ODC010001 MPLS Basic Knowledge ISSUE1.5.ppt

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ODC010001 MPLS Basic
Knowledge
ISSUE 1.5

HUAWEI TECHNOLOGIES CO., LTD. All rights reserved Page 1
The course introduce the basic concept
of MPLS, including MPLS label, MPLS
switching, MPLS label allocating.

Upon completion of this course, you
will be able to:
Describe the switching process of
MPLS, the method to allocate and
control the label.

Chapter 1 MPLS Overview
Chapter 2 Label and Label Stack
Chapter 3 Label Forwarding and Allocation

MPLS
 MPLS——Multi-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

Origin: To Integrate IP with ATM
Connectionless
control plane
Connectionless
forwarding plane
IP
Connection-oriented
control plane
Connection-oriented
forwarding plane
ATM
Connectionless
control plane
Connection-oriented
forwarding plane
MPLS

Connection-oriented Features
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
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
S2 S6
S4 S7
S3 S5
S1 S8
1
1
1
2 2
2
S2 S6
S4 S7
VC
S1 S8
S3 S5

Traditional IP Forwarding
 IP header is parsed 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.
Parse IP header
mapped to next hop
Parse IP header
mapped to next hop
Parse IP header
mapped to next hop

Virtual Channel Connection (VCC)
VC
switching
VC
switching
NNI NNI
VPI = 2
VCI = 44
VPI = 1
VCI = 1
VPI = 26
VCI = 44
VPI = 20
VCI = 30
UNI
UNI
ATM Switching Process
 Connection-oriented, N2 problem
 Routing depending on link layer, based on VPI/VCI or label
 Ensure QoS and real-time service

Technology Combining the Advantages of ATM and IP
+ X
R = X
Router ATM switch MPLS
Router
MPLS——multi-protocol label switching
 Layer 3 routing – scalable and flexible
 Layer 2 switching – High reliability and traffic engineering
management

Basic MPLS Concepts
 LSR: Label Switch Router
 LER: Label Edge Router
 LSP: Label Switch Path
LER
LER
LER
LER
LSR LSR
LSR
MPLS domain
IP
MPLS
LSP

Core LSR
Basic Working Process of MPLS
IP IP L1 IP L2 IP L3 IP
Traditional IP
forwarding
Traditional
IP forwarding Label forwarding
Edge LSR Edge LSR

MPLS Advantages
 Replace IP header with short and fixed-length labels as
forwarding basis to improve forwarding speed
 Better integrate IP with ATM
 Provide value-added service without prejudice to efficiency:
VPN
Traffic engineering
QOS

Exercise-1
1. What is MPLS?
2. Describe the core function of MPLS
3. Describe LER, LSR and LSP

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

Label Values
Special use and reserved range
− 0: IPv4 explicit null, label stack must be popped
− 1: Router alert, local processing operation
− 2: IPv6 explicit null, label stack must be popped
− 3: Implicit null, triggers label stack pop, overrides label swap
− 4 to 15: reserved
General use range
− 16 to 1,048,575

Label Position in Packet
Ethernet header
/PPP header Label Layer-3 data
Ethernet
/SONET
/SDH packet
ATM header Label Layer-3 data
Frame mode
ATM packet
Cell mode
ATM packet VPI/VCI Layer-3 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

Label Spaces and Identifiers
 Label space is a set of interface-specific or platform-wide labels
Platform-wide: One pool of labels is shared across all interfaces
Per-interface: Label values can be reused on each interface
 Label space
Label space ID—Zero for platform-wide label space (2 octets)
For example, 192.168.1.1:0 (platform-wide), 192.168.1.1:5 (per-interface)
MPLS based frame mode use Platform-wide label space, such as IP,
Ethernet.
MPLS based cell mode use Per-interface label space, such as ATM.

MPLS TTL Processing
Regard the entire MPLS domain as one hop
IP TTL --
MPLS TTL＝255 MPLS TTL -- IP TTL --
Ingress LER LSR Egress LER
Include MPLS TTL in IP TTL
IP TTL --
MPLS TTL＝IP TTL MPLS TTL --
MPLS TTL --
IP TTL＝MPLS TTL
Ingress LER LSR Egress LER

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
LSRA LSRB LSRC LSRX
1 2 3 32
LSRA LSRB
LSRA
LSRC
LSRA,LSRB
LSRA,LSRB…..
LSRA,LSRB…..LSRC

Label Stack
 Theoretically, label stack enables limitless nesting to provide
infinite service support. This is simply the greatest advantage
of MPLS technology.
MPLS
header
Layer2
header IP header Data
MPLS
header

Exercise-2
1. Describe MPLS label structure
2. Describe label position with different encapsulation mode
3. Describe two LSP loop detection methods

