ODA000015 MPLS Basic Knowledge

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ODA000015 MPLS Basic Knowledge

• 1.0

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Course Contents
Chapter 1 MPLS Overview Chapter 2 Label and Label
Stack Chapter 3 Label Forwarding and
Allocation Chapter 4 LDP and Configuration

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MPLS

• MPLSMulti-Protocol Label Switching
• Multi-Protocol
• Support multiple Layer-3 protocols, such as IP,
  IPv6, IPX, SNA
• Label Switching
• Label packets, and replace IP forwarding with
  label switching

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Origin To Integrate IP with ATM
IP
ATM
MPLS
Connectionless control plane
Connectionless control plane
Connection-oriented control plane
Connectionless forwarding plane
Connection-oriented forwarding plane
Connection-oriented forwarding plane
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Connection-oriented Features
S2
S6
S1
S8
S3
S5
VC
S4
S7

• connection-oriented cell switching
• VC S1, S4, S7, S8
• The data reach their destination in order along
  the same connection
• Fixed time delay, easy to control
• Connection types PVC SVC

• Connectionless packet route
• Path 1 S1, S2, S6, S8
• Path 2 S1, S4, S7, S8
• The data reach their destination out of order
  along different paths

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Traditional IP Forwarding
Parse IP header mapped to next hop
Parse IP header mapped to next hop
Parse IP header mapped to next hop

• IP header is parse at each hop, resulting in low
  efficiency.
• It is hard to deploy QoS and the efficiency is
  rather low.
• All routers are expected to know all routes in
  the entire network.

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ATM Switching Process
Virtual Channel Connection (VCC)
Virtual Path Connection(VPC)
UNI
UNI
NNI
NNI
VPswitching
VCswitching
VCswitching
VPI 2VCI 44
VPI 1VCI 1
VPI 26VCI 44
VPI 20VCI 30

• Connection-oriented
• Routing depending on link layer, based on VPI/VCI
  or label
• Ensure QoS and real-time service

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Technology Combining the Advantages of ATM and IP


X
R
X
Router ATM switch MPLS

Router

• Layer 3 routing scalable and flexible
• Layer 2 switching High reliability and traffic
  engineering management

MPLSmulti-protocol label switching
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MPLS Advantages

• Replace IP header with short and fixed-length
  labels as forwarding basis to improve forwarding
  speed
• Provide value-added service without prejudice to
  efficiency
• VPN
• Traffic engineering
• QOS

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Basic Working Process of MPLS
Core LSR
Edge LSR
Edge LSR
IP
IP
Traditional IP forwarding
Traditional IP forwarding
Label forwarding
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Basic MPLS Concepts
LSR Label Switch Router LER Label Edge
Router LSP Label Switch Path
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Course Contents
Chapter 1 MPLS Overview Chapter 2 Label and Label
Stack Chapter 3 Label Forwarding and
Allocation Chapter 4 LDP and Configuration

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MPLS Encapsulation Format and Label
20
0
23
24
31
Label
S
EXP
TTL
32 bits
Layer 2 header
MPLS header
IP header
Data

• Two types of MPLS encapsulation for ATM and FR
• shim encapsulation similar to other link layers
• Cell mode VC (VPI/VCI for ATM, DLCI for FR) is
  directly used as the label

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Label Position in Packet
Ethernet /SONET /SDH packet
Ethernet header /PPP header
Label
Layer-3 data
Cell mode ATM packet
VPI/VCI
Layer-3 data
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Label Stack
MPLS header
Layer2 header
MPLS header
IP header
Data
Theoretically, label stack enables limitless
nesting to provide infinite service support. This
is simply the greatest advantage of MPLS
technology.
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Course Contents
Chapter 1 MPLS Overview Chapter 2 Label and Label
Stack Chapter 3 Label Forwarding and
Allocation Chapter 4 LDP and Configuration

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Basic Concepts of Label Forwarding

