This is Video #1 of our MPLS Traffic Engineering series, which is designed to help viewers gain basic understanding of MPLS Traffic Engineering and how to configure TE tunnels in Cisco routers.

CCNA Training – Resources (Intense)

In this video, we will discuss the basic theory of MPLS Traffic Engineering.


Lecture Notes:

Review of MPLS

  • Commonly used in Service Provider environments and now in enterprise networks.
  • Layer 1 and Layer 2 agnostic, can be built with any kind of circuits.
  • Flexible is capable of providing L2 and L3 services. (e.g. MPLS VPNs, AToM, VPLS)
  • Resides in Layer 2.5 in the OSI Model.
  • Requires an IGP to work.
  • Uses labels to forward traffic across the network termed as “label switching”. Labels are assigned for each of the routes in the routing table. MPLS routers may assign different labels for an IP prefix.
  • LSP (Label Switch Path) is the path taken to reach one PE to another.
  • Packets are assigned to FECs (Forwarding Equivalence Class)
  • LER (Label Edge Routers) also known as PE routers connect to CE (Customer Edge) routers. LSR (Label Switch Routers) or P routers form the core of the MPLS network.

Problems MPLS TE Solve

  • IGP operates in the logic of shortest path so LSP takes the shortest path.
  • Shortest path is not always the best path! (It might be congested!)
  • This means other non-shortest path are not utilized and not efficiently used.
  • With MPLS TE these non-shortest paths can be utilized thus leading to improve traffic flows and less congestions.
  • MPLS TE also provides sub second failover times with its fast-reroute (FRR) feature.

Anatomy of an MPLS TE Tunnel

  • MPLS TE tunnels are represented as tunnel interfaces in the Cisco IOS – not much different from a regular GRE tunnel.
  • Unidirectional tunnel
  • Used CSPF (Constrained Shortest Path First) – shortest path fulfilling all conditions. (bandwidth, link color, TE metric)
  • Configured as “IP unnumbered”.
  • Head-end – is where the tunnel interface is configured. Can be on a PE or P router.
  • Mid-point – the multiple LSRs the tunnel LSP passes through.
  • Tail-end – The destination of the tunnel. Can terminate to any MPLS LSR.


  • CEF (Cisco specific only, for MPLS label imposition/disposition)
  • Link State Protocol (OSPF and IS-IS extensions to MPLS TE)
    • Flooding of information to build a whole view of the network
      • Link Identification
      • Bandwidth Information
      • TE Metric
      • Attribute Flags
    • LSP signaling and bandwidth reservation.
    • Path message is to request label information in the downstream direction.
    • RESV is to send back label information upstream.
  • LDP (optional)
    • Required to be configured on the TE tunnel interface when TE tunnel head-end and tail-end is on P routers.
    • Not required if MPLS TE is used in an IP only network. (No MPLS VPNS)