Label Distribution Protocol and Basic

MPLS Configuration
APNIC Technical Workshop

Issue Date: [201609]

Revision: [01]
Label Distribution
Protocol

2
MPLS Builders
Which protocols can set up
Label Switched Path?
Pure Signaling Routing Protocols
MPLS Protocols with Extensions
Most classic
and
widespread

LDP BGP

RSVP-TE IGP

3
Advantages of LDP
• Reliability
– LDP uses reliable TCP as the transport protocol for all but the
discovery messages.
• Auto provision
– Abilities to set up LSPs dynamically based on routing information
• Plug-and-play
– Simple deployment and configuration
• Support for a large number of LSPs

4
LDP Identifier
• An LDP Identifier is a six octet quantity used to identify an
LSR label space.
4 byte 2 byte

LSR ID Label Space ID

Label Space ID = 0
10.10.1.1 0 Label space is per platform
Label Space ID ≠ 0
20.20.20.2 6 Label space is per interface

R2#show mpls ldp discovery

Local LDP Identifier:
2.2.2.2:0
Discovery Sources:
Interfaces:
FastEthernet0/0 (ldp): xmit/recv
LDP Id: 3.3.3.3:0
Ethernet1/0 (ldp): xmit/recv
LDP Id: 1.1.1.1:0
Cisco IOS

5
Label Space – Per Platform
• In per-platform label space, one single label is assigned to a
destination network and announced to all neighbors. The label
must be locally unique and valid on all incoming interfaces.
Prefix Out Label
100.1.1.0/24 100
200.1.1.0/24 200

R1

E1/1 100.1.1.0/24
E1/2 200.1.1.0/24
R3
R2
In Label Prefix
Prefix Out Label 100 100.1.1.0/24
100.1.1.0/24 100 200 200.1.1.0/24
200.1.1.0/24 200

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Label Space – Per Interface
• In per-interface label space, local labels are assigned to IP
destination prefixes on a per-interface basis. These labels must
be unique on a per-interface basis.
Prefix Out Label
100.1.1.0/24 1/300
200.1.1.0/24 1/200

R1

ATM4/1 100.1.1.0/24
ATM4/2 200.1.1.0/24
R3
R2
In Label In Interface Prefix
Prefix Out Label 1/300 ATM 4/1 100.1.1.0/24
100.1.1.0/24 1/400 1/200 ATM 4/1 200.1.1.0/24
200.1.1.0/24 1/500 1/400 ATM 4/2 100.1.1.0/24
1/500 ATM 4/2 200.1.1.0/24

7
LDP Operations

Step 1

Neighbor Discovery

Step 2

Session Establishment

Step 3

Label Distribution

8
LDP Messages
Category Function Message Name
Discovery Announce and maintain the Hello
presence of an LSR in a network
Session Establish, maintain, and Initialization
terminate sessions between LDP
peers Keepalive
Label Create, change, and delete label Label Release
Distribution mappings for FECs
Label Request
Label Abort Request
Label Mapping
Label Withdrawal
Notification Provide advisory information and Notification
to signal error information
(Not list all the messages)

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LDP Neighbor Discovery (1)
• Basic Discovery – Directly connected peer
– LDP Hello messages are UDP messages that are sent on the links to
the “all routers on this subnet” multicast IP address - 224.0.0.2. The
UDP port used for LDP is 646.
UDP: Hello
(2.2.2.2:1064 → 224.0.0.2:646)

LDP
R2: 2.2.2.2
UDP: Hello
(1.1.1.1:1050 → 224.0.0.2:646)
NO_MPLS
LDP
R1: 1.1.1.1 R3: 3.3.3.3
UDP: Hello
(4.4.4.4:1027 → 224.0.0.2:646)

LDP
R4: 4.4.4.4

10
LDP Neighbor Discovery (2)
• Extended Discovery – Non-directly connected peer
– LDP sessions between non-directly connected LSRs are supported by LDP Extended
Discovery.

