mainly pops labels out of MPLS packets and forwards the packets that restore

the IP packet.

Upstream and Downstream

According to the direction of data transmission, LSRs are classified as follows:

l Upstream: Based on the specified LSR, in the direction of data flows, the LSRs that send

MPLS packets to the local LSR are upstream LSRs.

l Downstream: Based on the specified LSR, in the direction of data flows, the next-hop LSRs

that receive MPLS packets sent from the local LSR are downstream LSRs.

As shown in Figure 1-6, the data flows to 192.168.1.0/24. LSR A is the upstream LSR of LSR

B and the LSR B is the downstream of LSR A. Likely, LSR B is the upstream LSR of LSR C.

LSR C is the downstream LSR of LSR B.





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Figure 1-6Upstream and downstream

LSR-A LSR-B LSR-C

192.168.1.0/24

Downstreamdata flow

Downstreamdata flow

Label Distribution

Packets with the same destination address belong to an FEC. A label out of an MPLS label

resource pool is allocated to the FEC. LSRs record the relationship of the label and the FEC.

Then, LSRs send a message and advertises to upstream LSRs about the label and FEC

relationship in message. The process is called label distribution.

Figure 1-7Schematic diagram of the label distribution

LSR-A LSR-B LSR-C

192.168.1.0/24

Labels are

distributed upstream

Data flow

downstream

Labels are

distributed upstream

Data flow

downstream

As shown in Figure 1-7, LSR B and LSR C use an FEC respectively to identify packets withthe destination address as 192.168.1.0/24. Then, labels are allocated to FECs and their

relationship is advertised to upstream LSRs. Thus, labels are allocated by the downstream LSRs.

Label Distribution Protocols

Label distribution protocols are MPLS control protocols, namely, signaling protocols. They are

used to classify FECs, distribute labels, and create and maintain LSPs.

MPLS utilizes multiple label distribution protocols, such as Label Distribution Protocol (LDP),

Resource Reservation Protocol Traffic Engineering (RSVP-TE), and Multiprotocol Border

Gateway Protocol (MP-BGP).

MPLS Architecture

The MPLS architecture consists of a control plane and a forwarding plane.

Figure 1-8shows the MPLS architecture.





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Figure 1-8Schematic diagram of the MPLS architecture

IP Routing Protocol

Routing Information

Base (RIB)

MPLS IP Routing

Protocol

Label Forwarding

Information Base(LFIB)

Label Information Base

(LIB)

Control Plane

Forwarding Plane

l The control plane is connectionless and responsible for distributing labels, creating the label

forwarding table, and creating or deleting LSPs.

l The forwarding plane, also known as the data plane, is connection-oriented. It can apply

services and protocols of ATM, Frame Relay, and Ethernet networks. The forwarding plane

is mainly responsible for adding labels to and deleting labels from IP packets.

Simultaneously, it forwards the received packets according to the label forwarding table.

1.3.2 Establishing LSPs

Procedure of Establishing LSPs

Usually, MPLS allocates labels for packets and establish an LSP. Then, MPLS can forward

packets.

Labels are allocated and distributed by a downstream LSR to an upstream LSR. As shown in

Figure 1-9, the downstream LSR classifies FECs according to an IP routing table and then

allocates labels to specific FECs. Then, the downstream LSR notifies the upstream LSR through

label advertisement protocols to set up a label forwarding table and an LSP.

Figure 1-9Establishment of an LSP

To 3.3.3.3/32

Label=Z

To 3.3.3.3/32

Label=Y

To 3.3.3.3/32

Label=3

3.3.3.3/32

Ingress Transit Transit Egress





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The classification of LSPs is as follows:

l Static LSP: It is set up by the administrator.

l Dynamic LSP: It is set up by the routing protocol and label distribution protocol.

Establishing Static LSPs

You can allocate labels manually to set up static LSPs. The principle is that the value of the

outgoing label of the upstream node is equal to the value of the incoming label of the downstream

node.

The availability of a static LSP makes sense only for the local node that cannot sense the entire

LSP.

l On the ingress: A static LSP is configured, and the outgoing interface of the ingress is

enabled with MPLS. If the route is reachable, the state of the static LSP is Up regardless

of the existence of the transit or egress. A reachable route means that a route entry exists

whose destination address and the next hop address match those in the local routing table.

l On the transit: A static LSP is configured, and the incoming and outgoing interfaces of the

transit are enabled with MPLS. If the incoming and outgoing interfaces are Up on the

physical layer and protocol layer, the static LSP is Up, regardless the existence of the

ingress, egress, or other transits.

l On the egress: A static LSP is configured, the incoming interface of the egress is enabled

with MPLS. If the incoming interface is Up on the physical layer and protocol layer, the

static LSP is Up, regardless the existence of the ingress or the transit.

NOTE

A reachable route is required on the ingress only for setting up a static LSP, but not on the transit or egress.

A static LSP is set up without label distribution protocols or exchanging control packets. Thus,

the static LSP costs little and it is applicable to small-scale networks with simple and stable

topology. The static LSP cannot vary with the network topology dynamically. The administrator

needs to configure the static LSP.

