Multiprotocol Label Switching (MPLS) provides label switched path to deliver packets in networks. This is an introduction course to understand different terminologies and concepts associated with MPLS.

1. MPLS Intro
An Introduction to MPLS Networks and Applications
by Shawn Zandi

2. MPLS History
 1994: Toshiba presented Cell Switch Router as IETF
BOF
 1996: Ipsilon, Cisco and IBM supported the idea
 1997: Formation of the IETF MPLS working group
 IETF released RFC 2547 "BGP/MPLS VPNs" in
1999
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3. Terminology
Cisco
Terminology
New Terminology
Tag Switching MPLS
Tag Label
TDP LDP (Label Distribution Protocol)
TFIB LFIB (Label Forwarding Information
Base)
TSR LSR (Label Switch Router)
TSC LSC (Label Switch Controller)
TSP LSP (Label Switched Path)
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4. MPLS Applications
 MPLS VPN
 Most Popular Application of MPLS
 Traffic Engineering
 First called RRR or R3 (Routing & Resource
Reservation)
 AToM (Any Transport over MPLS)
 Point-to-point circuits over MPLS – Frame Relay, ATM,
PPP, HDLC, Ethernet and IEEE 802.1Q
 VPLS (Virtual Private LAN Service)
 Ethernet Supported in a multipoint fashion.
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5. Advantages of MPLS
 MPLS VPN
 VRF routing tables and ease of growth.
 Unified infrastructure
 Carrier for any technology - ATM, FR, PPP, Ethernet, IPv4 and
IPv6.
 Better IP over ATM than pervious solutions
 AAL5 - RFC1483, LANE, Multiprotocol over ATM - MPOA
 BGP-free core
 providers need IP routing but BGP is only required on edges.
 Optimal Traffic Flow
 Connections logically are fully mesh and no extra circuit
mapping is required.
 Traffic Engineering
 Different path from least cost path, Source-based routing &
Fast Re-Routing (FRR)
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6. MPLS Label
 32 bits header (4 Bytes) = each stack
 Unlimited Stacks supported, The last stack BOS flag=1
 Label: 20 bits
 EXP: 3 bits
 BOS: 1 bit (bottom of stack)
 TTL: 8 bits
Label EXP TTL
BOS6

