MPLS

Multi-Protocol Label Switching. A forwarding mechanism, not a routing protocol. Sits between Layer 2 and Layer 3 (“Layer 2.5”). Resides in service provider/carrier networks only.

Standard IP routing requires every router to perform a full destination-IP lookup per packet. Expensive at scale. ATM/Frame Relay switch faster using short labels (VCI/DLCI) but lack IP’s scalability. MPLS combines label-based switching speed with IP’s dynamic routing.

Routing protocols (OSPF, BGP-4, IS-IS) still determine paths. MPLS handles forwarding only.

Label Header

Inserted between the Layer 2 and IP headers. 32 bits total:

• Label: 20 bits forwarding identifier
• Traffic Class (TC): 3 bits QoS priority
• Bottom of Stack (S): 1 bit set on the innermost label in a stack
• TTL: 8 bits same semantics as IP TTL

Multiple labels can be stacked. LSRs read only the outermost label.

Mechanism

Label Switched Path

A predetermined sequence of LSRs from ingress to egress LER. Established before traffic flows.

A virtual circuit. Can be permanent or dynamically established via RSVP-TE or LDP.

Label Edge Router

Aka. LER. Entry and exit points of a MPLS network. Inspects IP header of incoming packets, attaches label, determines LSP. Strips label from the outgoing packet and delivers normal IP packet.

Label Switch Router

Aka. LSR. Core router. Swaps label at each hop. Never inspects the IP header.

Applications

• Network scalability
  Core routers skip per-packet IP lookups. Only edge routers consult IP routing tables.
• Traffic engineering
  Paths explicitly controlled, independent of dynamic routing. Provider load-balances and routes by priority or latency.
• VPNs
  Layer 3 VPNs via label stacking. Inner label identifies the customer circuit, outer label routes through the provider core. Customers are logically isolated on shared infrastructure.
• IP Multicast
  Multicast traffic carried along pre-established branching LSPs. Avoids per-router group membership state.
• IP CoS
  TC field in the label header marks packets with priority. LSRs queue and forward based on priority class.
• RSVP
  Signalling protocol for bandwidth reservation along a path. RSVP-TE establishes LSPs with guaranteed bandwidth and QoS.

Convergence

A single MPLS network carries ATM, Frame Relay, TDM, and IP traffic. Consolidates previously separate networks into one infrastructure.

Hybrid Position

Principle of MPLS is route at the edge, switch at the core.

• From IP
  Scalable, flexible, dynamic routing, inexpensive.
• From ATM/FR
  Performance, connection-oriented forwarding, traffic engineering, QoS, security.