Targeted LDP Session

R1:1.1.1.1 R2: 2.2.2.2 R3: 3.3.3.3 R4: 4.4.4.4

UDP: Targeted Hello

(1.1.1.1:1080 → 4.4.4.4:646)

UDP: Targeted Hello

(4.4.4.4:1012 → 1.1.1.1:646)

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LDP Session Establishment and Maintenance

My LSR-ID is After neighbor discovery, My LSR-ID is

smaller, I am in 2 neighbors start to larger, I am in
Passive role. establish session Active role.

R1: 1.1.1.1 R2: 2.2.2.2

Establish TCP Connection (Initiated by R2)

Initialization Message

Initialization Message

Keepalive Message

Keepalive Message

Session UP Session UP

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Label Distribution and Management
• After LDP sessions are established, labels will be distributed
between LDP peers. The label distribution mode used depends
on the interface and the implementation.

Label Distribution Ordered

Control Mode Independent

DoD
Label Advertisement (Downstream on Demand)
Mode DU
(Downstream Unsolicited)

Liberal
Label Retention Mode
Conservative

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Label Distribution Control Mode
- Ordered
• In Ordered control mode, an LSR would only assign a local label
for the IGP prefixes that are marked as directly connected in its
routing table or also for the IGP prefixes for which it has already
received a label from the nexthop router.

Upstream Downstream

LDP LDP LDP Loopback 2:

100.1.1.1/32

R1 R2 2
R3 1
R4
3
Label Mapping Label Mapping Label Mapping
To 100.1.1.1/32 To 100.1.1.1/32 To 100.1.1.1/32
Label=100 Label=200 Label=300

14
Label Distribution Control Mode - Independent
• In the independent mode, each LSR creates a local binding for a
particular FEC as soon as it recognizes the FEC. Usually, this
means that the prefix for the FEC is in its routing table.

Upstream Downstream

LDP LDP LDP

100.1.1.1/32

R1 R2 R3 2
R4
1
Label Mapping Label Mapping
To 100.1.1.1/32 To 100.1.1.1/32
Label=100 Label=300

15
Label Advertisement Mode - Downstream on Demand

• In the DoD mode, an LSR distributes labels to a specified

FEC only after receiving Label Request messages from its
upstream LSR.

Upstream Downstream
Label Request Label Request Label Request
for 100.1.1.1/32 for 100.1.1.1/32 for 100.1.1.1/32

LDP LDP LDP

100.1.1.1/32

R1 R2 R3 R4
Label Mapping Label Mapping Label Mapping
To 100.1.1.1/32 To 100.1.1.1/32 To 100.1.1.1/32
Label=100 Label=200 Label=300

16
Label Advertisement Mode - Downstream Unsolicited

• In the DU mode, each LSR distributes a label to its

upstream LSRs, without those LSRs requesting a label.

Upstream Downstream

LDP LDP LDP

100.1.1.1/32

R1 R2 R3 R4
Label Mapping Label Mapping Label Mapping
To 100.1.1.1/32 To 100.1.1.1/32 To 100.1.1.1/32
Label=100 Label=200 Label=300

17
Label Retention Mode - Liberal
• In the liberal mode, an LSR keeps all received remote
labels in the LIB, but not all are used to forward packets.
Upstream Downstream

Prefix Out Label To 100.1.1.1/32,

100.1.1.1/32 200 NH= R3

100.1.1.1/32 500(Liberal)
Label Mapping
To 100.1.1.1/32
Label=200

LDP LDP LDP

GE
100.1.1.1/32
GE

R1 R2 R3 R4

LDP
FE

R5 R6

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Label Retention Mode - Conservative
• An LSR that is running this mode does not store all remote
labels in the LIB, but it stores only the remote label that is
associated with the next-hop LSR for a particular FEC.
Upstream To 100.1.1.1/32,
Downstream
NH= R3
Prefix Out Label
100.1.1.1/32 200
Label Mapping
To 100.1.1.1/32
Label=200