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/null
 Link layer encapsulation types
 FTN (FEC to NHLFE): Map FEC to NHLFE
 ILM (Incoming Label Map): Map MPLS label to NHLFE

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

NHLFE
A:
…
Add label L1
E1
B
10.0.1.0/24
Others
Label operation
Transmitting interface
next hop
NHLFE
FEC
…
Remove the previous label and add L2
E1
C
L1
Others
label operation
Next hop
NHLFE
Ingress
label
B:
…
Remove the previous label and add L3
D
L2
Others
Label operation
Next hop
NHLFE
Ingress
label
C:
E1

Penultimate Hop Popping (PHP)
 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.
Parse IP header
Distribute FEC
Mapped to next hop
Label operation: pop
Parse IP header
FEC bound with LSP
FTN->NHLFE ILM->NHLFE ILM->NHLFE
Ingress LER LSR LSR Egress LER
Label operation: push
Label operation: swap

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: Label Distribution Protocol
 CR-LDP: Constrained Route LDP
 RSVP-TE
 MP-BGP
 PIM

Several Issues Concerning Label Distribution
 Label allocation mode
 DoD : downstream-on-demand
 DU: downstream unsolicited
 Label control mode
 Ordered
 Independent
 Label retention mode
 Conservative retention mode : upon receiving a label, if there is no
route destined for corresponding FEC, discard the label.
 Liberal mode: upon receiving a label, if there is no route destined for
the corresponding FEC, hold the label for later use.

Label Allocation Mode: DoD
Upstream Downstream
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.
171.68.10.0/24
171.68.40.0/24
LSR1 LSR2 LSR3
请求到目的地址
171.68.10/24 的标签
Requesting labels destined
for 171.68.10.0/24 的标签
Requesting labels
destined for 171.68.10.0/24
分配到171.68.10/24
的标签为
20
Label 20 is allocated
to 171.68.10.0/24
分配到171.68.10/24
的标签为18
Label 18 is
allocated to
171.68.10.0/24
Route triggering

Label Allocation Mode: DU
Upstream
Downstream
Route
triggering
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.
到171.68.10/24
可以使用标签 20
Label 20 can be used
to reach
171.68.10.0/24
171.68.10.0/24
171.68.40.0/24
Label 18 can be used
to reach
171.68.10.0/24

Label Control Mode: Ordered
Not until it receives a label mapping message from its downstream
LSP will it send the message upstream
Upstream Downstream
DOD+ Ordered
Upstream Downstream
DU+ Ordered

Label Control Mode: Independent
Upstream Downstream
Whether it receives a label mapping message from its downstream LSR, it will
send upstream a label mapping message immediately.
Upstream Downstream
DOD+ independent
DU+ independent

Label Retention: Conservative Retention Mode
 An LSR stores only the labels received from next-hop LSRs; all
other labels are ignored.
LSR1 LSR2 LSR3 LSR4
LSR5
172.16.2/24
mapping
label 20
mapping
label 30
mapping
label 17
mapping
label 16
Drop

Label Retention: Liberal Retention Mode
LSR1 LSR2 LSR3 LSR4
LSR5
172.16.2/24
mapping
label 20
mapping
label 30
mapping
label 17
mapping
label 16
store
 Every LSR stores the received label in its LIB, even when the label
is not received from a next-hop LSR.

Common Collocation 1: DoD + Ordered + Conservative
Upstream Downstream
 It is relatively easy to control the use of labels and the creation of LSPs
 ATM/FR frame mode can only use DoD

Common Collocation 2: DU + Ordered + Liberal
Upstream Downstream
 A waste of label resources
 Useless LSPs would be created
 Label merge is required at branches
 LSPs can be set up quickly and reliably

Label Forwarding Table
IN interface IN label Prefix/MASK OUT interface
(next hop)
OUT label
Serial0 50 10.1.1.0/24 Eth0（3.3.3.3） 80
Serial1 51 10.1.1.0/24 Eth0（3.3.3.3） 80
Serial1 62 70.1.2.0/24 Eth0（3.3.3.3） 52
Serial1 52 20.1.2.0/24 Eth1（4.4.4.4） 52
Serial2 77 30.1.2.0/24 Serial3（5.5.5.5) 3（pop）
 The “in” and “out” is correspond to the label swap，not the label
distribution.
 The in label is that I distribute to the others, I will not put it to
the packet
 The out label is the others distribute to me, I will put it to the
packet

Exercise-3
1. Describe MPLS label switching process
2. Describe MPLS label distribution work mode: allocation, control
and retention.

 Grasp the basic concepts and working
process of MPLS
 Grasp label allocation and distribution
Summary

ODC010001 MPLS Basic Knowledge ISSUE1.5.ppt

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