Restrictions For Implementing Cisco Mpls Forwarding; Information About Implementing Mpls Forwarding; Mpls Forwarding Overview; Label Switching Functions - Cisco ASR 9000 Series Configuration Manual

Restrictions for Implementing Cisco MPLS Forwarding

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

Label Switching Functions

In conventional Layer 3 forwarding mechanisms, as a packet traverses the network, each router extracts all
the information relevant to forwarding the packet from the Layer 3 header. This information is then used as
an index for a routing table lookup to determine the next hop for the packet.
In the most common case, the only relevant field in the header is the destination address field, but in some
cases, other header fields might also be relevant. As a result, the header analysis must be done independently
at each router through which the packet passes. In addition, a complicated table lookup must also be done at
each router.
In label switching, the analysis of the Layer 3 header is done only once. The Layer 3 header is then mapped
into a fixed-length, unstructured value called a label.
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Implementing MPLS Forwarding
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