LDP LDP LDP

ATM
100.1.1.1/32
ATM

R1 R2 R3 R4

LDP
ATM

R5 R6

19
Label Distribution Scheme Summary
• Cisco IOS can support:
Control Distribution Retention Label Space

Frame Mode Independent DU Liberal Per Platform

Cell Mode (LC ATM) Ordered DoD Conservation Per Interface

• Junos can support:

Control Distribution Retention

Ordered DU Liberal

DoD

• Huawei VRP can support:

Control Distribution Retention
Ordered DU Liberal By default
Ordered DoD Conservation Also support

20
LDP Session Protection (1)
• Without LDP session protection, if the link between R1 and R2 fails, the LDP
direct link adjacency fails.

LDP Session

R1 R2

R3

21
LDP Session Protection (2)
• MPLS LDP Session Protection uses LDP Targeted Hellos to
protect LDP sessions.
Targeted LDP Session

LDP Session

R1 R2

On R1 and R2:
1. Establish 2 • The LDP link adjacency is
LDP sessions
2. Enable LDP removed when the link goes
session down, but the targeted
protection
R3 adjacency keeps the LDP
session up.

22
Questions?
Basic MPLS
Configuration

24
Configuration Example
• Task: Configure MPLS LDP on Cisco IOS (Version 15.2) to
set up MPLS LSP only for loopback addresses.
• Prerequisite configuration:
– 1. IP address configuration on all the routers
– 2. IGP configuration on all the routers

Loopback0 Loopback0 Loopback0 Loopback0

10.0.0.1/32 10.0.0.2/32 10.0.0.3/32 10.0.0.4/32
Fa0/0 Fa0/0
E1/0 E1/0 10.23.1.0/30 E1/0 E1/0
.1 .2
.1 10.12.0.0/30 .2 10.23.2.0/30 .1 10.34.0.0/30 .2
Fa0/1 Fa0/1

R1 R2 R3 R4

MPLS Network

25
Step 1: Enable MPLS & LDP
• Configuration steps:
– 1. Configure basic MPLS and LDP on all the routers.
R1 configuration:
R1(config)# ip cef
To make MPLS work, CEF switching is mandatory.
R1(config)# mpls label range 100 199
Specifying the label range for this router start from
100 to 199.
R1(config)# mpls ldp router-id loopback 0 force
Forcing LDP router ID to be loopback 0 address.
R1(config)# interface ethernet 1/0
R1(config-if)# mpls ip
IP MPLS is enabled on the interface.
R1(config-if)# mpls label protocol ldp
Label distribution protocol is LDP.

26
Check MPLS Interface
• Using show mpls interfaces command verifies that interfaces have been
configured to use LDP:

R1#show mpls interfaces

Interface IP Tunnel BGP Static Operational
Ethernet1/0 Yes (ldp) No No No Yes

R2#show mpls interfaces

Interface IP Tunnel BGP Static Operational
FastEthernet0/0 Yes (ldp) No No No Yes
FastEthernet0/1 Yes (ldp) No No No Yes
Ethernet1/0 Yes (ldp) No No No Yes

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Check LDP Discovery
• Check LDP discovery

R2#show mpls ldp discovery

Local LDP Identifier:
10.0.0.2:0 Local LDP ID
Discovery Sources:
Interfaces:
FastEthernet0/0 (ldp): xmit/recv
LDP Id: 10.0.0.3:0 Neighbor’s LDP ID
FastEthernet0/1 (ldp): xmit/recv
LDP Id: 10.0.0.3:0
Ethernet1/0 (ldp): xmit/recv
LDP Id: 10.0.0.1:0 R2 has received Hello
messages from
routers whose ID are
10.0.0.3 and 10.0.0.1

28
Check LDP Neighbors
• Check LDP neighbors on R1

R1#show mpls ldp neighbor

Peer LDP Ident: 10.0.0.2:0; Local LDP Ident 10.0.0.1:0
TCP connection: 10.0.0.2.48548 - 10.0.0.1.646 LDP session is
State: Oper; Msgs sent/rcvd: 34/34; Downstream a TCP session
Up time: 00:09:57 (port = 646)
LDP discovery sources:
Ethernet1/0, Src IP addr: 10.12.0.2
Addresses bound to peer LDP Ident:
10.23.1.1 10.23.2.1 10.12.0.2 10.0.0.2

Operational is the
stable state of LDP
session.

