APNIC eLearning:
Introduction to MPLS
01 NOVEMBER 2017
01:00 PM AEST Brisbane (UTC+10)
Issue Date: 20 May 2016
Revision: 3.0
Introduction
• Presenter/s Jessica Bei Wei
Training Officer
jwei@apnic.net
Specialties:
Routing & Switching
MPLS， IPv6
QoS
• Reminder: please take time to fill-up the survey
2
Overview
• Definition of MPLS
• Advantages of MPLS
• MPLS Application
• MPLS Architecture
• MPLS Labels
• LSP Setup
• Forwarding Operations
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Definition of MPLS
• Multi Protocol Label Switching
– Multiprotocol, it supports ANY network layer protocol, i.e.
IPv4, IPv6, IPX, CLNP, etc.
– A short label of fixed length is used to encapsulate
packets
– Packets are forwarded by label switching instead of by IP
switching
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Initial Motivation of MPLS
• In mid 1990s, IP address lookup was considered more
complex and take longer time.
– Longest matching
IP Forwarding Table IP Forwarding Table IP Forwarding Table
Address Address Address
I/F I/F Prefix I/F
Prefix Prefix
128.89/16 1 128.89/16 0 128.89/16 0
171.69/16 0 171.69/16 1 171.69/16 1
… … …
128.89
1 0 0
128.89.25.4 Data 128.89.25.4 Data 128.89.25.4 Data 128.89.25.4 Data
A label-swapping protocol was the need for speed.
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Decoupling Routing and Forwarding
• MPLS can allow core routers to switch packets based on
some simplified header.
128.89
1 0 0
1
128.89.25.4 Data 20 128.89.25.4 Data 30 128.89.25.4 Data 128.89.25.4 Data
0
• But, hardware of routers became better and looking up
longest best match was no longer an
1 issue.
• More importantly, MPLS de-couples forwarding from
routing, and support multiple service models.
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MPLS vs IP over ATM
10.1.1.1 10.1.1.1 10.1.1.1 10.1.1.1
IP over ATM
10.1.1.1 • Layer 2 topology may be different
from Layer 3 topology, resulting in
suboptimal paths.
• Layer 2 devices have no
10.1.1.1
knowledge of Layer 3 routing –
10.1.1.1
virtual circuits must be manually
established.
10.1.1.1 L=3/1 L=2/4 L=4/8 10.1.1.1
MPLS
• Layer 2 devices run a Layer 3
routing protocol and establish
virtual circuits dynamically
based on Layer 3 information.
• MPLS provides a virtual full
mesh topology.
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MPLS VPN
VPNA
VPN B Site 1
Site 1 PE
PE CE
CE P
P
PE P VPN B
Site 2
CE
PE
VPNA
Site 2
CE
MPLS Core
VPN B
Site 3
CE
• MPLS Layer 3/ Layer 2 VPN
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Optimal Traffic Engineering
Tunnel 1 R3
R1 R2 R6
Tunnel 2
R4 R5
IP TE MPLS TE
Shortest path Determines the path at the source based on additional
parameters (available resources and constraints, etc.)
Equal cost load balancing Load sharing across unequal paths can be achieved.
9
MPLS QoS
• MPLS does NOT define a new QoS architecture.
