Chapter 8: The MPLS Conceptual Model

Multiprotocol Label Switching (MPLS) is defined in RFC 3031.

Terminology

• Label - Identifies a group of networks sharing a common destination
• Label stack - An ordered set of independent labels attached to a packet header
• Label swap - Forwarding operation based on label lookups
• Label-Switched Hop (LSH) - The hop between two MPLS nodes *Label-Switched Path (LSP) - The path taken through multiple LSRs
• Label Switching Router (LSR) - An MPLS node capable of forwarding labeled packets

MPLS Features

Label switching is not dependent on L3 routing functionality.

MPLS is designed to forward packets on the minimum amount of information required (a short label rather than an entire IP header).

Packets are grouped by destination into Forwarding Equivalence Classes (FECs).

Packets are assigned to an FEC by applying a label at the ingress MPLS node. Packets are relabeled at each Label Switching Router (LSR).

Service providers use MPLS technologies to isolate each customers' routing information, forming an MPLS VPN.

A Penultimate Hop Pop (PHP) occurs when an LSR directly before the egress edge LSR remove the label, so that the egress edge LSR only has to make a routing decision (versus a label and routing decision).

Router Switching Mechanisms

• Process Switching - Each packet is processed individually; very resource intensive
• Cache-driven (Fast) Switching - The first packet to a destination is process-switched and the routing decision is cached; subsequent packets are forwarded based on the cache entry
• Topology-driven (CEF) Switching - A Forwarding Information Base (FIB) is built in parallel to the routing table; provides high-speed layer 3 switching

MPLS is CEF switched.