Establishing Dynamic LSPs

Dynamic LSPs are set up automatically by the label distribution protocol. The following label

distribution protocols are applicable to an MPLS network.

l LDP

The Label Distribution Protocol (LDP) is specially defined for distributing labels. When

LDP sets up an LSP in hop-by-hop mode, LDP identifies the next hop along the LSP

according to the routing forwarding table on each LSR. Information contained in the routing

forwarding table is collected by IGP and BGP. LDP is not directly assotiated with routing

protocols, but indirectly uses routing information.

LDP is not the only label distribution protocol. BGP and RSVP can also be extended to

distribute MPLS labels.

l RSVP-TE

The Resource Reservation Protocol (RSVP) is designed for the integrated service module

and is used to reserve resources on nodes along a path. RSVP works on the transport layer

and does not transmit application data. RSVP is a network control protocol, similar to theInternet Control Message Protocol (ICMP).





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RSVP is extended to support the setting up of a Constraint-based Routed LSP (CR-LSP).

The extended RSVP is called the RSVP-TE signaling protocol. It is used to set up TE

tunnels.

Different from LDP LSPs, RSVP-TE tunnels are characteristic as follows:

Bandwidth reservation requests

Bandwidth constraint

Link colors

Explicit paths

l MP-BGP

The Multiprotocol Extensions for BGP (MP-BGP) is an extended protocol of BGP. MP-

BGP imports the community attribute. MP-BGP supports label distribution for MPLS VPN

routes and labeled inter-AS VPN routes.

1.3.3 MPLS Forwarding

Basic Concepts of MPLS Forwarding

l Tunnel ID

To provide the same interface of a tunnel used by upper layer applications such like the

VPN and route management, the system automatically allocates an ID to each tunnel,

namely, tunnel ID. The tunnel ID is valid locally.

The tunnel ID is in a length of 32 bits. The length of the fields varies according to tunnel

types.

Figure 1-10shows the structure of a tunnel ID.

Figure 1-10Structure of a tunnel ID

The description of each field is as follows.

Table 1-2Description of each field of a tunnel ID

Field Description

Token Indicates the field that is used to search an MPLS forwarding

table for specified MPLS forwarding entries. The token is an

index number for searching forwarding information.

Sequence Number Indicates the sequence number of a tunnel ID.

Slot Number Indicates the slot number of an outgoing interface sending

packets.





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Field Description

Tunnel Type Indicates the type of a tunnel. The tunnel types of MPLS are as

follows:

l LSP: indicates the LSP tunnel that is set up dynamicallythrough LDP without any constraints.

l CRLSP: indicates the LSP tunnel that is set up dynamically

through CR-LDP or RSVP-TE with constraints.

l MPLS Local IFNET: indicates the tunnel that is set up by the

External BGP (EBGP) on MPLS interfaces of AS Boundary

Routers (ASBRs).

On inter-AS VPN Option B and Option C, besides information

on L2VPN label blocks, VPN routing information sent from an

ASBR to the BGP peer must contain tunnel information. No

tunnel, however, is set up between ASBRs. Thus, the MPLS local

IFNET tunnel is required between ASBRs to send AS-externalrouting information to the peer within the AS.

Allocation Method Indicates the method to allocate tokens. The methods are as

follows:

l Global: All tunnels share the same public global token space.

Two tokens cannot have the same value.

l Global with reserved tokens: This method is similar to the

global method. Differently, this method can reserve tokens

that tunnels cannot use. That is, the token of the tunnel begins

at a specified value.

l Per slot: Each slot has independent tokens. Tokens in thesame slot are different and tokens of different slots may be

the same.

l Per slot with reserved tokens: This method is similar to the

per slot method. Differently, this method can reserve tokens

that tunnels cannot use. That is, the token of a tunnel begins

at a specified value.

l Per slot with different avail value: This method is similar to

the per slot method. Differently, the token values of different

slots range differently.

l Mixed: Space can be created in either Global method or Per

slot method. The method to be selected depends on theinterface type. For the VLANIF or the interface of a backbone

network, the global method is adopted. Otherwise, the per slot

method is adopted.

l Mixed with 2 global space: Space can be created in global

space 1 method, global space 2 method, and per slot method.

l 2 global space: Space can be created in global space 1 method

and global space 2 method.

NOTEThe allocation method adopted by devices depends on license files.





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l NHLFE

The next hop label forwarding entry (NHLFE) can guide the MPLS packet forwarding.

An NHLFE contains the following information.

Tunnel ID

Outgoing interface

Next hop

Outgoing label

Label operation

l ILM

The incoming label map (ILM) indicates the mapping between an incoming label and a set

of NHLFEs.

The ILM contains the following information.

Tunnel ID

incoming label

incoming interface

Label operation

The ILM on a transit can bind the labels to NHLFEs. The function of an ILM table is similar

to the FIB that is searched according to destination IP addresses. Thus, you can obtain all

label forwarding information from searching an ILM table.

l FTN

FTN is a short form of FEC-to-NHLFE. The FTN indicates the mapping between an FEC

and a set of NHLFEs.

Details about the FTN can be obtained by searching the token values that are not 0x0 in a

FIB. The FTN is available on the ingress only.

