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1© 2001, Cisco Systems.
MPLS Architecture Overview
MPLS Architecture Overview
Jay [email protected] [email protected]
Adopted from Stefano Previdi’s presentation
22001 Cisco Systems, Inc.
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AgendaAgenda
• MPLS Concepts
• LSRs and labels
• Label assignment and distribution
• Label Switch Paths
• LDP overview
• Day in the Life of a Packet
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MPLS ConceptsMPLS Concepts
• MPLS: Multi Protocol Label Switching
• MPLS is a layer 2+ switching
• Developed to integrate IP and ATM
• MPLS forwarding is done in the same way as in ATM switches
• Packet forwarding is done based on Labels
42001 Cisco Systems, Inc.
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MPLS ConceptsMPLS Concepts
• Unlike IP, classification/label can be based on:
Destination Unicast address
Traffic Engineering
VPN
QoS
• FEC: Forwarding Equivalence Class
• A FEC can represent a: Destination address prefix, VPN, Traffic Engineering tunnel, Class of Service.
52001 Cisco Systems, Inc.
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Agenda
• MPLS Concepts
• LSRs and labels
• Label assignment and distribution
• Label Switch Paths
• LDP overview
• Summary
62001 Cisco Systems, Inc.
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LSRs and LabelsLSRs and Labels
• LSR: Label Switch Router
• Edge-LSR: LSRs that do label imposition and disposition
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LSRs and LabelsLSRs and Labels
IGP domain with a label distribution protocol
• An IP routing protocol is used within the routing domain (e.g.:OSPF, i-ISIS)
• A label distribution protocol is used to distribute address/label mappings between adjacent neighbors
• The ingress LSR receives IP packets, performs packet classification, assign a label, and forward the labelled packet into the MPLS network
• Core LSRs switch packets/cells based on the label value
• The egress LSR removes the label before forwarding the IP packet outside the MPLS network
82001 Cisco Systems, Inc.
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LSRs and LabelsLSRs and Labels
• More than one Label is allowed -> Label Stack
• MPLS LSRs always forward packets based on the value of the label at the top of the stack
Label = 20 bits Exp = Experimental, 3 bits S = Bottom of stack, 1bitTTL = Time to live, 8 bits
0 1 2 30 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 | Exp|S| TTL
92001 Cisco Systems, Inc.
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LSRs and LabelsLSRs and Labels
PPP HeaderPPP Header Layer 3 HeaderLayer 3 HeaderShim HeaderPPP Header(Packet over SONET/SDH)
Ethernet HdrEthernet Hdr Layer 3 HeaderLayer 3 HeaderShim HeaderEthernet
FR HdrFR Hdr Layer 3 HeaderLayer 3 HeaderShim HeaderFrame Relay
ATM Cell Header HECHEC DATADATACLPCLPPTIPTIVCIVCIGFCGFC VPIVPI
Label
HECHEC DATADATACLPCLPPTIPTIVCIVCIGFCGFC VPIVPI
Label
Subsequent cells
102001 Cisco Systems, Inc.
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AgendaAgenda
• MPLS Concepts
• LSRs and labels
• Label assignment and distribution
• Label Switch Paths
• LDP overview
• Day in the Life of a Packet
112001 Cisco Systems, Inc.
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Label Assignment and Distribution
Label Assignment and Distribution
• Labels have link-local significanceEach LSR binds his own label mappings
• Each LSR assign labels to his FECs
• Labels are assigned and exchanged between adjacent neighboring LSR
• Applications may require non-adjacent neighbors
122001 Cisco Systems, Inc.
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Label Assignment and Distribution
Label Assignment and Distribution
• Rtr-C is the downstream neighbor of Rtr-B for destination 171.68.10/24
• Rtr-B is the downstream neighbor of Rtr-A for destination 171.68.10/24
• LSRs know their downstream neighbors through the IP routing protocol
Next-hop address is the downstream neighbor
171.68.10/24
Rtr-BRtr-A Rtr-C
171.68.40/24
Upstream and Downstream LSRs
132001 Cisco Systems, Inc.
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Label Assignment and Distribution
Label Assignment and Distribution
• LSRs distribute labels to the upstream neighbors
171.68.10/24
Rtr-BRtr-A Rtr-C
171.68.40/24
NextNext--HopHop
In In LabLab
--
......
Address Address PrefixPrefix
171.68.10171.68.10
......
OutOutI/FI/F
11
......
Out Out LabLab
3030......
In In I/FI/F
00
...... NextNext--HopHop
In In LabLab
3030
......
Address Address PrefixPrefix
171.68.10171.68.10
......
OutOutI/FI/F
11
......
Out Out LabLab
4040......
In In I/FI/F
00
......
NextNext--HopHop
In In LabLab
4040
......
Address Address PrefixPrefix
171.68.10171.68.10
......
OutOutI/FI/F
11
......
Out Out LabLab
--......
In In I/FI/F
00
......
