Prerequisites For Implementing Cisco Mpls Forwarding; Restrictions For Implementing Cisco Mpls Forwarding; Information About Implementing Mpls Forwarding; Mpls Forwarding Overview - Cisco CRS Configuration Manual

Prerequisites for Implementing Cisco MPLS Forwarding

Prerequisites for Implementing Cisco MPLS Forwarding
These prerequisites are required to implement MPLS Forwarding:
• You must be in a user group associated with a task group that includes the proper task IDs. The command
• Router that runs Cisco IOS XR software.
• Installed composite mini-image and the MPLS package, or a full composite image.

Restrictions for Implementing Cisco MPLS Forwarding

• Label switching on a Cisco router requires that Cisco Express Forwarding (CEF) be enabled.
• CEF is mandatory for Cisco IOS XR software and it does not need to be enabled explicitly.

Information About Implementing MPLS Forwarding

To implement MPLS Forwarding, you should understand these concepts:

MPLS Forwarding Overview

MPLS combines the performance and capabilities of Layer 2 (data link layer) switching with the proven
scalability of Layer 3 (network layer) routing. MPLS enables service providers to meet the challenges of
growth in network utilization while providing the opportunity to differentiate services without sacrificing the
existing network infrastructure. The MPLS architecture is flexible and can be employed in any combination
of Layer 2 technologies. MPLS support is offered for all Layer 3 protocols, and scaling is possible well beyond
that typically offered in today's networks.
Based on routing information that is stored in the VRF IP routing table and VRF CEF table, packets are
forwarded to their destination using MPLS.
A PE router binds a label to each customer prefix learned from a CE router and includes the label in the
network reachability information for the prefix that it advertises to other PE routers. When a PE router forwards
a packet received from a CE router across the provider network, it labels the packet with the label learned
from the destination PE router. When the destination PE router receives the labeled packet it pops the label
and uses it to direct the packet to the correct CE router. Label forwarding across the provider backbone, is
based on either dynamic label switching or traffic engineered paths. A customer data packet carries two levels
of labels when traversing the backbone:
• Top label directs the packet to the correct PE router
• Second label indicates how that PE router should forward the packet to the CE router
Related Topics
Configuring the Size of the Local Label, on page 160
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
156
reference guides include the task IDs required for each command. If you suspect user group assignment
is preventing you from using a command, contact your AAA administrator for assistance.
Implementing MPLS Forwarding