Process of MPLS Forwarding

Take the LSP that supports the PHP as an example to describe how MPLS packets are forwarded.

Figure 1-11MPLS label distribution and packet forwarding

IP Packet

To 3.3.3.3

Label=Z Label=Y

PUSH SWAP

PHP

Label distributing

IP Packet

To 3.3.3.3IP Packet

To 3.3.3.3

IP Packet

To 3.3.3.3

Ingress EgressTransit Transit

To 3.3.3.3/32

Label=Z

To 3.3.3.3/32

Label=Y

To 3.3.3.3/32

Label=3

3.3.3.3/32

Ingress EgressTransit Transit

3.3.3.3/32

IP Packet

To 3.3.3.3

Packet transmitting

POP





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As shown in Figure 1-11, an LSP whose FEC is identified by the destination address 3.3.3.3/32

is set up on an MPLS network. MPLS packets are forwarded as follows:

1. The ingress receives an IP packet destined for 3.3.3.3/32. Then, the ingress adds Label Zto the packet and then forwards the packet.

2. The transit receives the labeled packet and swaps labels by popping Label Z out and pushing

Label Y into the packet.

3. The transit at the penultimate hop receives the packet with Label Y. The value of Label 3

is allocated by the egress. The transit performs the PHP to pop out Label Y and forwards

the packet. From the penultimate hop to the egress, the packet is transmitted as an IP packet.

4. Then, the egress receives the IP packet and forwards it to 3.3.3.3/32.

MPLS Forwarding Flow

When an IP packet enters an MPLS domain, the ingress searches the FIB to check whether the

tunnel ID corresponding to the destination IP address is 0x0.

l If the tunnel ID is 0x0, the packet is forwarded along the IP link.

l If the tunnel ID is not 0x0, the packet is forwarded along an LSP.

Figure 1-12shows the MPLS forwarding flow.

Figure 1-12MPLS forwarding flow

Ingress

Egress

Transit

FIB NHLFE

ILM NHLFE

InLabel

ILM

FEC Tunnel ID Tunnel ID Out Interface Next Hop OutLabel Operation

InLabel Tunnel ID Tunnel ID Out Interface Next Hop OutLabel Operation

MPLS packets are forwarded as follows on nodes along an LSP.

1. The ingress searches the FIB and NHLFE tables to forward MPLS packets.

2. The transit searches the ILM and NHLFE tables to forward MPLS packets.

3. The egress searches the ILM table to forward MPLS packets.

During the MPLS forwarding, FIB entries, ILM entries, and NHLFEs are associated with each

other through the token field in the tunnel ID.

l Forwarding on the Ingress

The ingress processes as follows to forward MPLS packets:





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1. Searches the FIB and finds the tunnel ID corresponding to the destination IP address.

2. Finds the NHLFE corresponding to the tunnel ID in the FIB and associates the FIB

entry with the NHLFE entry.

3. Checks the NHLFE for information about the outgoing interface, next hop, outgoing

label, and label operation type. The label operation type is Push.

4. Pushes the obtained label into IP packets, processes the EXP field according to QoS

policy and the TTL field, and then sends the encapsulated MPLS packets to the next

hop.

l Forwarding on the Transit

The transit processes as follows to forward the received MPLS packets:

1. Checks the ILM table corresponding to an MPLS label and finds the token.

2. Finds the NHLFE corresponding to the token in the ILM table.

3. Checks the NHLFE for information about the outgoing interface, next hop, outgoing

label, and label operation type.

4. MPLS packets are processed distinctively according to the specific label value.

If the label value is equal to or greater than 16, a new label replaces the label in

the MPLS packet. At the same time, the EXP field and TTL field are processed.

Then, the MPLS packet with the new label is forwarded to the next hop.

If the label value is 3, the label is popped out from the MPLS packet. At the same

time, the EXP field and TTL field are processed. Then, the packet is forwarded

through IP routes or according to its next layer label.

l Forwarding on the Egress

When the egress receives MPLS packets, the egress checks the ILM table for the label

operation type. At the same time, the egress processes the EXP field and TTL field.

When the S field in the label is equal to 1, the label is the stack bottom label and the

packet is directly forwarded through IP routes.

When the S field is equal to 0 in the label, a next layer label exists and the packet is

forwarded according to the next layer label.

Processing MPLS TTL

An MPLS label has a TTL field in the length of 8 bits. The TTL field is the same as that in an

IP packet header. MPLS processes the TTL to prevent loops and implement traceroute.

RFC 3443 defines two modes in which MPLS processes the TTL, that is, uniform mode and

pipe mode. By default, MPLS processes the TTL in Uniform mode.

l Uniform Mode

When IP packets enter an MPLS network, on the ingress, the IP TTL decreases by one and

is mapped to an MPLS TTL field. Then, the TTL field in MPLS packets is processed in

the standard mode. As shown in Figure 1-13, on the egress, the MPLS TTL decreases by

one and is mapped to the IP TTL field.





