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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 isis

Tunnel1: R1 R2  R3  R4  R5 Tunnel2: R1 R6  R7  R4

Autoroute Topology (OSPF and ISIS)

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.