Cisco CRS Configuration Manual page 185

Implementing MPLS Traffic Engineering
MPLS-TE tunnels are calculated at the LSP headend router, based on a fit between the required and available
resources (constraint-based routing). The IGP automatically routes the traffic to these LSPs.
Typically, a packet crossing the MPLS-TE backbone travels on a single LSP that connects the ingress point
to the egress point. MPLS-TE is built on these mechanisms:
Tunnel interfaces
From a Layer 2 standpoint, an MPLS tunnel interface represents the headend of an LSP. It is configured
with a set of resource requirements, such as bandwidth and media requirements, and priority. From a
Layer 3 standpoint, an LSP tunnel interface is the headend of a unidirectional virtual link to the tunnel
destination.
MPLS-TE path calculation module
This calculation module operates at the LSP headend. The module determines a path to use for an LSP.
The path calculation uses a link-state database containing flooded topology and resource information.
RSVP with TE extensions
RSVP operates at each LSP hop and is used to signal and maintain LSPs based on the calculated path.
MPLS-TE link management module
This module operates at each LSP hop, performs link call admission on the RSVP signaling messages,
and performs bookkeeping on topology and resource information to be flooded.
Link-state IGP (Intermediate System-to-Intermediate System [IS-IS] or Open Shortest Path First
[OSPF]—each with traffic engineering extensions)
These IGPs are used to globally flood topology and resource information from the link management
module.
Enhancements to the shortest path first (SPF) calculation used by the link-state IGP (IS-IS or OSPF)
The IGP automatically routes traffic to the appropriate LSP tunnel, based on tunnel destination. Static
routes can also be used to direct traffic to LSP tunnels.
Label switching forwarding
This forwarding mechanism provides routers with a Layer 2-like ability to direct traffic across multiple
hops of the LSP established by RSVP signaling.
One approach to engineering a backbone is to define a mesh of tunnels from every ingress device to every
egress device. The MPLS-TE path calculation and signaling modules determine the path taken by the LSPs
for these tunnels, subject to resource availability and the dynamic state of the network.
The IGP (operating at an ingress device) determines which traffic should go to which egress device, and steers
that traffic into the tunnel from ingress to egress. A flow from an ingress device to an egress device might be
so large that it cannot fit over a single link, so it cannot be carried by a single tunnel. In this case, multiple
tunnels between a given ingress and egress can be configured, and the flow is distributed using load sharing
among the tunnels.
Related Topics
Building MPLS-TE Topology, on page 213
Creating an MPLS-TE Tunnel, on page 216
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
Overview of MPLS Traffic Engineering
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