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Figure 1-13TTL process in Uniform mode

CE CEPE P PE

IP TTL

255

IP TTL

252

IP TTL

254

MPLS

TTL 253

IP TTL

254

MPLS

TTL 254

MPLS

l Pipe Mode

As shown in Figure 1-14, on the ingress, the IP TTL decreases by one and the MPLS TTL

is constant. Then, MPLS TTL is processed in the standard mode. On the egress, IP TTL

decreases by one. That is, when IP packets enter an MPLS network, the IP TTL only

decreases by one respectively on the ingress and egress.

Figure 1-14TTL process in Pipe mode

CE CEPE P PE

IP TTL

255

IP TTL

253

MPLS

MPLS

TTL 100

MPLS

TTL 100

IP TTL

254

MPLS

TTL 100

MPLS

TTL 99

IP TTL

254

1.3.4 MPLS Ping/Traceroute





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Overview

On an MPLS network, when data fails to be transmitted across an LSP, the MPLS control plane

cannot detect the transmission failure. Thus, the network maintenance is difficult to carry out.

MPLS ping and MPLS traceroute functions provide a mechanism to detect LSP faults and locatethe failed node timely.

MPLS ping is used to test the network connectivity and the host accessibility. MPLS traceroute

is used to check the network connectivity and locate network faults.

Similar to the IP ping and IP traceroute, the MPLS ping and the MPLS traceroute detect the LSP

availability through MPLS Echo Request and MPLS Echo Reply messages. These two messages

are sent through UDP. The port number is 3503. Thus, the receiver can recognize theses two

messages according to the received UDP port number.

The MPLS Echo Request message contains information about the FEC of the LSP to be detected.

The message is sent like other packets that belong to the FEC along the LSP. In this manner, theLSP is detected. The MPLS Echo Request message is forwarded to the destination by MPLS;

the MPLS Echo Reply message is forwarded to the source through an IP link.

The destination address in the IP header of the Echo Request message is set to 127.0.0.1/8 and

the IP TTL is set to 1. This can prevent the egress from forwarding the message to other nodes.

MPLS Ping

Figure 1-15MPLS network

5.5.5.5/32 4.4.4.4/32

1.1.1.1/30

1.1.1.2/30

2.2.2.1/30

2.2.2.2/30

3.3.3.1/30

3.3.3.2/30CX-A CX-B CX-C CX-D

LSP

As shown in Figure 1-15, an LSP whose FEC is identified with the destination being CX-D is

set up on CX-A. CX-A uses the MPLS ping feature to detect the LSP as follows:

1. CX-A checks whether the LSP exists. For a TE tunnel, CX-A checks whether the tunnel

interface exists and whether a CR-LSP is set up successfully. If the LSP does not exist, an

error message is returned and CX-A stops pinging. If the LSP exists, CX-A performs the

following actions continuously.

2. CX-A constructs an MPLS Echo Request packet. The destination address is 127.0.0.1/8 in

the IP packet header and the IP TTL is set to 1. CX-A searches the corresponding LSP and

pushes a label (its TTL is 255) of the LSP into the packet. Then, CX-A sends the packet to

CX-B.

3. CX-B and CX-C that serve as transits forward the MPLS Echo Request packet as a common

MPLS packet.

If a transit fails to forward the packet, the transit returns a reply message carrying the errorcode.





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4. When the MPLS forwarding path works normally, transits forward the packet successfully

to CX-D, namely, the egress of the LSP. CX-D processes the packet and replies with an

MPLS Echo Reply packet.

MPLS Traceroute

As shown in Figure 1-15, CX-A uses the MPLS traceroute feature to detect an LSP with the

destination address 4.4.4.4/32 as follows:

1. CX-A checks whether an LSP exists.

l If the LSP exists, CX-A performs the following actions continuously.

l If the LSP does not exist, an error message is returned and CX-A stops tracing the route.

2. CX-A constructs an MPLS Echo Request packet. The destination address is 127.0.0.1/8 in

the IP packet header and the IP TTL is 1. CX-A searches for a corresponding LSP and

pushes a label (its TTL is 1) of the LSP into the packet. Then, CX-A sends the packet to

CX-B. CX-B receives this packet and the TTL of the label times out. Then, an MPLS EchoReply message is returned. The destination UDP port and the destination IP address of the

MPLS Echo Reply message is the source UDP port and the source IP address of the MPLS

Echo Request packet. In additional, the IP TTL is 255.

3. After receiving the MPLS Echo Reply message, CX-A sends an MPLS Echo Request

packet. The TTL of the label is 2. CX-B forwards this packet as a common MPLS packet.

CX-C receives this packet and the TTL of the label times out. Then, an MPLS Echo Reply

message is returned.

If a transit fails to forward the packet, no MPLS Echo Reply message is returned.

4. After receiving the MPLS Echo Reply message, CX-A sends an MPLS Echo Request

packet. The TTL of the label is 3. CX-B and CX-C forward this packet as a common MPLS

packet. CX-D receives the packet and finds that the destination address of the packet is a

local loop IP address. Then, CX-D returns an MPLS Echo Reply message.

1.4 Applications

1.4.1 MPLS-based VPN

The traditional VPN can transmit data of private networks over the public network through

tunneling protocols, such as the Generic Routing Encapsulation (GRE), Layer 2 Tunneling

Protocol (L2TP), and Point to Point Tunneling Protocol (PPTP).