29
Check LDP Neighbors
• Check LDP neighbors on R2
R2#show mpls ldp neighbor
Peer LDP Ident: 10.0.0.3:0; Local LDP Ident 10.0.0.2:0
TCP connection: 10.0.0.3.28664 - 10.0.0.2.646
State: Oper; Msgs sent/rcvd: 36/36; Downstream
Up time: 00:12:12
LDP discovery sources: Multiple links
FastEthernet0/0, Src IP addr: 10.23.1.2 between two routers
FastEthernet0/1, Src IP addr: 10.23.2.2 still mean single
Addresses bound to peer LDP Ident:
10.23.1.2 10.23.2.2 10.34.0.1 10.0.0.3
LDP session.
Peer LDP Ident: 10.0.0.1:0; Local LDP Ident 10.0.0.2:0
TCP connection: 10.0.0.1.646 - 10.0.0.2.48548
State: Oper; Msgs sent/rcvd: 36/36; Downstream
Up time: 00:11:40
LDP discovery sources:
Ethernet1/0, Src IP addr: 10.12.0.1
Addresses bound to peer LDP Ident:
10.12.0.1 10.0.0.1

Fa0/0 Fa0/0
E1/0 E1/0 10.23.1.0/30 E1/0 E1/0
.1 .2
.1 10.12.0.0/30 .2 10.23.2.0/30 .1 10.34.0.0/30 .2
Fa0/1 Fa0/1

R1 R2 R3 R4

30
Check LDP Label Information Base
• Check LDP LIB
R2#show mpls ldp bindings
lib entry: 10.0.0.1/32, rev 24
local binding: label: 206
remote binding: lsr: 10.0.0.3:0, label: 302
remote binding: lsr: 10.0.0.1:0, label: imp-null
lib entry: 10.0.0.2/32, rev 18
local binding: label: imp-null
remote binding: lsr: 10.0.0.3:0, label: 301
remote binding: lsr: 10.0.0.1:0, label: 101
lib entry: 10.0.0.3/32, rev 22
local binding: label: 205
remote binding: lsr: 10.0.0.3:0, label: imp-null
remote binding: lsr: 10.0.0.1:0, label: 100
...........(omitted)

R2#show mpls ldp bindings 10.0.0.4 32

lib entry: 10.0.0.4/32, rev 20
local binding: label: 204
remote binding: lsr: 10.0.0.3:0, label: 300
remote binding: lsr: 10.0.0.1:0, label: 107

31
Check Label Forwarding Table
• Check label forwarding table

R2#show mpls forwarding-table

Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
203 Pop Label 10.34.0.0/30 0 Fa0/0 10.23.1.2
Pop Label 10.34.0.0/30 0 Fa0/1 10.23.2.2
204 300 10.0.0.4/32 0 Fa0/0 10.23.1.2
300 10.0.0.4/32 0 Fa0/1 10.23.2.2
205 Pop Label 10.0.0.3/32 0 Fa0/0 10.23.1.2
Pop Label 10.0.0.3/32 0 Fa0/1 10.23.2.2
206 Pop Label 10.0.0.1/32 0 Et1/0 10.12.0.1

32
Step 2: Configure Conditional Label Distribution
• Only set up LSP for loopback addresses.

– 2.1 Create prefix-list on each router, all the loopback addresses are
in 10.0.0.0/24 block.
R1(config)#ip prefix-list ALL-LOOPBACK seq 5 permit
10.0.0.0/24 le 32

– 2.2 Apply the prefix-list

R1(config)#mpls ldp label
R1(config-ldp-lbl)#allocate global prefix-list ALL-LOOPBACK

Allocate labels for the routes matching ALL-LOOPBACK

prefix-list.

