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MPLS TE Overview. Understanding MPLS TE Components. Outline. Overview Traffic Tunnels: Concepts Traffic Tunnels: Characteristics Traffic Tunnels: Attributes Network Links and Link Attributes Constraint-Based Path Computation TE Processes

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mpls te overview

MPLS TE Overview

Understanding MPLS TE Components

  • Overview
  • Traffic Tunnels: Concepts
  • Traffic Tunnels: Characteristics
  • Traffic Tunnels: Attributes
  • Network Links and Link Attributes
  • Constraint-Based Path Computation
  • TE Processes
  • Role of RSVP in Path Setup and Trunk Admission Control
  • Forwarding Traffic to a Tunnel
  • Summary
traffic tunnels concepts
Traffic Tunnels: Concepts
  • The concept of MPLS TE traffic tunnels was introduced to overcome the limitations of hop-by-hop IP routing:
    • A tunnel is an aggregation of traffic flows that are placed inside a common MPLS label-switched path.
    • Flows are then forwarded along a common path within a network.
traffic tunnels concepts cont
Traffic Tunnels: Concepts (Cont.)
  • Unidirectional single class of service modelencapsulates all of the traffic between an ingress and an egress router.
  • Different classes of service model assigns traffic into separate tunnels with different characteristics.
traffic tunnels characteristics
Traffic Tunnels – Characteristics
  • A traffic tunnel is distinct from the MPLS LSP through which it traverses:
    • More than one TE tunnel can be defined between two points:
      • Each tunnel may pick the same or different paths through the network
      • Each tunnel will use different MPLS labels
    • A traffic tunnel can be moved from one path onto another based on resources in the network.
  • A traffic tunnel is configured by defining its required attributes and characteristics.
traffic tunnels attributes
Traffic Tunnels – Attributes
  • Attributes are explicitly assigned through administrative action.
  • A traffic tunnel is characterized by:
    • Its ingress (headend) and egress (tailend) label switch routers
    • The forwarding equivalence class that is mapped onto it
    • A set of attributes that determine its characteristics
traffic tunnels attributes cont
Traffic Tunnels–Attributes (Cont.)
  • The administrator enters the relevant information (attributes) at the headend of the traffic tunnel:
    • Traffic parameter—Resources required for tunnel (for example, required bandwidth)
    • Generic path selection and management—Path can be administratively specified or computed by the IGP
    • Resource class affinity—Include or exclude certain links for certain traffic tunnels
    • Adaptability—Should the traffic tunnel be reoptimized?
    • Priority and preemption—Importance of a traffic tunnel and possibility for a preemption of another tunnel
    • Resilience—Desired behavior under fault conditions
network links and link attributes
Network Links and Link Attributes
  • Resource attributes (link availability) are configured locally on the router interfaces:
    • Maximum bandwidth
      • The amount of bandwidth available
    • Link affinitystring
      • To allow the operator to administratively include or exclude links in path calculations
    • Constraint-based specific metric
      • TE default metric
constraint based path computation
Constraint-Based Path Computation
  • Constraint-based routing is demand-driven.
  • Resource-reservation-aware routing paradigm:
    • Based on criteria including, but not limited to, network topology
    • Calculated at the edge of a network:
      • Modified Dijkstra’s algorithm at tunnel headend (CSPF [Constraint-based SPF] or PCALC [path calculation]).
      • Output is a sequence of IP interface addresses (next-hop routers) between tunnel endpoints.
constraint based path computation cont
Constraint-Based Path Computation (Cont.)
  • Constraint-based routing takes into account:
    • Policy constraints associated with the tunnel and physical links
    • Physical resource availability
    • Network topology state
  • Two types of tunnels can be established across those links with matching attributes:
    • Dynamic—Using the least-cost path computed by OSPF or IS-IS
    • Explicit—Definition of a path by using Cisco IOS configuration commands
traffic engineering processes
Traffic Engineering Processes
  • Information distribution
  • Path selection and calculation
  • Path setup
  • Trunk admission control
  • Forwarding traffic on to tunnel
  • Path maintenance
role of rsvp in path setup procedures
Role of RSVP in Path Setup Procedures
  • When the path has been determined, a signaling protocol is needed:
    • To establish and maintain label-switched paths (LSPs) for traffic tunnels
    • For creating and maintaining resource reservation states across a network (bandwidth allocation)
  • The Resource Reservation Protocol (RSVP) was adopted by the MPLS workgroup of the IETF.
path setup with rsvp
Path Setup with RSVP
  • When the path has been calculated, it must be signaled across the network.
    • Reserve any bandwidth to avoid “double booking” from other TE reservations.
    • Priority can be used to preempt low priority existing tunnels.
  • RSVP is used to set up TE LSP.
    • PATH message (from head to tail) carries LABEL_REQUEST.
    • RESV message (from tail to head) carries LABEL.
  • When RESV messages reaches headend, tunnel interface is up.
  • RSVP messages exist for LSP teardown and error signaling.
rsvp and trunk admission control
RSVP and Trunk Admission Control
  • On receipt of PATH message:
    • Router checks whether there is bandwidth available to honor the reservation.
    • If bandwidth is available, then RSVP is accepted.
  • On receipt of a RESV message:
    • Router actually reserves the bandwidth for the TE LSP.
    • If preemption is required, lower priority LSPs are torn down.
  • OSPF or IS-IS updates are triggered.
forwarding traffic to a tunnel
Forwarding Traffic to a Tunnel
  • IP routing is separate from LSP routing and does not see internal details of the LSP.
  • The traffic has to be mapped to the tunnel:
    • Static routing—The static route in the IP routing table points to an LSP tunnel interface.
    • Policy routing—The next-hop interface is an LSP tunnel.
    • Autoroute—SPF enhancement:
      • The headend sees the tunnel as a directly connected interface (for modified SPF only).
      • The default cost of a tunnel is equal to the shortest IGP metric regardless of the used path.
ip forwarding database modification with autoroute
IP Forwarding Database Modification with Autoroute
  • Autoroute feature enables the headend to see the LSP as a directly connected interface:
    • This feature is used only for the SPF route determination, not for the constraint-based path computation.
    • All traffic directed to prefixes topologically behind the tunnel endpoint (tailend) is forwarded onto the tunnel.
  • Autoroute affects the headend only; other routers on the LSP path do not see the tunnel.
  • Tunnel is treated as a directly connected link to the tailend:
    • When tunnel tail is seen in PATH list during IGP SPF, replace outgoing physical interface with tunnel interface.
    • Inherit tunnel to all downstream neighbors of tailend.
autoroute topology ospf and isis1
Autoroute Topology (OSPF and ISIS)

From R1 Router Perspective: Next hop to R5 is Tunnel1. Next hop to R4 and R8 is Tunnel2.

All nodes behind tunnel are routed via tunnel.



  • Traffic tunnels are configured with a set of resource requirements, such as bandwidth and priority.
  • CSPF augments the link cost by considering other factors such as bandwidth availability or link latency when choosing a path.
  • RSVP with TE extensions is used for establishing and maintaining LSPs.
  • TE tunnels do not appear in the IP routing table.