• FEC (Forwarding Equivalence Class) Import the
  packets with identical characteristics into the
  same LSP
• NHLFE (Next Hop Label Forwarding Entry)
  Describe label operations
• next hop
• label operation types push/pop/swap
• FTN (FEC to NHLFE) Map FEC to NHLFE
• ILM (Incoming Label Map) Map MPLS label to NHLFE

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Label Forwarding
label operation pop
Label operation push
ILM-gtNHLFE Parse IP header distribute FEC mapped
to next hop
Label operation swap
Label operation swap
Parse IP header FEC bound with LSP FTN-gtNHLFE
ILM-gtNHLFE
ILM-gtNHLFE
A
B
C
D
Egress LER
Ingress LER
LSR
LSR

• The traditional routing protocol and Label
  Distribution Protocol (LDP) serve to create
  routing table and label mapping table (FEC-Label
  mapping) in each LSR for FECs with service
  requirement, i.e. create LSP successfully.
• Ingress LER receives a packet, determines the FEC
  that the packet belongs to, and label the packet
• In MPLS domain, packets are forwarded in
  accordance with labels and label forwarding table
  via the forwarding unit
• Egress LER removes the label and continues
  forwarding the packet

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NHLFE
A
NHLFE
FEC
Others
Label operation
next hop
Transmitting interface

Add label L1
E1
10.0.1.0/24
B
B,C
NHLFE
Ingress label
Transmitting interface
Next hop
Others
label operation
C
L1

E1
Remove the previous label and add L2
D
NHLFE
Ingress label
Others
Transmitting interface
Label operation
Next hop

Remove label
D
L2
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PHP
Label operation push
Label operation swap
Parse IP header Distribute FEC Mapped to next hop
Label operation pop
Parse IP header FEC bound with LSP FTN-gtNHLFE
ILM-gtNHLFE
ILM-gtNHLFE
LSR
Ingress LER
LSR
Egress LER

• The label at the outmost layer does not make any
  sense to the last hop. Thus, it is advisable to
  pop the label at the last hop but one to ease the
  burden of the last hop.
• If there is only one layer of label, the last hop
  will perform IP forwarding directly otherwise,
  it will perform the internal label forwarding.

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Creating LSP

• LSP drive modes
• Driven by stream incoming packets drive LSP
  creation
• Driven by topology topology information (route)
  drives LSP creation
• Driven by application application (like QoS)
  drives LSP creation
• Signaling protocol is used to distribute labels
  between LSRs and establish LSP
• LDP
• CR-LDP (Constraint-based Routing LDP)
• RSVP-TE (Resource Reservation Protocol)
• MP-BGP
• PIM

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Several Issues Concerning Label Distribution

• Label allocation mode
• DoD downstream-on-demand
• DU downstream unsolicited
• Label control mode
• Ordered
• Independent
• Label hold mode
• Conservative retention mode upon receiving a
  label, if there is no route destined for the
  corresponding FEC, hold the label for later use
• Liberal mode upon receiving a label, if there is
  no route destined for corresponding FEC, discard
  the label

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Label Allocation Mode DoD
Label 18 is allocated to 171.68.10/24
Route triggering
171.68.10/24
???
171.68.10/24
???
Label 20 is allocated to 171.68.10/24
20
18
????
????
171.68.10/24
171.68.40/24
LSR1 LSR2
LSR3
Downstream
Upstream
Requesting labels destined for 171.68.10/24
???????
Requesting labels destined for 171.68.10/24
171.68.10/24
???
???
The upstream LSR sends a label request
(containing FEC description information) to the
downstream LSR. The downstream LSR allocates a
label to this FEC and feeds back the bound label
to the upstream LSR via the label mapping message.
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Label Allocation Mode DU
Route triggering
171.68.10/24
?
171.68.10/24
?
Label 18 can be used to reach 171.68.10/24
Downstream
Label 20 can be used to reach 171.68.10/24
Upstream
20
18
??????
??????
171.68.10/24
171.68.40/24
Once the LDP session is set up successfully, the
downstream LSR will initiatively advertise the
label mapping message to its upstream LSR. The
upstream router will save the label in the label
mapping table.
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Label Control Mode Ordered
Downstream
Upstream
Not until it receives a label mapping message
from its downstream LSP will it send the message
upstream
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Label Control Mode Independent
Downstream
Upstream
Whether it receives a label mapping message from
its downstream LSR, it will send upstream a label
mapping message immediately.
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Common Collocation 1 DoD Ordered Liberal
Downstream
Upstream