7. Label Stack
 Top label and bottom label on a stack:
Label EXP TTL0
Label EXP TTL0
Label EXP TTL1
…
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8. Label Stack (cont.)
 Some MPLS applications like MPLS VPNs require
more than one label in the label stack to forward the
packets.
 MPLS VPNs and AToM put two labels in the label
stack.
 MPLS is not a Layer 2 Protocol, not even Layer 3
 MPLS is viewed as a Layer 2.5 protocol.
Label 0 Label 1 IPv4PPP
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9. Protocol Identifier
 Data Link Layer Protocol Identifier
 ATM uses a different way for encapsulating the Label.
Layer 2 Protocol Identifier Field Value (hex)
PPP Protocol Field 0281
Ethernet SNAP Ether-type 8847
HDLC Protocol 8847
Frame Relay NLPID 80
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10. Label Switch Router
 LSR is a router that supports MPLS.
 Ingress LSR
 Inserts a label (push) and sends packet to MPLS network.
 Egress LSR
 Removes the label (pop) and sends packet on a data link.
 Intermediate LSR
 Modifies the label (swap) and switches the packets.
 Edge LSR = Ingress and Egress LSRs
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11. MPLS Network
Label Switch Router (cont.)
Ingress
LSR
Intermediate LSR Egress LSR
Imposing LSR Disposing
LSR
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12. MPLS VPN
Label Switch Router (cont.)
Provider Edge PE CECustomer Edge P
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13. MPLS Network
Label Switched Path
Ingress
LSR
LSR LSR LSR Ingress
LSR
LSP
LSP : Sequence of LSRs – a path through the MPLS
network. (Unidirectional)
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14. MPLS Network
Nested LSP
Ingress
LSR
LSR LSR LSR Ingress
LSR
LSP
LSP inside LSP – The second Label spans the entire
MPLS network.
LSP
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15. Forwarding Equivalence Class
 FEC is group or flow of packets classified for a
forwarding decision and have similar EXP.
 Ingress LSR decides which packet belongs to which
FEC.
 All packets with same FEC get the same label imposed
by the ingress LSR
 Same FEC = Same Label.
 Same Label <> Same FEC. (might have different
FEC)
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16. FEC Classification
 Ingress LSR Classifies traffic (FEC) based on:
 Certain multicast group
 Based on DSCP or Precedence
 Based on VC (sub-interface)
 Based on Destination IP
 Based on BGP Prefixes pointing to the same Next-hop.
 In this case all traffic for an Egress LSR (iBGP Peer) can be
forwarded through a specific LSP.
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17. Label Distribution
 Adjacent LSRs must agree to use specific Label for
a specific IP Prefix.
 Labels are local and have no global meaning.
 Labels are between adjacent LSRs.
 A label distribution protocol is required.
 Using IP Routing Protocol (EIGRP, ISIS, OSPF)
 Using Label Distribution Protocol (TDP, LDP, RSVP)
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18. Label Distribution with Routing Protocol
 Advantage:
 Does not need a new label distribution protocol.
 Routing and Label distribution are always in sync.
 EIGRP implementation is straight forward.
 Disadvantage:
 Link state routing protocols do not function this way.
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19. Label Distribution with LDP
 Label Information Base (LIB) holds remote and local
label bindings.
 One local binding per prefix.
 Label Space:
 Per platform
 Per interface (LC-ATM)
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20. LIB LFIB
 LFIB is Label Forwarding Instance Base, a table
used to forward incoming and outgoing labels for
LSPs.
1. All remote bindings LIB
2. Only one possible outgoing label in LIB LFIB
LDP
Static
MPBGP
RSVP
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21. MPLS Payload
 The MPLS has no Network Level Protocol Identifier.
 NLPID exists in all Layer 2 protocols (different
names)
 Intermediate LSRs do not need to know what
payload is.
 Egress LSR should know what the payload is, to
forward.
 Egress LSR is the one who created label binding for
FEC.
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22. LDP Modes
 Label Distribution Mode
 DOD Downstream-on-Demand (pull mode – LC-ATM)
 UD Unsolicited Downstream (push mode – Default)
 Label Retention Mode
 LLR Liberal Label Retention (keep all bindings in LIB -
Default)
 CLR Conservative Label Retention (LC-ATM)
 LSP Control Mode
 Independent LSP (immediate local binding for FEC -
Default)
 Ordered LSP (IOS ATM switches)
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23. LFIB Forwarding Commands
 show ip cef x.x.x.x
 show adjacency table
 show mpls forwarding-table
 show mpls forwarding-table x.x.x.x
 show mpls forwarding-table x.x.x.x detail
 show mpls forwarding-table vrf …
 show mpls interfaces … detail
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24. Label Operation
 Pop
 Swap
 Push
 Untagged/No Label
 Aggregate
 Remove the label stack then perform IP Lookup.
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25. IOS Label Range
 Default Label Range 16 to 100,000
 Router(config)# mpls label range 16 1048575
 Unknown Label: drop
 Reserved Label 0 to 15:
 Implicit NULL Label (3)
 Set by egress LSR for connected and summarized prefixes to
penultimate LSR to not send Label. “penultimate hop popping”
PHP
 Explicit NULL Label(0, for IPv6=2)
 Like implicit NULL but send label=0 to retain EXP value.
 Router Alert Label (1)
 Perform software Lookup instead of hardware
 OAM Alert Label (14) RFC 3429 – not supported on IOS
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26. TTL and MPLS
 TTL (-1) is copied from IP header to MPLS and vice
versa.
 Don’t copy if TTL value is greater than packet TTL.
Label to
IP
Igress
LSR
LSR LSR
TTL=25
4
TTL=25
3
TTL=25
3
TTL=25
3
TTL=25
3
TTL=25
2
TTL=25
3
TTL=25
1
TTL=25
2
IP to
Label
TTL=25
3
Label to
Label
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27. TTL – Label to Label
Intermediate LSR does not change TTL in IP header or underlying
labels.
POP
LS
R
LSR LSR
TTL=25
3
TTL=25
3
TTL=24
9
TTL=24
8
TTL=25
3
TTL=25
3
TTL=25
1
TTL=25
3
TTL=25
3
TTL=25
1
TTL=25
1
TTL=25
0
SWAP
TTL=25
2
TTL=25
1
PUSH
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28. TTL Expire
 ICMP “time exceeded” (type 11 code 0) is forwarded along the LSP
because interim LSR might have no route to the originator of packet.
Ingress Egres
s
TTL=2 TTL=1
TTL=1
ICMP
Time
Exceede
d
TTL=25
5
TTL=25
5
TTL
Expired
!
ICMP
Time
Exceede
d
TTL=25
4
ICMP
Time
Exceede
d
TTL=25
3
ICMP
Time
Exceede
d
TTL=25
1
TTL=25
3ICMP
Time
Exceede
d
TTL=25
3
TTL=25
2ICMP
Time
Exceede
d
TTL=25
0
LSR LSRLSR
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29. MPLS MTU
 Maximum size of packet that can be sent to data link
without fragmentation.
 MRU Maximum Receive Unit used in LFIB for
neighbors.
 A value per FEC (or prefix) not based on interface.
 On LSR configure MTU to 1508 (1500 + 2 Labels)
 (config-if)# mpls mtu 1508
 If on switches MTU is not increased = baby giant
drops.
 (config)# system jumbomtu
 (config)# system mtu 1508
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30. Fragmentation
 Fragmentation <> Performance
 LSR strips the label and fragments payload.
 Path MTU Discovery
 Modern hosts send IP with DF bit set (Don’t Fragment) to
receive ICMP type 3 code 4 “Fragmentation needed”
 Process continues with lowering the size till no error is
received and correct MTU achieves.
 LSR sends ICMP type 3 code 4 along with LSP (just
like TTL exceeded)
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31. Label Distribution Protocol
 Discovery of LSRs (hello message –
224.0.0.2/UDP/646)
 Session Establishment – TCP
 Label Mapping Advertisement
 Notifications
 LDP needs “ip cef”
 Interface configration: “mpls ip”
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32. LDP - LSR Discovery
 LDP hello messages are UDP 646 to 224.0.0.2
 “all routers on this subnet”
 show mpls ldp discovery [detail]
 show mpls interfaces
 (config-if)# mpls ldp discovery
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