Transcript Slide 1

MPLS
FUNDAMENTALS
DINESH BHATT
Manager (MPLS)
1
Pre-requisites knowledge for
understanding MPLS
OSI & TCP/IP layered architecture
TCP/IP protocol suite
Switch, Router & various protocols they
support
IP addressing & routing methodology
2
TCP/IP and OSI Model
TCP/IP has simple hierarchical design &
clear corresponding relations with OSI
reference model is as below 7
Application layer
6
Presentation layer
5
Session layer
4
Transport layer
Transport layer
3
Network layer
Internet layer
2
Data link layer
Network Interface
1
Physical layer
OSI reference model
Application layer
Physical layer
TCP/IP
3
ubnet
Mask
IP Addressing & Network Mask
32 bits
Dotted
Decimal
Maximum
Network
255
Host
255
255
255
32
IP Add.
172
16
122
128
64
32
16
8
4
2
1
11111111 11111111
128
64
32
16
8
4
2
1
128
64
32
16
8
4
2
1
11111111 11111111
128
64
32
16
8
4
2
1
Binary
204
Example 10101100 00010000 01111010 11001100
Binary
255
11111111
255
11111111
0
00000000
0
00000000
Also written as “/16” where 16 represents the number of 1s in the
mask. Hence the network of the above IP Add is 172.16.0.0./16
4
TCP/IP Protocol Stack
Application
Layer
HTTP, Telnet, FTP,
TFTP, Ping, etc
TCP/UDP
Transport Layer
Internet Layer
Network
Interface Layer
Physical Layer
IP
Routing
protocols
Provide application program
network interfaces
Establish terminal to
terminal connection
ICMP
ARP/RARP
Addressing and
route selecting
Ethernet, 802.3, PPP,
HDLC, FR, etc
Interfaces and
wires/cables
Physical media access
Binary data flow transmission
5
Hub, Switches Routers
Ethernet
Hub
10
One device
sending at
a time. Hub
works at layer 1
All nodes share 10 Mbps
Switched Ethernet
Backbone
Switch
Each node has 10 Mbps
Router
10
Multiple
devices
sending at the
same time.
Switch uses MAC address (L2) to filter the
network. They do not look at the Network
layer header and hence faster (LAN)
Router works at Layer 3, i.e. Network layer, uses IP
addresses for facilitating communications amongst
the switches or WAN communications ( for which it
6
is connected to other Router)
MPLS- Definition
•
•
•
•
•
•
•
It stands for Multi-Protocol Label Switching.
It is the technique that provides virtual path capability to packet(label)
switches.
It aim is to avoid some drawbacks of both circuit switching and packet
switching and to increase the utilization of bandwidth.
MPLS is basically deployed to manage the traffic within the ISP .
It combines the benefits of both Circuit switching and packet switching .
It uses Circuit switching within ISP. and IP based packet switching within
ISPs.
The general idea behind MPLS is to attach a discrete set of labels to IP
packets to perform a specific function, without forcing routers and switches
to dive into IP addresses or other information in each packet to obtain
instructions relating to that particular function.
It efficiently enables Traffic Engineering & quality of service in networks.
7
MPLS and ISO model
IETF main goal is
that when a layer is
added, no
modification is
needed on the
existing layers.
All new protocol
must be backward
compatible
7
to
5
Applications
TCP
PPP
PPP
UDP
IP
MPLS
Frame
4
3
ATM (*)
ATM (*)
2
Physical (Optical - Electrical)
1
FR
Relay
8
MPLS Advantages 1. MPLS provides all the required convergence of all
type of networks be it IP-network, Next Generation
network or our traditional legacy (TDM) network.
2. By collapsing multiple existing backbone service
delivery platform into a single MPLS enable backbone
–CONVERGENCE can be achieved.
3. Reduces CapEx & OpEx by reduction of number of
network element.
4. Increase relaibility.
5. Seamless Inter-works & Inter-operate with other N/W’s.
6. IP Rich services can be deployed with minimal CapEx
and faster way.
7. Provides VPN (L3 & L2 intranet, extranet), IPsec, internet.
9
MPLS: Multi Protocol Label Switching, a Layer 2+ switching, is a
versatile solution to address the problems faced by present day
Network- Speed, Scalability, Quality of Services(QoS) & Traffic
engineering
In conventional packet forwarding every router opens the IP datagram
and looks at IP header to find out destination IP address and then with
the help of its routing table takes independent decision to forward the
packet.Handling a bulky IP header and then reconstructing it before
forwarding to next router compromises with the speed of entire
operation. This operation takes place at layer-3.
Unlike conventional forwarding of IP packets, MPLS classifies each
packet and attaches a small label with IP datagram at the ingress point
of MPLS network. subsequent, routers only look at the label and route
the packet after swapping the label with new one.
Developed to integrate IP and ATM & Layer-2 protocols (e.g. Ethernet,
ATM, PPP, Frame Relay etc.) .
Packet forwarding is done based on Labels.
Support multiple Layer-3 protocols, such as IP, IPv6, IPX, SNA, OSPF
, BGP, static , RIP etc.
10
MPLS Elements / Terms...
LER - Label Edge Router ( PE- Provider Edge)
LSR - Label Switch Router (P- Provider or Core Router)
FEC - Forward Equivalence Class
Label - Associates a packet to a FEC
Label Stack - Multiple labels containing information on how a
packet is forwarded.
Shim - Header containing a Label Stack
Label Switch Path - path that a packet follows for a specific
FEC
LDP - Label Distribution Protocol, used to distribute Label
information between MPLS-aware network devices
Label Swapping - manipulation of labels to forward packets
towards the destination.
11
Origin: To Integrate IP with ATM
IP
MPLS
Connectionless
control plane
Connectionless
control plane
Connectionless
forwarding plane
Connection-oriented
forwarding plane
ATM
Connection-oriented
control plane
Connection-oriented
forwarding plane
12
Connection-oriented Features
S2
1
S6
1
S2
S6
1
S1
S8
S3
S1
S5
S8
VC
S5
2
S3
2
S4
2
S4
S7
S7
Connectionless: packet route
connection-oriented: cell switching
•
Path 1 = S1, S2, S6, S8
VC = S1, S4, S7, S8
•
Path 2 = S1, S4, S7, S8
•
•
The data reach their destination in
order along the same connection
The data reach their destination
out of order along different paths
•
Fixed time delay, easy to control
•
Connection types: PVC SVC
13
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.
14
Basic Working Process of
MPLS
Core LSR
Edge LSR
Edge LSR
IP
IP
Traditional
IP forwarding
L1
IP
L2
Label forwarding
IP
L3
IP
Traditional IP
forwarding
15
Basic MPLS Concepts
LER
IP
LER
LSR
MPLS domain
LSR
LER
LSP
MPLS
LSR
LER
LSR: Label Switch Router
LER: Label Edge Router
LSP: Label Switch Path
16
FEC Classification
•A packet can be mapped to a particular FEC based on the following criteria:
•destination IP address,
•source IP address,
•TCP/UDP port,
•in case of inter AS-MPLS, Source-AS and Dest-AS,
•class of service,
•application used,
•…
•any combination of the previous criteria.
Ingress Label
6
FEC
Egress Label
138.120.6/24 - xxxx
9
•FECs are manually initiated by the operator
•A FEC is associated with at least one Label
Ingress
Label
Ingress
Label
FEC FEC
Attribute
Egress
Label
Attribute
Egress
Label
6
138.120.6/24 - xxxx
A
9
6
138.120.6/24 - xxxx
B
12
17
MPLS Encapsulation Format
and
Label
0
20
23 24
31
Label
Layer 2
header