33
Verify the Results of Conditional Label Distribution
• Before the configuration.
R1#show mpls ldp bindings
lib entry: 10.0.0.1/32, rev 51
local binding: label: imp-null
remote binding: lsr: 10.0.0.2:0, label: 206
lib entry: 10.0.0.2/32, rev 52
local binding: label: 101
remote binding: lsr: 10.0.0.2:0, label: imp-null
lib entry: 10.0.0.3/32, rev 53 Entries for all the
local binding: label: 100 prefixes in IP routing
remote binding: lsr: 10.0.0.2:0, label: 205 table.
lib entry: 10.0.0.4/32, rev 54
local binding: label: 107
remote binding: lsr: 10.0.0.2:0, label: 204
lib entry: 10.12.0.0/30, rev 71
local binding: label: imp-null
remote binding: lsr: 10.0.0.2:0, label: imp-null
lib entry: 10.23.1.0/30, rev 72
local binding: label: 102
remote binding: lsr: 10.0.0.2:0, label: imp-null
lib entry: 10.23.2.0/30, rev 73
local binding: label: 103
remote binding: lsr: 10.0.0.2:0, label: imp-null
lib entry: 10.34.0.0/30, rev 75
local binding: label: 104
remote binding: lsr: 10.0.0.2:0, label: 200

34
Verify the Results of Conditional Label Distribution
• After configure on all the routers.

R1#show mpls ldp bindings

lib entry: 10.0.0.1/32, rev 51
local binding: label: imp-null
remote binding: lsr: 10.0.0.2:0, label: 206 Only the entries for
lib entry: 10.0.0.2/32, rev 52 loopback addresses.
local binding: label: 101
remote binding: lsr: 10.0.0.2:0, label: imp-null
lib entry: 10.0.0.3/32, rev 53
local binding: label: 100
remote binding: lsr: 10.0.0.2:0, label: 205
lib entry: 10.0.0.4/32, rev 54
local binding: label: 107
remote binding: lsr: 10.0.0.2:0, label: 204

35
Check the LSP
• Check the LSP for 10.0.0.4 from R1 to R3
R1#show mpls forwarding-table 10.0.0.4 32
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
107 204 10.0.0.4/32 0 Et1/0 10.12.0.2

R2#show mpls forwarding-table 10.0.0.4 32

Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
204 300 10.0.0.4/32 0 Fa0/0 10.23.1.2
300 10.0.0.4/32 0 Fa0/1 10.23.2.2

R3# show mpls forwarding-table 10.0.0.4 32

Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id If I want
Switched to use an
interface
300 Pop Label 10.0.0.4/32 outgoing
0 label at the
Et1/0 10.34.0.2
penultimate hop for keeping
QoS info. What can I do?
Implicit Null

36
Explicit-null Label
• Explicit Null label can be used to keep the QoS information.

Explicit Null (IPv4) = Label 0 Explicit Null (IPv6) = Label 2

Prefix: 10.0.0.4/32 Prefix: 10.0.0.4/32 Prefix: 10.0.0.4/32 Prefix: 10.0.0.4/32

Local Label Null Local Label 204 Local Label 300 Local Label Explicit-n

Out Interface E1/0 Out Interface Fa0/0 Fa0/1 Out Interface E1/0 Out Interface --

Out Label 204 Out Label 300 Out Label Explicit-n Out Label --

Operation Push Operation Swap Operation Swap Operation POP

204 IP:10.0.0.4 300 IP:10.0.0.4 0 IP:10.0.0.4

Push Swap Swap Pop

Fa0/0 Fa0/0
E1/0 E1/0 E1/0 E1/0

Fa0/1 Fa0/1

R1 R2 R3 R4

37
Additional Task: Using Explicit-null Label
• Explicit-null configuration on R4:

R4(config)# mpls ldp explicit-null

• After configuring this command, check the label forwarding

table on R3.

R3#show mpls forwarding-table 10.0.0.4 32

Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
300 explicit-n 10.0.0.4/32 0 Et1/0 10.34.0.2

Out label is
explicit-null

38
Questions?