The MPLS-based VPN can be a private network whose security is similar to that of the FR

network. Because packets are not encapsulated or encrypted, the IPSec technology and GRE or

L2TP tunnels are not required on the device. In addition, the network delay is minimized.

As shown in Figure 1-16, the MPLS-based VPN integrates private network branches through

an LSP to form a unified network. The MPLS-based VPN controls the interconnection between

VPNs. Figure 1-16shows the devices applied in the MPLS-based VPN.

l Customer Edge (CE) is an edge device in a customer network. The CE can be a router, a

switch, or a host.

l Provider Edge (PE) is an edge device on a service provider network.





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l Provider (P) is a backbone device on an SP network. A P is not directly connected to CEs.

Ps only need to possess basic MPLS forwarding capabilities and do not maintain

information about a VPN.

Figure 1-16MPLS-based VPN

CE1

VPN

branch 1

PE1

Backbone network

VPN

branch 3

VPN

branch 2

PE3

PE2

CE3

CE2

The characteristics of MPLS-based VPN are as follows:

l PEs are responsible for managing VPN users, setting up LSPs between PEs, and allocating

routes to sites of a VPN.

l The route allocation between PEs are implemented by LDP or MBGP.

l The MPLS-based VPN supports the IP address multiplexing between sites and the

interconnection of different VPNs.

1.4.2 PBR to an LSP

The policy-based routing (PBR) means to select a route according to a user-defined policy for

security and load balancing. The CX600 supports the PBR to an LSP. In an MPLS network, IP

packets that meet the filtering policy can be forwarded through a specified LSP.

In Figure 1-17, CX-A, CX-B, CX-C, CX-D, and CX-E are in the original network. CX-F and

CX-G are added to provide new services. The traffic is forwarded as follows:

l The traffic for original services is forwarded through the original network.

l The traffic for new services is forwarded by CX-F and CX-G.





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Figure 1-17Application of the PBR to an LSP

CX-A CX-C

CX-B

CX-F

CX-D

CX-G

CX-E

To forward part of the traffic of new services through the original network, you can configure

the PBR to an LSP on CX-A. In this manner, the traffic meeting the specific policy can be

forwarded through the original network.

You can also use the PBR to the LSP together with LDP FRR to divert some traffic to the backup

LSP for load balancing when the backup LSP may be idle relatively.


Terms

Terms Explanation

Label space A value range of labels.

ILM The incoming label map (ILM) indicates the

mapping between an incoming label and a set

of NHLFEs. The ILM contains the following

information: Tunnel ID, incoming label and

incoming interface.

LDP peer Two LSRs with an LDP session that use LDP

to exchange label or FEC mapping

information.

LDP identifier A value that is used to identify a specified

LSR label space.





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Terms Explanation

NHLFE The next hop label forwarding entry

(NHLFE) can guide the MPLS packet

forwarding. An NHLFE contains the

following information: Tunnel ID, outgoing

interface, next hop, outgoing label and label

operation.

PHB PHB describes how the packets with the same

DSCP value are forwarded to the next hop.

The PHB records certain traffic attributes,

such as latency and packet loss ratio. At

present, the IETF defines three standardized

PHBs, that is, expedited forwarding (EF),

assured forwarding (AF), and best-effort

(BE). The BE is the default PHB.

Control plane The control plane is connectionless and

responsible for distributing labels, creating

the label forwarding table, and creating or

deleting LSPs.

Forwarding plane The forwarding plane, also known as the data

plane, is connection-oriented. It can apply

services and protocols of ATM, Frame Relay,

and Ethernet networks. The forwarding plane

is mainly responsible for adding labels to and

deleting labels from IP packets.

Simultaneously, it forwards the receivedpackets according to the label forwarding

table.

Abbreviation

Abbreviation Full Spelling

DoD Downstream-on-Demand

DU Downstream Unsolicited

LSP Label switched path

FEC Forwarding Equivalence Class

ILM Incoming Label Map

LAM Label Advertisement Mode

LDP Label Distribution Protocol

LER Label Edge Router

LFIB Label Forward Information Base





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Abbreviation Full Spelling

LSP Label Switched Path

LSR Label Switching Router

MPLS Multiprotocol Label Switching

NHLFE Next Hop Label Forwarding Entry

PHP Penultimate Hop Popping





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2MPLS LDPAbout This Chapter

2.1 Introduction to LDP

2.2 References

2.3 Principles



Feature Description - MPLS 2 MPLS LDP



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2.1 Introduction to LDP

Definition

Label Distribution Protocol (LDP) is a control protocol of Multiprotocol Label Switching

(MPLS). It is like a signaling protocol of a traditional network. It classifies Forwarding

Equivalence Classes (FECs), distributes labels, and establishes and maintains the Label

Switched Path (LSP). In addition, LDP defines the messages in and procedures for distributing

labels.

Purpose

MPLS supports multiple labels and its forwarding plane is connection-oriented, and thus this

excellent scalability enables the MPLS/IP-based network to provide various services. Through

LDP, Label Switching Routers (LSRs) directly map routing information at the network layer tothe switched paths at the data link layer, and thus establish LSPs at the network layer. LDP

features simple networking and configurations, supports route topology-driven establishment of

LSPs, and supports large-capacity LSPs, and thus is widely used to provide VPN services.