EXP
MPLS header IP header
S
32 bits
TTL
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 :
Exp :
S:
TTL :
Label value
Experimental Use ( Class of Service)
Bottom of Stack (set to 1 for the last entry in the label)
Time To Live
18
MPLS TTL Processing
Regard the entire MPLS domain as one hop
IP TTL -MPLS TTL＝255
MPLS TTL --
Ingress LER
IP TTL --
LSR
Egress LER
MPLS TTL --
MPLS TTL -IP TTL＝MPLS TTL
LSR
Egress LER
Include MPLS TTL in IP TTL
IP TTL -MPLS TTL＝IP TTL
Ingress LER
19
Label Position in Packet
Ethernet
/SONET
/SDH packet
Frame mode
ATM packet
Cell mode
ATM packet
Ethernet header
/PPP header
ATM header
VPI/VCI
Label
Layer-3 data
Label
Layer-3 data
Layer-3 data
20
Label Stack
Layer2
header
MPLS
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.
21
Label Switched Path
Ingress Ingress
Interface Label
1
5
Ingress Ingress
Interface Label
FEC Egress Egress
Interface Label
138.120
3
1
12
FEC Egress Egress
Interface Label
138.120
4
x
12
MPLS switch
3
1
4
138.120
1
127.20
2
MPLS switch
1
3
2
3
2
3
1
MPLS switch 192.168
2
MPLS switch
Ingress Ingress
Interface Label
1
x
FEC Egress Egress
Interface Label
5
3
138.120
22
Hop by Hop IP forwarding
Ingress Ingress
Interface Label
1
Default
Ingress Ingress
Interface Label
FEC Egress Egress
Interface Label
3
None
1
Default
FEC Egress Egress
Interface Label
4
None
x
Default
??
MPLS switch
??
3
1
1
127.20
2
MPLS switch
1
3
2
138.120.6.12
??
1
138.120
138.120.6.12
3
3
2
4
MPLS switch 192.168
2
MPLS switch
Ingress Ingress
Interface Label
1
x
FEC Egress Egress
Interface Label
None
3
Default
23
IP forwarding using LSP
Ingress Ingress
Interface Label
1
5
Ingress Ingress
Interface Label
FEC Egress Egress
Interface Label
3
138.120
1
12
FEC Egress Egress
Interface Label
138.120
4
x
12
MPLS switch
3
1
4
1
127.20
1
138.120.6.12
2
MPLS switch
3
138.120
3
2
3
MPLS switch 192.168
138.120.6.12
2
1
2
MPLS switch
Ingress Ingress
Interface Label
1
x
FEC Egress Egress
Interface Label
138.120
July 29, 2000 TECON 2000
3
5
24
24
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
25
Label Forwarding
Stack label operation: pop
Label operation: push
Label operation: swap
Parse IP header
FEC bound with LSP
FTN->NHLFE
A
Ingress LER
ILM->NHLFE
Label operation: swap
ILM->NHLFE
B
C
LSR
LSR
ILM->NHLFE
Parse IP header
distribute FEC
mapped to next hop
D
Egress LER

