Understanding MPLS Khurram Waheed Systems Engineer Session Goals • Understand the problems MPLS is addressing • Understand major MPLS technology components • Understand typical MPLS applications • Understand benefits of deploying MPLS Agenda • Introduction • MPLS Basics • MPLS Layer-3 VPNs • MPLS Layer-2 VPNs • MPLS Traffic Engineering • Summary Introduction Why Multi-Protocol Label Switching? • SP/Carrier perspective • Reduce costs (CAPEX/OPEX); consolidate networks and maximise utilisation of resources. • Consolidated network for multiple Layer-2/3 services over same infrastructure • Support increasingly stringent SLAs (Voice + Video etc.) • Enterprise/end-user perspective • Campus/LAN • Need for network segmentation (users, applications, etc.) What is MPLS? Brief Summary • It’s all about labels … • Use the best of both worlds • Layer-2: efficient forwarding and traffic engineering • Layer-3: flexible and scalable • MPLS forwarding plane • Use of labels for forwarding Layer-2/3 data traffic • Labeled packets are switched; instead of routed • Leverage layer-2 forwarding efficiency • MPLS control/signalling plane • Use of existing IP control protocols extensions + new protocols to exchange label information • Leverage layer-3 control protocol flexibility and scalability MPLS Basics Topics Basics of MPLS Signalling and Forwarding • MPLS Reference Architecture Service (Clients) • MPLS Labels Layer-3 VPNs Layer-2 VPNs • MPLS Signalling and Forwarding Operations Transport IP/MPLS (LDP/RSVP-TE/BGP) MPLS Forwarding MPLS Reference Architecture Different Types of Nodes in an MPLS Network • P (Provider) router • Label switching router (LSR) MPLS Domain • Switches MPLS-labeled packets P P • PE (Provider Edge) router CE PE PE CE • Edge router (LER) • Imposes and removes MPLS labels CE CE • CE (Customer Edge) router • Connects customer network to MPLS PE P P PE network Label switched traffic MPLS Labels Label Definition and Encapsulation MPLS Label • 4 Bytes (32Bits) in size 0 1 2 3 • Labels used for making forwarding decision 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 • Multiple labels can be used for MPLS packet encapsulation TC = Traffic Class: 3 Bits; S = Bottom of Stack; TTL = Time to Live • Creation of a label stack MPLS Label Encapsulation • Outer label always used for switching MPLS packets in network LAN MAC Label Header MAC Header Label Layer 3 Packet • Remaining inner labels used for services (VPNs etc.) MPLS Label Stack LAN MAC Label Header MAC Header Label Label S Layer 3 Packet Bottom of Stack Bit Set Basic MPLS Forwarding Operations How Labels are being used to establish End-to-End Connectivity • Label imposition (PUSH) Label Imposition Label Disposition Label Swap Label Swap • By ingress PE router; classify and label packets (Push) (PoP) • Based on Forwarding Equivalence Class (FEC) L1 L1 L2 L2 L3 L3 • Label swapping or switching L2/L3 (SWAP) Packet P P • By P router; forward packets using labels; CE PE PE CE indicates service class & destination • Label disposition (POP) CE CE • By egress PE router; remove label and forward original packet to destination CE PE P P PE Traffic Flow MPLS Path (LSP) Setup and Traffic Forwarding Overview • IGP (OSPF/ISIS) • Learns MPLS Loopback Interfaces (loopback0 IP MPLS etc.) Destination address based Label based Forwarding Forwarding table learned Forwarding table learned • LSP signalling from control plane from control plane • Either LDP or RSVP TTL support TTL support • Leverages IP routing (RIB from IGP) OSPF, IS-IS, BGP Control Plane OSPF, IS-IS, BGP LDP, RSVP • Exchange of labels Packet IP Header One or more labels • Label bindings Encapsulation • Downstream MPLS node advertises what label to use QoS 8 bit TOS field in IP header 3 bit TC field in label • MPLS forwarding OAM IP ping, traceroute MPLS OAM • MPLS Forwarding table (FIB) MPLS Path (LSP) Setup Signalling Options • LDP signalling LDP RSVP • Leverages existing routing (RIB) LSP or TE Tunnel Forwarding path LSP • RSVP signalling Primary and, optionally, backup Based on TE topology • Aka MPLS RSVP/TE database Forwarding Based on IP routing database Shortest-path and/or other • Enables enhanced capabilities, such as Fast Calculation Shortest-Path based Re-Route (FRR) constraints (CSPF calculation) Packet Single label One or two labels Encapsulation Initiated by head-end node towards tail-end node By each node independently Uses routing protocol Signalling Uses existing routing extensions/information protocols/information Supports bandwidth reservation Supports link/node protection IP Packet Forwarding Example Basic IP Packet Forwarding • IP routing information exchanged Forwarding Table Address Forwarding Table Forwarding Table I/F between nodes Prefix Address Address I/F I/F • Via