2.2 References

The following table lists the references of this document:

Document Description Remarks

RFC5036 LDP Specification Does not support loop detection.

RFC3215 LDP State Machine -

RFC5443 LDP IGP Synchronization Supports all messages except the

end-of-lib message.

RFC3478 Graceful Restart Mechanism

for Label Distribution Protocol

-

RFC1321 The MD5 Message-Digest

Algorithm

-

RFC3037 LDP Applicability -

RFC3988 Maximum Transmission Unit

Signalling Extensions for the

Label Distribution Protocol

-

2.3 Principles





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2.3.1 Concepts

The MPLS architecture involves multiple label distribution protocols, of which LDP (Label

Distribution Protocol) is widely used.

LDP defines the messages of label distribution and procedures for processing the messages.

LSRs associated by the incoming labels, next hop nodes, and outgoing labels for a specified

forwarding equivalence class (FEC) according to local forwarding table; thus LSPs are formed.

For details about LDP, refer to LDP Specification in RFC 5036.

LDP Adjacency

When an LSR receives a Hello message from the peer, it indicates that an LDP peer may exist.

Under this situation, the LSR will create an LDP adjacency for maintaining the existence of the

peer. There are two types of LDP adjacencies: the local adjacency and remote adjacency.

LDP Peers

LDP peers refer to two LSRs that use LDP to set up an LDP session and then exchange Label

messages.

LDP peers learn the labels of each other through LDP session between them.

LDP Sessions

In an LDP session, LSRs exchange messages such as Label Mapping and releasing. LDP sessions

are classified into the following types:

l Local LDP session: an LDP session between the two LSRs that are directly connected.l Remote LDP session: an LDP session between the two LSRs that are directly or indirectly

connected.

Local and remote LDP sessions can exist together.

LDP Dynamic Capability Announcement Function

The LDP dynamic capability announcement function allows an LDP extension to be dynamically

enabled or disabled on a device during an LDP session is working, ensuring the stability of an

LSP.

Relationships Between LDP Adjacencies, Peers, and Sessions

LDP maintains the existence of peers by using adjacencies. The type of peers depends on the

type of corresponding adjacencies. A peer can be maintained by multiple adjacencies. If a peer

is maintained by both local and remote adjacencies, the type of the peer supports coexistence of

the local and remote adjacencies. An LDP session can be established only when peers are present.

Type of LDP Messages

LDP messages have the following types:

l Discovery message: used to notify or maintain the existence of an LSR on a network.

l Session message: used to establish, maintain, and terminate sessions between LDP peers.





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l Advertisement message: used to create, modify, and delete label mappings for FECs.

l Notification message: used to provide advisory information and error information.

To ensure the reliability of message transmission, LDP uses the TCP transport for Session,

Advertisement, and Notification messages and uses the UDP transport for transmitting theDiscovery message only.

Label Spaces and LDP Identifiers

l Label space

Label space refers to the value range of labels that are distributed for LDP peers. The types

are as follows:

Per-Platform Label Space: indicates that the entire LSR uses one label space.

Per-Interface Label Space: indicates that each interface of the LSR is assigned with a

label space.

l LDP ID

The LDP identifier identifies the range of the label space of a specified LSR. The format

is :. The length is 6 bytes. Where,

LSR ID: indicates the LSR identifier and is of 4 bytes.

Label space ID: indicates the identifier of the label space, with a length of 2 bytes.

2.3.2 LDP Sessions

LDP Discovery Mechanism

The LDP discovery mechanism is used by an LSR to discover potential LDP peers. LDP

discovery mechanisms are classified into the following types:

l Basic discovery mechanism: used to discover directly-connected LSR peers on a link.

An LSR periodically sends LDP Hello messages to implement the basic discovery

mechanism and establish a local LDP session.

The Hello message contains the LDP identifier and other information (such as the hold time

and transport address). If the LSR receives an LDP Hello message on an interface, it

indicates that LDP peers are connected to the interface.

l Extended discovery mechanism: used to discover indirectly-connected LSR peers on a link.

The LSR periodically sends Targeted Hello messages to a specified address to implement

the extended discovery mechanism and establish a remote LDP session.

The Targeted Hello message, a UDP message, is sent to the specified address to LDP

interface 646. The Targeted message contains the LDP identifier and other information

(such as the transport address and hold time). If the LSR receives a Targeted Hello message

on an interface, it indicates that LDP peers are connected to the interface.

Procedures for Establishing an LDP Session

Two LSRs send Hello messages to each other to trigger the establishment of an LDP session.

Figure 2-1shows the procedures for establishing an LDP session.