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
26
NHLFE
A:
NHLFE
FEC
next hop
10.0.1.0/24
Transmitting interface
E1
B
Label operation
Others
Add label L1
…
B:
NHLFE
Ingress
label
Next hop
L1
C
Transmitting
interface
E1
label operation
Remove the previous label and add L2
Others
…
C:
NHLFE
Ingress
label
Next hop
L2
D
Transmitting
interface
E1
Label operation
Remove label
Others
…
27
Pop at Last Hop But One (PHP)
Label operation: push
Label operation: swap
Parse IP header
FEC bound with LSP
FTN->NHLFE
Ingress LER
Label operation: pop
ILM->NHLFE
ILM->NHLFE
LSR
LSR
Parse IP header
Distribute FEC
Mapped to next hop
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.
28
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
29
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
30
Label Allocation Mode: DoD
Route triggering
Label 18 is
分配到171.68.10/24
allocated to
的标签为18
171.68.10/24
171.68.10/24
分配到
Label
20
is allocated
to 171.68.10/24
的标签为20
171.68.40/24
Upstream
171.68.10/24
LSR1
LSR2
请求到目的地址
Requesting
labels destined
for 171.68.10/24
171.68.10/24
的标签
LSR3 Downstream
Requesting labels
destined for的标签
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.
31
Label Allocation Mode: DU
Route
triggering
Upstream
171.68.40/24
Label 18 can be used
to reach 171.68.10/24
到 171.68.10/24
Label
20 can be used
20
to
reach 171.68.10/24
可以使用标签
Downstream
171.68.10/24
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.
32
Label Control Mode: Ordered
Not until it receives a label mapping message from its downstream
LSP will it send the message upstream
DOD+ Ordered
Upstream
Downstream
DU+ Ordered
Upstream
Downstream
33
Label Control Mode: Independent
Whether it receives a label mapping message from its downstream LSR, it will
send upstream a label mapping message immediately.
DOD+ independent
Upstream
Downstream
DU+ independent
Upstream
Downstream
34
Label Retention: Conservative
Retention Mode
An LSR stores only the labels
received from next-hop LSRs; all
other labels are ignored.
mapping
label 20
mapping
label 30
172.16.2/24
LSR2
LSR1
mapping
label 16
LSR3
LSR4
mapping
label 17
Drop
LSR5
35
Label Retention: Liberal Retention Mode
Every LSR stores the received label in
its LIB, even when the label is not
received from a next-hop LSR.
mapping
label 20
mapping
label 30
172.16.2/24
LSR2
LSR1
mapping
label 16
LSR3
LSR4
mapping
label 17
store
LSR5
36
Common Collocation 2: DU +
Ordered + Conservative
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
37
Common Collocation 1: DoD +
Ordered + Liberal
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
38
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
39
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
40
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.
41
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
42
LDP Message Switching
UDP-Hello
Discovery
stage
UDP-Hello
TCP connection establishment
Session creation
and maintenance
Session initialization
Label request
LSP creation and
maintenance
FEC
Label
Label mapping
43
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
44
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