– Similar parts with IP DiffServ: functional components and where they
are used.(such as marking and traffic policing at network edge, etc)
– Difference: packets are differentiated by MPLS Traffic Class bits
MPLS Header IP Packet
IP Packet
Traffic Class
DSCP
VPN Site
IP Domain CE PE P P
MPLS Domain
- Packet
QoS in MPLS VPN Architecture
10
Technology Comparison
IP Native Ethernet MPLS
• Destination address • Destination address • Label based
based based • Forwarding table
Forwarding • Forwarding table learned • Forwarding table learned from control
from control plane learned from data plane plane
• TTL support • No TTL support • TTL support
Ethernet loop avoidance Routing protocols
Control Plane Routing protocols
and signaling protocols Label distribution protocols
Packet
IP header 802.3 header MPLS Header
Encapsulation
QoS 8 bit TOS in IP header 3 bit 802.1p in VLAN tag 3 bit TC in label
OAM IP Ping, traceroute E-OAM MPLS Ping, traceroute
11
Evolution of MPLS
• Technology Evolution and Main Growth Areas
Today
1996, Ipsilon, Cisco and IBM announced label
switching plans, till now, there are over 280
RFCs of MPLS tech. Optimize MPLS
for Cloud
Optimize MPLS for
packet transport
Optimize MPLS for video
Complete base MPLS portfolio
Bring MPLS to Market
Large Scale
L2VPN
First MPLS
First L2VPN Deployments
L3VPN &TE
Deployments
Deployed
Large Scale Large
Formation of First MPLS L3VPN Scale First LSM First MPLS
the IETF MPLS RFCs Deployments MPLS TE Deployme TP
working group Released Deployed nts Deployments
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015
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MPLS Application Scenario
MPLS CORE
TE Main Path for PE1-PE3 Enterprise
L3VPN
PE1 PE3
Enterprise P P
Enterprise
P P
L2VPN L2VPN
PE2 TE Backup Path for PE1-PE3 PE4
Enterprise
QoS Operations : QoS Operations:
QoS Operations:
Traffic marking, Traffic marking,
Congestion
police, shaping police, shaping
management,
congestion avoidance
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MPLS Architecture
Routing Information Label Binding and
Exchange with other Exchange with other
routers routers
Control Plane
IP Routing
Protocols
Routing Label Label
Information Distribution Information
Base (RIB) Protocols Base (LIB)
Data Plane
Incoming Incoming
IP Packet Forwarding Label Forwarding Labeled Packet
Information Information
Base (FIB) Base (LFIB)
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MPLS Topology
MPLS Domain
IP Packet Label IP Packet Label IP Packet Label IP Packet IP Packet
IP Domain Edge IP Domain
Edge LSR LSR
LSR LSR
• LSR (Label Switch Router) is a router that supports MPLS.
• LER (Label Edge Router), also called edge LSR, is an LSR that operates at
the edge of an MPLS network.
• LSP (Label Switched Path) is the path through the MPLS network or a part
of it that packets take.
15
MPLS Label
MPLS Label Encapsulation
Datalink Layer Header MPLS Label Layer 2/ Layer 3 Packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Label - 20bits TC S TTL-8bits
TC = Traffic Class: 3 Bits; S = Bottom of Stack: 1 Bit; TTL = Time to Live
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MPLS Label Stacking
LAN MAC Label Header
MAC Header Label S Label S Layer 3 Packet
S=0 S=1
Bottom of Stack Bit Set
MPLS Label Stack
• Multiple labels can be used for MPLS packet encapsulation.
network. This is done by packing the labels into a stack.
• Some MPLS applications (VPN, etc.) actually need more
than one labels in the label stack to forward the labeled
packets.
17
LSP Setup Overview
• Before forwarding packets, labels must be allocated to
establish an LSP.
• Protocols for label distribution: LDP, RSVP-TE, MP-BGP.
Upstream Downstream
To 100.1.1.1/32 To 100.1.1.1/32 To 100.1.1.1/32
Label=100 Label=200 Label=300
100.1.1.1/32
LSP
R1 R2 R3 R4
Establishing an LSP
Labels are allocated from downstream LSRs to upstream LSRs.
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Basic Concepts of MPLS Forwarding
• FEC
– Forwarding Equivalence Class, is a group or flow of packets that are
forwarded along the same path and are treated the same with regard to
the forwarding treatment.
– For example, packets with Layer 3 destination IP address matching a
certain prefix.
• Push
– A new label is added to the packet between the Layer 2 header and the
IP header or to the top of the label stack.
• Swap
– The top label is removed and replaced with a new label.