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Figure 2-1Procedures for establishing an LDP session

LSR-B (passive role)

192.168.1.1/32LSR-A (active role)

192.168.1.2/32

Hello message

The actor sends an Initialization

message to negotiate about parameters

When the parameters are received,

an Initialization message and a

Keepalive message are sent

When the parameters are received,

a Keepalive message is sent

Step1

Step2

Step3

Step4

Step5

TCP Connection

1. Two LSRs send a Hello message to each other. The Hello message contains the transport

address that the two parties use to establish an LDP session. The role with the larger

transport address starts to establish a TCP connection as the active role. As shown in Figure

2-1, LSRA starts to establish the TCP connection as the active role and LSRB waits for the

TCP connection as the passive role.

2. After the TCP connection is successfully established, the active role LSRA sends an

Initialization message to negotiate parameters used for establishing the LDP session with

the passive role. These parameters include the LDP version, label distribution mode, value

of the Keepalive timer, maximum length of the PDU, and label space.

3. After receiving the Initialization message, if the passive role LSRB rejects certain

parameters, it sends a Notification message to terminate the establishment of the LDP

session. If the passive role LSRB accept all parameters, it sends an Initialization message

and a Keepalive message to the active role LSRA.

4. After receiving the Initialization message, if the active role LSRA cannot accept certain

parameters, it sends a Notification message to the passive role LSRB to terminate the

establishment of the LDP session. If the active role LSRB can accept all parameters, it

sends a Keepalive message to the passive role LSRB.

After both the two parties receives the Keepalive message from each other, the LDP session is

successfully established.

2.3.3 Advertising and Managing Labels

After the LDP session is established, an LDP exchanges messages, such as the label mapping

message, to establish LSP. RFC 5036 defines the label advertisement mode, label distribution

control mode, and label retention mode to determine how the LSR advertises and manages labels.

The CX600 supports the combination of the following modes:





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l Combination of the DU label advertisement mode, ordered label control mode, and liberal

label retention mode

l Combination of the DoD label advertisement mode, ordered label control mode, and

conservative label retention mode

Label Advertisement Mode

On an MPLS network, an LSR distributes labels for FECs and notifies its upstream LSRs of the

distributed labels. That is, labels are distributed from downstream to upstream on the MPLS

network.

Label advertisement modes are as follows:

l DU label advertisement mode

Downstream Unsolicited (DU): An LSR distributes a label for an FEC without having to

receive the Label Request message from its upstream LSR.

As shown in Figure 2-2, for the FEC destined for 192.168.1.1/32, the establishment of the

LSP is triggered in host mode. The egress sends an unsolicited Label Mapping message to

the upstream transit node to advertise the label of the host route to 192.168.1.1/32.

Figure 2-2DU mode

Distribute labels

upstream voluntarily

Ingress Transit Egress

Distribute labels

upstream voluntarily192.168.1.1/32

l Downstream on Demand

Downstream on Demand (DoD): An LSR distributes labels for FECs after receiving an

Label Request messages from its upstream LSRs.

As shown in Figure 2-3, the downstram egress triggers the establishment of an LSP

destined for the FEC 192.168.1.1/32 in host mode. The upstream ingress sends the Label

Request message, the downstream egress receives this message, and then sends the Label

Mapping message to the downstream.

Figure 2-3DoD mode

Ingress Transit Egress

192.168.1.1/32The upstream requests

the downstream for labels

The upstream requests the

downstream for labels

The label is

distributed after the

request is received





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The label advertisement mode on an upstream LSR and that on an downstream LSR must be

consistent.

Label Distribution Control Mode

The label distribution control mode refers to the processing mode used by an LSR when the LSR

distributes labels during the setup of an LSP.

The label distribution control modes are classified into the following modes:

l Independent label distribution control

Independent label distribution control refers to that a local LSR can distribute a label bound

to a FEC and then inform the upstream LSR, without waiting for the label distributed by

the downstream LSR.

As shown in Figure 2-2, if the label distribution mode is DU and the label distribution

control mode is Independent, the transit LSR distributes labels for the ingress without

waiting for labels of the egress. As shown in Figure 2-3, if the label distribution mode is DoD and the label distribution

control mode is Independent, the directly-connected transit of the ingress that sends the

Label Request message directly replies with labels without waiting for labels of the

egress.

l Ordered label distribution control

Ordered label distribution control: An LSR advertises the mapping between a label and a

FEC to its upstream LSRs only when it is the outgoing node of the FEC or when it receives

the Label Mapping message of the next hop for the FEC.

As shown in Figure 2-2, the label distribution mode is DU and the label distribution

control mode is ordered. Consequently, the LSR (transit in the diagram) must receive

a label mapping message from the downstream (egress in the diagram). Then, it candistribute a label to the upstream (ingress in the diagram).

As shown in Figure 2-3, if the label distribution mode is DoD and the label distribution

control mode is Ordered, the directly-connected transit of the ingress that sends the

Label Request message must receive a label mapping message from the downstream

(egress in the diagram). Then, it can distribute a label to the upstream (ingress in the

diagram).

Label Retention Mode

The label retention mode refers to the way an LSR handles processes the label mapping that it

receives but does not use in a short time.

The label mapping that an LSR receives maybe from the next hop, maybe not.

The label retention modes are classified into the following modes:

l Liberal label retention mode

An LSR preserves the Label Mapping message received from a neighbor LSR regardless

of whether the neighbor LSR is its next hop or not.

l Conservative label retention mode

An LSR retains the label mapping received from a neighbor LSR only when the neighbor

LSR is its next hop.