IGP (e.g., OSFP, IS-IS) Prefix Prefix 128.89 0 128.89 128.89 1 0 … 171.69 1 171.69 1 … • Packets being forwarded based on … … destination IP address • Lookup in routing table (RIB) R3 128.89 0 R1 R2 0 128.89.25.4 Data 1 128.89.25.4 Data 1 128.89.25.4 Data 128.89.25.4 Data 171.69 R4 MPLS Path (LSP) Setup Step 1: IP Routing (IGP) Convergence • Exchange of IP routes • OSPF, IS-IS, etc. Forwarding Table Forwarding Table Forwarding Table In Address Out Out In Address Out Out In Address Out Out Label Prefix I’face Label Label Prefix I’face Label Label Prefix I’face Label 128.89 1 128.89 0 128.89 0 • Establish IP reachability 171.69 1 171.69 1 … … … … … … R3 0 128.89 R2 0 R1 1 You Can Reach 128.89 Thru Me You Can Reach 128.89 and 1 171.69 Thru Me Routing Updates You Can Reach 171.69 Thru Me R4 (OSPF, IS-IS, …) 171.69 MPLS Path (LSP) Setup Step 2A: Assign Local Labels • Each MPLS node assigns a local label to each route in local routing table Forwarding Table Forwarding Table Forwarding Table In Address Out Out In Address Out Out In Address Out Out • In label Label Prefix I’face Label Label Prefix I’face Label Label Prefix I’face Label - 128.89 1 20 128.89 0 30 128.89 0 - - 171.69 1 21 171.69 1 … … … … … … … … … … … … R3 0 128.89 0 1 0 R1 R2 1 R4 171.69 MPLS Path (LSP) Setup Step 2B: Assign Remote Labels • Local label mapping are sent to In Address Out Out In Address Out Out In Address Out Out connected nodes Label Prefix I’face Label Label Prefix I’face Label Label Prefix I’face Label - 128.89 1 20 20 128.89 0 30 30 128.89 0 - • Receiving nodes update forwarding - 171.69 1 21 21 171.69 1 36 table … … … … … … … … … … … … • Out label R3 128.89 R2 0 R1 1 0 Use Label 30 for 128.89 Use Label 20 for 128.89 and 1 Use Label 21 for 171.69 Label Distribution Use Label 36 for 171.69 R4 Protocol (LDP) 171.69 (Downstream Allocation) MPLS Traffic Forwarding Hop-by-Hop Traffic Forwarding Using Labels • Ingress PE node adds label to packet In Address Out Out In Address Out Out In Address Out Out (PUSH) Label Prefix I’face Label Label Prefix I’face Label Label Prefix I’face Label • Via forwarding table - 128.89 1 20 20 128.89 0 30 30 128.89 0 - - 171.69 1 21 21 171.69 1 36 • Downstream node use label for … … … … … … … … … … … … forwarding decision (SWAP) • Outgoing interface R3 0 128.89 0 R1 R2 • Out label 128.89.25.4 Data 1 30 128.89.25.4 Data • Egress PE removes label and 128.89.25.4 Data 20 128.89.25.4 Data 1 forwards original packet (POP) R4 Forwarding based on Label 171.69 MPLS Penultimate Hop Popping Forwarding Efficiency Gains with PHP • Downstream node signals POP label (Implicit NULL) In Address Out Out In Address Out Out In Address Out Out Label Prefix I’face Label Label Prefix I’face Label Label Prefix I’face Label • Last LSR (P) removes outer label - 128.89 1 20 20 128.89 0 POP POP 128.89 0 - before sending to LER (PE) R3 0 128.89 • Improves LER (PE) performance by 0 R1 R2 not performing multiple label lookups 128.89.25.4 Data 1 to forward to final packet POP 128.89.25.4 Data 128.89.25.4 Data 20 128.89.25.4 Data • Explicit-NULL can be used for QoS requirements Outer label removed (PHP) Implicit-NULL signaled (POP) Summary…So Far... Key Takeaways • MPLS networks consist of PE routers at in/egress and P routers in core • Traffic is encapsulated with label(s) at ingress (PE router) • Labels are removed at egress (PE router) • MPLS forwarding operations include label imposition (PUSH), swapping (SWAP), and disposition (POP) • Penultimate Hop Popping (PHP) is used to improve LER performance. • LDP and/or RSVP can be used for signalling label mapping information to set up an end-to-end Label Switched Path (LSP) MPLS Virtual Private Networks MPLS Virtual Private Networks Topics • Definition of MPLS VPN service Service (Clients) • Basic MPLS VPN deployment scenario Layer-3 VPNs Layer-2 VPNs • Technology options Transport IP/MPLS (LDP/RSVP-TE/BGP) MPLS Forwarding What is a Virtual Private Network? Definition • Set of sites which communicate with each other • Typically over a shared public or private network infrastructure • Defined by a set of administrative policies • Policies established by VPN customers themselves (DIY) • Policies implemented by VPN service provider (managed/unmanaged) • Different inter-site connectivity schemes possible • Full mesh, partial mesh, hub-and-spoke, etc. • VPN sites may be either within the same or in different organisations • VPN can be either intranet (same org) or extranet (multiple orgs) MPLS VPN Example Basic Building Blocks • VPN policies PE-CE VPN Signalling PE-CE • Configured on PE routers (manual operation) Link Link • VPN signalling PE P VPN P PE • Between PEs CE CE • Exchange of VPN policies VPN VPN Policy Policy VPN VPN • VPN traffic forwarding Policy Policy CE CE