MPLS: Multi-Site WAN Architecture

MPLS (Multi-Protocol Label Switching) is the established protocol for carrier and enterprise WANs. It provides fast label switching, L2/L3 VPN support, Traffic Engineering, and per-LSP QoS. It remains widely deployed despite competition from SD-WAN. 2026 MPLS architecture guide: L3 VPN, VPLS, Segment Routing, and convergence.

MPLS Concepts

• Label: 20 bits + TC (3 QoS bits) + S (1 bottom-of-stack bit) + TTL (8 bits)
• LSR (Label Switching Router): MPLS core routers
• LER (Label Edge Router): ingress/egress routers (ingress/egress PE)
• LSP (Label-Switched Path): path precomputed through label swapping
• PE (Provider Edge): connects customers through CE (Customer Edge) devices
• P (Provider): backbone router that processes only labels

Control Plane

• LDP (Label Distribution Protocol): distributes labels for MPLS forwarding
• RSVP-TE: label distribution + Traffic Engineering (TE)
• Segment Routing (SR-MPLS): newer standard in which labels represent IGP segments, eliminating the need for LDP/RSVP
• MP-BGP: distributes L3 VPN routes (VPNv4, VPNv6)

MPLS L3VPN

The most common use case. Multi-customer isolation is implemented through VRFs:

• VRF: routing table isolated per customer on the PE
• RD (Route Distinguisher): 64 bits, making prefixes unique across customers
• RT (Route Target): BGP communities used to import/export routes between VRFs
• VPNv4 = RD:prefix (96 bits) + RT

Cisco IOS-XR PE Configuration (L3VPN Example)

• vrf CLIENT-A
• address-family ipv4 unicast
• import route-target 65001:100
• export route-target 65001:100
• router bgp 65001
• vrf CLIENT-A
• rd 65001:100
• address-family ipv4 unicast
• neighbor 192.168.1.2 remote-as 65100
• interface GigabitEthernet0/0/0/0
• vrf CLIENT-A
• ipv4 address 192.168.1.1/30

MPLS L2VPN (VPWS / VPLS)

• VPWS (Virtual Private Wire Service): point-to-point L2 over MPLS (Ethernet pseudowire)
• VPLS (Virtual Private LAN Service): multipoint L2 over MPLS (emulates an Ethernet switch)
• EVPN: modern replacement for VPLS, offering greater scalability and active-active multihoming

Segment Routing MPLS (SR-MPLS)

Modern successor to LDP/RSVP-TE:

• Labels distributed by the IGP (OSPF/IS-IS extensions)
• Source routing: the headend adds a stack of labels (segments) to the packet
• Node SID: identifies a router
• Adjacency SID: identifies a link
• No need for RSVP-TE to implement Traffic Engineering
• SR Policy: programmable TE policies

Traffic Engineering

• RSVP-TE LSPs: reserved bandwidth and explicit paths
• SR-TE: SR-based equivalent
• Fast Reroute (FRR): precomputed backup path, switching in <50ms if a failure occurs
• IGP metric = TE cost by default and can be customized

Regional ISP Sizing

• 2× PE per POP (Point of Presence)
• 4× redundant P core routers in a mesh topology
• Inter-POP links: 100G minimum (400G in 2026)
• Typical hardware: Juniper MX204/MX480, Cisco ASR 9000/NCS 540

MPLS vs SD-WAN

• MPLS: guaranteed carrier QoS (CoS), contractual SLA, and high cost (€15-30/Mbps)
• Internet SD-WAN: no carrier QoS, best-effort SLA, and 10× lower cost (~€2/Mbps)
• Hybrid: SD-WAN with MPLS prioritized for voice/video + Internet for bulk traffic

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