When the next hop of an LSR changes due to change of the network topology:





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l If the LSR uses the liberal label retention mode, it can use the previous labels sent from the

non-next hop to fast re-establish an LSP (For the information about establishment of LDP

LSP , refer to Establishment of LDP LSP). This requires more memory and label spaces

than the conservative mode.

lIf the LSR uses the conservative label retention mode, it retains only labels received fromthe next hop. This saves memory and the label space but the LSP is re-set up slowly.

Conservative label retention mode is usually used together with DoD on the LSRs that have

limited label spaces.

The LSP is distributed with a label but not established successfully called Liberal LSP.

2.3.4 Establishment of LDP LSP

The LSP establishment is the process of binding an FEC with the label and advertising the

binding relationship to LSRs on the LSP. The procedures for establishing an LSP in DU label

distribution mode and ordered label control mode are described as follows:

1. When the route of the network changes, if a label edge router (LER) finds a new destination

address in its routing table and the address does not belong to any existing FEC, the LER

creates an FEC for the address.

2. If the egress of an MPLS network has available labels for distribution, it distributes labels

for FECs and actively sends the Label Mapping message to the ingress. The Label Mapping

message contains distributed labels and bound FECs.

3. After receiving the Label Mapping message, the LSR adds mapping to its label forwarding

table and then actively sends the Label Mapping message of the specified FEC to the ingress

LSR.

4. After receiving the Label Mapping message, the ingress LSR also adds the mapping to its

label forwarding table. An LSP is thus established, and the packets classified as the FECcan be forwarded on the basis of the label.

2.3.5 LDP Extension for Inter-Area LSP

This feature enables LDP to establish inter-area LDP LSPs to provide tunnels that traverse the

public network.





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(such as the carried FEC is 1.3.0.1/32) from Area 10, LSRA searches for a route according to

the longest match rule defined in RFC 5283. Then, LSRA finds information about the aggregated

route 1.3.0.0/24, and uses the outbound interface and next hop of this route as those of the route

1.3.0.1/32. In this manner, LDP can establish inter-area LDP LSPs.

2.3.6 Outbound and Inbound LDP Policies

By default, an LSR receives and sends label mapping messages for all FECs, resulting in the

establishment of a large number of LDP LSPs. The establishment of a large number of LDP

LSPs consumes a great amount of system resources of an LSR, and as a result, the LSR may be

overburdened. In this case, an outbound or inbound LDP policy needs to be configured to reduce

the number of label mapping messages to be sent or received, reducing the number of LSPs to

be established and saving memory.

Outbound LDP Policy

The outbound LDP policy filters label mapping messages to be sent. The outbound LDP policydoes not take effect with L2VPN label mapping messages, which means that all the L2VPN

label mapping messages can be sent. In addition, the ranges of FECs to which the labeled BGP

routes and non-BGP routes are mapped can be configured separately.

If FECs in the label mapping messages to be sent to an LDP peer group or all LDP peers are in

the same range, the same outbound policy is applicable to the LDP peer group or all LDP peers.

In addition, the outbound LDP policy supports split horizon. After split horizon is configured,

an LSR distributes labels only to its upstream LDP peers.

An LSR checks whether an outbound policy mapped to the labeled BGP route or non-BGP route

is configured before sending a label mapping message for a FEC.

l If no outbound policy is configured, the LSR sends the label mapping message.

l If an outbound policy is configured, the LSR checks whether the FEC in the label mapping

message is within the range defined in the outbound policy. If the FEC is within the FEC

range, the LSR sends a label mapping message for the FEC; if the FEC is not in the FEC

range, the LSR does not send a label mapping message.

If the FEC to which the route mapped fails to pass any outbound policy, no transit LSP or egress

LSP can be established.

Inbound LDP Policy

The inbound LDP policy filters label mapping messages to be received. The inbound LDP policydoes not take effect with L2VPN label mapping messages, which means that all the L2VPN

label mapping messages can be received. In addition, the range of FECs to which the non-BGP

routes are mapped is configurable.

If FECs in the label mapping messages to be received by an LDP peer group or all LDP peers

are in the same range, the same inbound policy is applicable to the LDP peer group or all LDP

peers.

An LSR checks whether an inbound policy mapped to a FEC is configured before receiving a

label mapping message for the FEC.

l If no inbound policy is configured, the LSR receives the label mapping message.

l

If an inbound policy is configured, the LSR checks whether the FEC in the label mappingmessage, is within the range defined in the inbound policy. If the FEC is within the FEC





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range, the LSR receives the label mapping message for the FEC; if the FEC is not in the

FEC range, the LSR does not receive the label mapping message.

If the FEC fails to pass any outbound policy on an LSR, the LSR receives no label mapping

message for the FEC.

In this case, one of the following results may occur:

l If a DU LDP session is established between an LSR and its peer, a liberal LSP is established.

In addition, this liberal LSP cannot function as a backup LSP after LDP FRR is enabled.

l If a DoD LDP session is established between an LSR and its peer, the LSR sends a Release

message to tear dow

Documents

3.2.7 Feature Description - MPLS