• Pop
– The top label is removed. The packet is forwarded with the remaining
label stack or as an unlabeled packet.
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MPLS Forwarding Operations
Prefix: 100.1.1.1/32 Prefix: 100.1.1.1/32 Prefix: 100.1.1.1/32 Prefix: 100.1.1.1/32
Local Label Null Local Label 100 Local Label 200 Local Label 300
Out Interface E1 Out Interface E1 Out Interface E1 Out Interface --
Out Label 100 Out Label 200 Out Label 300 Out Label --
Operation Push Operation Swap Operation Swap Operation POP
100 IP:100.1.1.1 200 IP:100.1.1.1 300 IP:100.1.1.1
Push Swap Swap Pop
E1 E0 E1 E0 E1 E0
R1 R2 R3 R4 100.1.1.1/32
Loopback0
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Why PHP?
Review what R4 hasPrefix:
Prefix: 100.1.1.1/32 done: 100.1.1.1/32 Prefix: 100.1.1.1/32 Prefix: 100.1.1.1/32
Local1.LabelFirst,Nulllookup the label in the
Local Label 100
LFIB; Local Label 200 Local Label 300
Remove
Out Interface E1
the label
Out Interface E1 Out Interface E1 Out Interface --
Out Label 200 Out Label --
2. Then,
Out Label 100
IP lookup and forward IP packet.
Out Label 300
Operation Push Operation Swap Operation Swap Operation POP
Is the first lookup
100 IP:100.1.1.1 200 IP:100.1.1.1 300 IP:100.1.1.1
necessary?
Can
Pushwe simplify it? Swap Swap Pop
E1 E0 E1 E0 E1 E0
R1 R2 R3 R4 100.1.1.1/32
Loopback0
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Penultimate Hop Popping
Prefix: 100.1.1.1/32 Prefix: 100.1.1.1/32 Prefix: 100.1.1.1/32 Prefix: 100.1.1.1/32
Local Label Null Local Label 100 Local Label 200 Local Label imp-null
Out Interface E1 Out Interface E1 Out Interface E1 Out Interface --
Out Label 100 Out Label 200 Out Label imp-null Out Label --
Operation Push Operation Swap Operation Pop Operation --
100 IP:100.1.1.1 200 IP:100.1.1.1 IP:100.1.1.1
Push Swap Pop
E1 E0 E1 E0 E1 E0
R1 R2 R3 R4 100.1.1.1/32
Loopback0
The implicit NULL label is the label that has a value of 3, the label 3
will never be seen as a label in the label stack of an MPLS packet.
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MPLS TTL Processing (1)
Only the TTL in the
top level decreased
Decreased & Copied Decreased & Copied
TTL=251 TTL=250 TTL=250 TTL=249 TTL=250 TTL=248 TTL=247
IP Domain Edge IP Domain
Edge LSR LSR
LSR LSR
MPLS Domain
• MPLS processes the TTL to prevent loops and implement
traceroute.
• By default, TTL propagation is enabled as above.
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MPLS TTL Processing (2)
After disabled TTL propagation
Decreased Only the TTL in the
top level decreased
Set 255 Decreased
TTL=251 TTL=255 TTL=250 TTL=254 TTL=250 TTL=249 TTL=248
IP Domain Edge IP Domain
Edge LSR LSR
LSR LSR
MPLS Domain
• TTL propagation can be disabled to hide the MPLS network topology.
• Disabling TTL propagation makes routers set the value 255 into the TTL
field of the label when an IP packet is labeled.
