What Is MPLS?
J.D. Wegner, Global Knowledge Instructor, Certified
Cisco Instructor
Excerpted from the "MPLS: What is it? What can it do for me?"
white paper. Download the complete paper
from our Knowledge Center.
Traditional routers forward packets by examining the
network-layer header (typically the destination IP address),
searching for the best matching entry in the routing table, and
forwarding the packet through the specified interface to the next
hop router. This process is time-consuming and is repeated for each
packet at each router along the path. Because no state is
maintained from packet to packet, the system is highly scalable,
but inefficient.
Multiprotocol Label Switching (MPLS) began life as a way for
routers to short-cut the process of treating each packet
independently. In an MPLS network, the ingress router does a
standard lookup and assigns a numeric label to the packet.
The MPLS label is assigned by the ingress edge router based on
a Forwarding Equivalence Class (FEC) which represents a series of
packets to be forwarded in the same manner, over the same path, to
the same destination. In basic IP routing, for example, a label is
assigned to each target IP network that the MPLS domain knows
about. An FEC could also be associated with specific classes of
service for QoS processing, or with a VPN.
Core routers then examine the label and forward the packet
according to the label. All packets with the same label are
forwarded the same way. This relieves the core routers of much
processing, making the overall network more efficient.
The beauty of MPLS is that the label itself has no meaning other
than what the software gives it. Because the labels are just
numbers, they can be assigned according to any criteria the router
software supports. This feature give MPLS an extraordinary ability
to support many networking applications.
The label can be used to implement any forwarding treatment
that comes to mind. By allowing multiple labels to be stacked
within a packet, MPLS permits multiple applications, such as QoS
and VPNs, to be combined. Once a packet has entered the MPLS
domain, the routers use a simple and fast label lookup process to
forward the packet to its next hop. In the Cisco implementation,
MPLS leverages the Cisco Express Forwarding (CEF) feature to
optimize label lookup and forwarding.
For MPLS to operate, labels must be generated, stored, and
distributed. The Label Distribution Protocol (LDP) handles the
generation and distribution of labels for basic IP forwarding and
other applications, and the Label Information Base (LIB) stores the
labels generated locally and received from LDP neighbors. LDP is
generally enabled on each MPLS interface. Other labels, as required
by various applications, are generated and distributed through
other protocols. For example, Multi-Protocol Border Gateway
Protocol (MP-BGP) assigns and distributes labels used for VPN
forwarding, and the Resource Reservation Protocol (RSVP) does the
same for traffic engineering.
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Related Courses
MPLS - Implementing Cisco MPLS v2.2
MPLST - MPLS Traffic Engineering and Other Features
MPLS ENT - Enterprise Networks over Service Provider MPLS
AMPLS - Advanced Implementing and Troubleshooting MPLS VPN
Networks