• It is relatively easy to control the use of
  labels and the creation of LSPs

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Common Collocation 2 DU Ordered Conservative
Upstream
Downstream

• A waste of label resources
• Useless LSPs would be created
• LSPs can be set up quickly

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LSP Loop Detection

• Path looping shall be avoided even in setting up
  LSP within the MPLS domain.
• LSP path looping can be avoided in two ways
• Maximum hop number
• Path vector

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Course Contents
Chapter 1 MPLS Overview Chapter 2 Label and Label
Stack Chapter 3 Label Forwarding and
Allocation Chapter 4 LDP and Configuration

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Basic Concepts of LDP

• LDP is a MPLS control and signaling protocol
• Main functions
• Release Label-FEC mapping
• Create and maintain label switching path
• LDP serves to distribute and maintain label
  mapping messages between peers in the form of
  message.
• LDP uses the TCP transmission service.

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LDP Message Types

• Discovery message Used to discover LDP
  adjacencies in the network
• Session message Used to set up, maintain and
  terminate a session between LDP peers
• Distribution message Used to create, change and
  delete label mappings related to FEC
• Notification message Used to provide
  recommendation or error notification information

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LDP Message Switching

• Discovery stage

TCP connection establishment
Session creation and maintenance
LSP creation and maintenance
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Basic MPLS Configurations (1)

• Designate ID for LSR
• It is necessary to configure the LSR with an ID
  before configuring other MPLS commands. The ID is
  generally in the format of IP address, and shall
  be unique within the domain.
• mpls lsr-id X.X.X.X
• Note make configurations in the system view.
• Activate/deactivate the LDP or enter the LDP view
• To configure LDP, first activate the LDP and
  enter the LDP view
• mpls ldp
• Note make configurations in the system view

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Basic MPLS Configurations (2)

• Enable interface LDP
• mpls ldp enable
• Note make configurations in the interface view
• LDP loop detection control
• Enable loop detection
• Loop-detect
• Set the maximum hot number for loop detection
• hops-count hop-number

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MPLS Debugging

• MPLS display commands
• Display information about LDP and LSR
• display mpls ldp
• Display information about LDP-enabled interface
• display mpls ldp interface
• Display information about all LSPs established in
  the public network
• display mpls lsp

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Configuration Example

• Suppose a network consists of four NE routers,
  where Router B is connected to Router C via SDH,
  while Router B is connected to Router A and
  Router D via Ethernet.
• The four routers all support MPLS. LSP can be set
  up between any two routers. The operational
  routing protocol is OSPF

Router C is configured with Quidway interface
pos 7/0/0 Quidway-Pos7/0/0 ip address
100.10.1.1 255.255.255.0 Quidway router id
172.16.1.2 Quidway ospf Quidway-ospf area
0 Quidway-ospf-area-0.0.0.0 network 100.10.1.0
0.0.0.255 Quidway mpls lsr-id
172.16.1.2 Quidway mpls ldp Quidway-Pos7/0/0
mpls ldp enable

• Configuration procedure
• Configure ip address for the interface
• Configure the ospf protocol
• Configure the MPLS LDP

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Summary

• Grasp the basic concepts and working process of
  MPLS
• Grasp label allocation and distribution
• Grasp MPLS LDP configuration

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