Introduction to MPLS¶

What is MPLS?¶

Multi-Protocol Label Switching is a new paradigm in the way routers make forwarding decisions.
Multiprotocol: MPLS is capable of transporting many L2 and L3 protocols
Label Switching: Packets are switched based on labels, not destination IP!
Labels - Ultimately determine the destiny of a packet through an MPLS network

Why MPLS?¶

Unified network infrastructure
ATM concepts integrated into an IP world
Scalable and secure L3 VPNs using a peer-to-peer model
BGP-free core
Traffic engineering capabilities
WAN Quality of Service (QoS) requirements

MPLS Key Terms¶

MPLS Labels¶

MPLS is often described as a shim or layer 2.5 protocol
An MPLS packet contains a label stack, consisting of one or more MPLS labels

Label - the MPLS label itself, determines destiny of the packet
EXP- Experimental (not so much any more), used for QoS markings
S - Boolean, if set to 1, it’s bottom of the label stack. If 0, there’s another label on the stack
TTL - Time-to-live for loop prevention and hop count (as it is in IP)

Push vs Pop vs Swap - The swap operation means that the top label in the label stack is replaced with another - The push operation means that the top label is replaced with another and then one or more additional labels are pushed onto the label stack - The pop operation means that the top label is removed

Label Switched Routers¶

A label switched router (LSR) is a router running MPLS
LSR can be an ingress edge, intermediate or egress edge LSR
A Label Switched Path (LSP) is the series of LSRs that switch a labeled packet

Forwarding Equivalency Class (FEC)¶

Group of packets that are forwarded the same way through the MPLS network
Every packet in the same FEC enters the MPLS network with the same label
FEC can theoretically be based on many different things L3 destination prefix, QoS, L2 PVC, BGP next-hop, TE tunnel, link color, etc…