MPLS Basics

MPLS Study Notes Terminology

Penultimate Hop Popping

RFC 3031

Transport diff payloads
-Ethernet, FRLY, ATM,IPv4, IPv6

Layer 3 MPLS VPN (RFC2547) will be the focus of the 4.0 R&S Exam.


Label Switching

  • Labels used to route(switch)traffic instead of normal IPv4/IPv5 route.
  • Similar to how FRLY DLCI or ATM VPI/VCI works….
  • Labels are locally significant between each adjacent neighbor.
  • MPLS TE(Traffic Engineering), fast convergence…
  • SP no longer has to hold 300k routes on every router in their network.

How MPLS Works

  • Layer2 or Layer3 traffic enters SP network.
  • MPLS label is added to incoming traffic at SP edge(PE) or (LSR).
  • SP core switches traffic towards exit point using MPLS label
  • MPLS label is removed as traffic exits SP network
  • Traffic is “tunneled” from SP entry to SP exit since SP core does not inspect payload.

How Labels Work

Each label represents a unique IP prefix

  • Label to IP binding is called a Forwarding Equivalence Class(FEC)

Labels are dynamically advertised using a Label Distribution Protocol. LDP and TDP use IS-IS or OSPF.

  • Tag Distribution Protocol(TDP)
  • >>  Legacy and Cisco Proprietary
  • Label Distribution Protocol(LDP)…Most common(TCP based)
  • >>  Open Standard
  • BGP
  • >>  Multiprotocol Extensions…MP-BGP or MBGP
  • Resource Reservation Protocol(RSVP)
  • >>  Used for MPLS TE

We are going to look at LDP and BGP for the 4.0 Exam.

How Switching Works
Label + Prefix binding forms Label Forwarding Information Base (LFIB)
When traffic is received, LFIB is consulted in order to perform one of 3 operations

  • Label push(ADD)
    Add a label to an incoming packet
    AKA label imposition
  • Label swap(Change)
    Replace the label on an incoming packet
  • Label pop(Remove)
    Remove the label from an outgoing packet
    AKA label disposition

MPLS Terms
Customer Edge (CE) Router

  • CPE equip that typically does not run MPLS
  • Can be layer 2 only or layer 3 aware

Provider Edge (PE) Router

  • Receives traffic from Customer Edge(CE) devices, adds MPLS label(push), and forwards into core
  • Receives traffic from core, removes MPLS labels(pop), and forwards towards Customer Edge(CE) devices.

Provider (P) Router

  • Connects only to PE’s and other P routers
  • Switches traffic between interfaces based on MPLS labels(swap)
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