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Chapter Goals
• Understand the advantages of MPLS.
• Learn the components of an MPLS system.
• Compare and contrast MPLS and hop-by-hop routing.
• Describe the two methods of label distribution.
• Explain the purpose of MPLS traffic engineering.
MPLS/Tag Switching
Background
In a normally routed environment, frames pass from a source to a destination in a hop-by-hop basis.
Transit routers evaluate each frame’s Layer 3 header and perform a route table lookup to determine the
next hop toward the destination. This tends to reduce throughput in a network because of the intensive
CPU requirements to process each frame. Although some routers implement hardware and software
switching techniques to accelerate the evaluation process by creating high-speed cache entries, these
methods rely upon the Layer 3 routing protocol to determine the path to the destination.
Unfortunately, routing protocols have little, if any, visibility into the Layer 2 characteristics of the
network, particularly in regard to quality of service (QoS) and loading. Rapid changes in the type (and
quantity) of traffic handled by the Internet and the explosion in the number of Internet users is putting
an unprecedented strain on the Internet’s infrastructure. This pressure mandates new traffic-management
solutions. MPLS and its predecessor, tag switching, are aimed at resolving many of the challenges facing
an evolving Internet and high-speed data communications in general.
To meet these new demands, multiprotocol label switching (MPLS) changes the hop-by-hop paradigm
by enabling devices to specify paths in the network based upon QoS and bandwidth needs of the
applications. In other words, path selection can now take into account Layer 2 attributes. Before MPLS,
vendors implemented proprietary methods for switching frames with values other than the Layer 3
header. (MPLS is described in more detail in a later section.)
Based upon Cisco’s proprietary tag-switching protocol, the IETF is defining MPLS as