Use label 40 for destination 171.68.10/24
Use label 30 for destination 171.68.10/24
IGP derived routes
Unsolicited Downstream Distribution
142001 Cisco Systems, Inc.
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Label Assignment and Distribution
Label Assignment and Distribution
• Upstream LSRs request labels to downstream neighbors
• Downstream LSRs distribute labels upon request
171.68.10/24
Rtr-BRtr-A Rtr-C171.68.40/24
Use label 30 for destination 171.68.10/24
Use label 40 for destination 171.68.10/24
Request label for destination 171.68.10/24
Request label for destination 171.68.10/24
On-Demand Downstream Distribution
152001 Cisco Systems, Inc.
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Label Assignment and Distribution
Label Assignment and Distribution
Several protocols for label exchange
• LDP
Maps unicast IP destinations into labels
• RSVP, CR-LDP
Used in traffic engineering
• BGP
External labels (VPN)
• PIM
For multicast states label mapping
162001 Cisco Systems, Inc.
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AgendaAgenda
• MPLS Concepts
• LSRs and labels
• Label assignment and distribution
• Label Switch Paths
• LDP overview
• Day in the Life of a Packet
172001 Cisco Systems, Inc.
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Label Switch Path (LSP)Label Switch Path (LSP)
• LSPs are derived from IGP routing information
• LSPs may diverge from IGP shortest pathLSP tunnels (explicit routing) with TE
• LSPs are unidirectionalReturn traffic takes another LSP
LSP follows IGP shortest path LSP diverges from IGP shortest path
IGP domain with a label distribution protocol
IGP domain with a label distribution protocol
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Label Switch Path (LSP) Penultimate Hop PoppingLabel Switch Path (LSP)
Penultimate Hop Popping
• The label at the top of the stack is removed (popped) by the upstream neighbor of the egress LSR
• The egress LSR requests the “popping”through the label distribution protocol
•Egress LSR advertises implicit-null label
• The egress LSR will not have to do a lookup and remove itself the label
•One lookup is saved in the egress LSR
192001 Cisco Systems, Inc.
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Label Switch Path (LSP) Penultimate Hop PoppingLabel Switch Path (LSP)
Penultimate Hop Popping
0 1
Summary route for 171.68/16
01
171.68.10/24
NextNext--HopHop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
44 171.68/16171.68/16 22 poppop00
...... ...... ...... ............NextNext--HopHop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
-- 171.68/16171.68/16 11 4400
...... ...... ...... ............
Egress LSR summarises morespecific routes and advertises a label for the new FEC
Summary route is propagate through the IGP and label is assigned by each LSR
Use label “implicit-null”for FEC 171.68/16
Summary route for 171.68/16
Use label 4 for FEC 171.68/16
171.68.44/24
Address Address Prefix and maskPrefix and mask
171.68.10/24171.68.10/24
NextNext--HopHop
171.68.9.1171.68.9.1
InterfaceInterface
Serial1Serial1
171.68.44/24171.68.44/24 171.68.12.1171.68.12.1 Serial2Serial2
171.68/16171.68/16 ...... NullNull
Egress LSR needs to do an IP lookup for finding more specific route
Egress LSR need NOT receive a labelled packet
202001 Cisco Systems, Inc.
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AgendaAgenda
• MPLS Concepts
• LSRs and labels
• Label assignment and distribution
• Label Switch Paths
• LDP overview
• Day in the Life of a Packet
212001 Cisco Systems, Inc.
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LDP ConceptsLDP Concepts
• Label Distribution Protocol
• Labels map to FECs for Unicast Destination Prefix
• LDP works between adjacent/non-adjacent peers
• LDP sessions are established between peers
222001 Cisco Systems, Inc.
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LDP MessagesLDP Messages
• Discovery messages
• Used to discover and maintain the presence of new peers
• Hello packets (UDP) sent to all-routers multicast address
• Once neighbor is discovered, the LDP session is established over TCP
232001 Cisco Systems, Inc.
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LDP MessagesLDP Messages
• Session messages
• Establish, maintain and terminate LDP sessions
• Advertisement messages
• Create, modify, delete label mappings
• Notification messages
• Error signalling
242001 Cisco Systems, Inc.
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• MPLS Concepts
• LSRs and labels
• Label assignment and distribution
• Label Switch Paths
• ATM LSRs
• Loops and TTL
• LDP overview
• Day in the Life of a Packet
AgendaAgenda
252001 Cisco Systems, Inc.
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Day in the life of a PacketDay in the life of a Packet
0
Summary route for 171.68/16
01
171.68.10/24
NextNext--HopHop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
77 171.68/16171.68/16 22 poppop00
...... ...... ...... ............NextNext--HopHop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
-- 171.68/16171.68/16 11 4400
...... ...... ...... ............
Egress LSR summarises morespecific routes and advertises a label for the new FEC
Summary route is propagate through the IGP and label is assigned by each LSR
Use label “implicit-null”for FEC 171.68/16
Summary route for 171.68/16
Use label 4 for FEC 171.68/16
Egress LSR needs to do an IP lookup for finding more specific route
171.68.44/24
Address Address Prefix and maskPrefix and mask
171.68.10/24171.68.10/24
NextNext--HopHop
171.68.9.1171.68.9.1
InterfaceInterface
Serial1Serial1
171.68.44/24171.68.44/24 171.68.12.1171.68.12.1 Serial2Serial2
171.68/16171.68/16 ...... NullNull
Use label 7 for FEC 171.68/16
NextNext--HopHop
In In LabLab
Address Address PrefixPrefix
OutOutI/FI/F
Out Out LabLab
In In I/FI/F
44 171.68/16171.68/16 11 7700
...... ...... ...... ............
CE
PEP P PE
0
0
21
262001 Cisco Systems, Inc.
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SummarySummary
• LSRs forward packet based on label information
• IP header and forwarding decision have been de-coupled for better flexibility
• Label information can derive from different sources
IP routing protocols (destination based unicast routing)
Multicast
Traffic Engineering
QoS
VPN
272001 Cisco Systems, Inc.
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SummarySummary
• MPLS allows flexible packet classification and network resources optimisation
• Labels are distributed by different protocols
•LDP, RSVP, BGP, PIM
• Different distribution protocols may co-exist in the same LSR
• Label have local (LSR) significanceNo need for global (domain) wide label allocation/numbering
282001 Cisco Systems, Inc.
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Thank You!Thank You!