Set the maximum value for the path vector
path-vectors pv-number
Note: make configurations in the LDP view
45
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
46
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 B
Router A
ethernet1/0/0
168.1.1.2
pos2/0/1
ethernet8/0/0 100.10.1.2
168.1.1.1
pos7/0/0
100.10.1.1

Configuration procedure
 Configure ip address for the
interface
 Configure the ospf protocol
 Configure the MPLS LDP
Router C
ip route-static 171.68.0.0 255.255.0.0 Serial0
ip route-static vpn-instance VPN-A 0.0.0.0 0.0.0.0
192.168.1.1 public
ethernet1/0/1
172.17.1.1
Router D
ethernet2/0/1
172.17.1.2
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
47
TRAFFIC ENGINEERING
48
QUALITY OF SERVICE
MPLS VPN Network Structure
VPN_A
VPN_A
iBGP sessions
10.2.0.0
CE
CE
VPN_B
11.5.0.0
VPN_A
10.2.0.0 CE
PE
P
P
P
P
PE
CE
10.1.0.0
VPN_A
11.6.0.0
VPN_B
CE
PE
PE
CE
VPN_B
10.3.0.0
10.1.0.0 CE

CE (Custom Edge): The user equipment directly connected with the service
provider.

PE (Provider Edge Router): The edge router on the backbone network, connected
with CE and mainly responsible for access of the VPN service.

P (Provider Router): The core router on the backbone network, mainly responsible
for the routing and fast forwarding functions.
50
CR-LDP and RSVP-TE
CR-LDP and RSVP-TE are both signaling mechanisms used to support
Traffic Engineering across an MPLS backbone. RSVP is a QoS signaling
protocol that is an IETF standard and has existed for quite some time.
RSVP-TE extends RSVP to support label distribution and explicit routing
while CR-LDP proposed to extend LDP (designed for hop-by-hop label
distribution to support QoS signaling and explicit routing). MPLS Traffic
Engineering tunnels are not limited to IP route selection procedures and
thus will spread network traffic more uniformly across the backbone taking
advantage of all available links. A signaling protocol is required to set up
these explicit MPLS routes or tunnels.
There are many similarities between CR-LSP and RSVP-TE for constraintbased routing. The Explicit Route Objects that are used are extremely
similar. Both protocols use ordered Label Switched Path (LSP) setup
procedures. Both protocols include some QoS information in the signaling
messages to enable resource allocation and LSP establishment to take
place automatically.
At the present time CD-LDP development has ended and RSVP-TE has
emerged as the "winner" for traffic engineering protocols.
51
VPNv4 and IPv4 Address Families
VPNV4 address structure:
Route Distinguisher (8 bytes) IPv4 address
To enable different VPNs to use the same address space, a new
address family, i.e. VPNv4, is introduced. The original standard
address family is called IPv4.
VPNv4 address family mainly serves to transfer VPN routes between
PE routers.
RD is unique among different VPNs. If two VPNs use the same IP
address, PE router will add different RDs for them and convert the
address into a unique VPN-v4 address without causing conflict of the
address space.
The standard route received by PE from CE is the IPv4 route. To
import VRF routing tables and distribute them to other routers, a RD is
needed. It is suggested that the RDs of the same VPN be configured
the same.
52
MPLS/VPN RD
RD structure:
TYPE (2-byte)
Administrator Field
2-byte ASN
1
4-byte IP address
Assigned Number Field
4-byte assigned number
2-byte assigned number
RD format:

16-bit Autonomous System Number (ASN): 32-bit user-defined number, e.g. 100:1

32-bit IP address: 16-bit customized number, e.g. 172.1.1.1:1
Usually, each site is assigned with a unique RD, which is the identifier of
VRF.
Difference between the routing table of public network and the routing table
of private network:


The routing table of public network is generated by the IGP routes, which may
include the BGP-4 (IPv4) route, but not the VPN route.
VRF routing table includes the specific VPN routes. It may include the routes
redistributed from MP-iBGP route to VRF, or the route obtained from CE by the vrf
route instance.
53
VRF- VPN Routing & Forwarding
VRF can be regarded as a virtual router structured as follows:

It is associated with some interfaces and has a forwarding table based on these
interfaces.

A set of rules is available to control import of the route into VPN or export of the
route from VPN.

The route can be redistributed to the routing table (static route, RIP instance, BGP)
via some routing protocols.

VRF is configured on PE and exchange the route with CE. The route independently
exists in the VRF routing table (routing table of the private network).
PE maintains a separate forwarding table for each site.
Each site has a unique VRF.
If (and only if) two sites have identical forwarding table, they share a VRF.
The interface/sub-interface connected with CE is mapped to VRF.
The routes in VRF will be distributed to the sites (usually connected on other
PEs) belonging to the same VPN.
54
Distribution of VRF Routes
P Router
CE Router
Site

PE
PE
MP-iBGP
CE Router
Site
The PE router distributes the local VPN route information via the
MPLS/VPN backbone network.

The transmitting PE exports the local VRF routes via MP-iBGP
(with the export-target attribute).

The receiving PE imports the route to the VRF where it belongs
(with the matched import-target attribute).
55
Basic Intranet Model
VPN A
SITE -1
MPLS/VPN Backbone
SiteSite-1 & Site -2 routes
RT=VPN -A
VPN A
SiteSite-3 & SiteSite-4 routes
RT=VPN -A
SITE -3
MP-iBGP
P Router
SITE -2
VPN A
SiteSite-1 routes
SiteSite-2 routes
SiteSite-3 routes
SiteSite-4 routes
SiteSite-1 routes
SiteSite-2 routes
SiteSite-3 routes
SiteSite-4 routes
SITE -4
VPN A
56
MPLS/VPN Packet Forwarding-1
In Label
-
FEC
Out Label
197.26.15.1/32
VPN-A VRF
149.27.2.0/24,
NH=197.26.15.1
Label=(28)
PE-1
41
Beijing
41
28
149.27.2.27
149.27.2.27
Shanghai
149.27.2.0/24
When the ingress PE receives an ordinary IP packet from CE, PE adds it
to the corresponding VPN forwarding table based on the VRF to which
the ingress interface belongs, and searches for the next hop and label.
57
MPLS/VPN Packet Forwarding-2
In Label
28(V)
FEC
149.27.2.0/24
VPN-A VRF
149.27.2.0/24,
NH=beijign
Out Label
-
In Label
FEC
Out Label
41
197.26.15.1/32
POP
VPN-A VRF
149.27.2.0/24,
NH=197.26.15.1
Label=(28)
PE-1
149.27.2.27
28
149.27.2.27
41
28
Beijing
149.27.2.0/24

149.27.2.27
149.27.2.27
Shanghai
The second last hop router pops up the external layer label and sends it
to the egress PE according to the next hop.

The egress PE router judges the CE that the packet will go to based on
the internal layer label.

Pop up the internal layer label and forward the packet to the destination
CE as an ordinary IP packet.
58
59