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MPLS LSP Ping
R1#ping mpls ipv4 4.4.4.4/32
Sending 5, 100-byte MPLS Echos to 4.4.4.4/32,
timeout is 2 seconds, send interval is 0 msec:
Codes: '!' - success, 'Q' - request not sent, '.' - timeout,
'L' - labeled output interface, 'B' - unlabeled output interface,
'D' - DS Map mismatch, 'F' - no FEC mapping, 'f' - FEC mismatch,
'M' - malformed request, 'm' - unsupported tlvs, 'N' - no label entry,
'P' - no rx intf label prot, 'p' - premature termination of LSP,
'R' - transit router, 'I' - unknown upstream index,
'l' - Label switched with FEC change, 'd' - see DDMAP for return code,
'X' - unknown return code, 'x' - return code 0
Type escape sequence to abort.
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/14/16 ms
Total Time Elapsed 128 ms Cisco IOS
MPLS Echo MPLS Domain
Request
4.4.4.4/32
R1 R2 R3 MPLS Echo R4
Reply
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MPLS LSP Trace
R1#traceroute mpls ipv4 4.4.4.4/32
Tracing MPLS Label Switched Path to 4.4.4.4/32, timeout is 2 seconds
Codes: '!' - success, 'Q' - request not sent, '.' - timeout,
'L' - labeled output interface, 'B' - unlabeled output interface,
'D' - DS Map mismatch, 'F' - no FEC mapping, 'f' - FEC mismatch,
'M' - malformed request, 'm' - unsupported tlvs, 'N' - no label entry,
'P' - no rx intf label prot, 'p' - premature termination of LSP,
'R' - transit router, 'I' - unknown upstream index,
'l' - Label switched with FEC change, 'd' - see DDMAP for return code,
'X' - unknown return code, 'x' - return code 0
Type escape sequence to abort.
0 12.1.1.1 MRU 1500 [Labels: 200 Exp: 0]
L 1 12.1.1.2 MRU 1500 [Labels: 19 Exp: 0] 16 ms
L 2 23.1.1.2 MRU 1504 [Labels: implicit-null Exp: 0] 12 ms
! 3 34.1.1.2 12 ms Cisco IOS
MPLS Domain
4.4.4.4/32
R1 R2 R3 R4
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IP MTU
• MTU indicates the maximum size of the IP packet that can
still be sent on a data link, without fragmenting the packet.
TCP MSS
IP MTU
Ethernet MTU
TCP
IP Header Payload
Header
20 byte 20 byte 1460 byte
IP Packet
size=1500 PASS PASS PASS
DF=1
R1 R2 R3 R4
MTU=1500 MTU=1500 MTU=1500
IP Domain
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MPLS MTU Issue
• In MPLS L3VPN network, 2 labels are added into the packet, the
labeled packets are slightly bigger than the IP packets. This
would lead to the need to fragment the packet.
TCP MSS
IP MTU
Ethernet MTU
LDP VPN TCP
IP Header Payload
Label Label Header
4 byte 4 byte 20 byte 20 byte 1460 byte
Labeled Packet
size=1508 DROP
DF=1
R1 R2 R3 R4
MTU=1500 MTU=1500 MTU=1500
MPLS Domain
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How to Optimize Fragmentation?
• Solution 1. Change MPLS MTU: Make sure that you
configure this value on all the links in the path so that the
packets are not dropped.
R1(config)#interface ethernet1/0
R1(config-if)#mpls mtu 1508
R1#show mpls interfaces Ethernet 1/0 detail
Interface Ethernet1/0:
IP labeling enabled
LSP Tunnel labeling not enabled
BGP labeling not enabled
MPLS not operational
MTU = 1508
• Solution 2. Change the TCP MSS to be smaller:
R1(config)#interface ethernet 1/0
R1(config-if)#ip tcp adjust-mss 1452
For detailed, please refer to: https://blog.apnic.net/2014/12/15/ip-mtu-and-tcp-mss-missmatch-an-evil-for-network-performance/
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Questions
• Please remember to fill out the
feedback form
– https://www.surveymonkey.com/r/a
pnic-20171101-eL2
• Slides are available for download
from APNIC FTP.
• Acknowledgement to Cisco
System.
30
APNIC Helpdesk Chat
Thank You!
END OF SESSION
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