An introduction to MPLS networks and applications
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The document provides an overview of MPLS (Multiprotocol Label Switching), detailing its history, terminology, main applications such as MPLS VPN and traffic engineering, and key advantages like optimal traffic flow and unified infrastructure. It outlines the structure of MPLS labels, the roles of Label Switch Routers (LSRs), and methods for label distribution among adjacent LSRs. Additionally, it addresses MPLS network operations, forwarding equivalence classes, and the importance of TTL in label processing.
![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
32](https://image.slidesharecdn.com/mplsnetworksintroduction1-161023205553/85/An-introduction-to-MPLS-networks-and-applications-32-320.jpg)
An introduction to MPLS networks and applications
- 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 2
- 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) 3
- 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. 4
- 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) 5
- 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 … 7
- 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 8
- 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 9
- 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 10
- 11. MPLS Network Label Switch Router (cont.) Ingress LSR Intermediate LSR Egress LSR Imposing LSR Disposing LSR 11
- 12. MPLS VPN Label Switch Router (cont.) Provider Edge PE CECustomer Edge P 12
- 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) 13
- 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 14
- 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) 15
- 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. 16
- 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) 17
- 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. 18
- 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) 19
- 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 20
- 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. 21
- 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) 22
- 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 23
- 24. Label Operation Pop Swap Push Untagged/No Label Aggregate Remove the label stack then perform IP Lookup. 24
- 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 25
- 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 26
- 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 27
- 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 28
- 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 29
- 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) 30
- 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” 31
- 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 32