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Configuring the MPLS Module
Conceptually, label switching is straightforward. A label is a relatively short,
fixed-length identifier that is used to forward packets received from a given link. The
label value is locally significant to a particular link and is assigned by the receiving
entity.
Because labels are relatively short (for example, 20 bits in a MPLS shim header), the
label of a received packet can be used as an index into a linear array containing the
forwarding database. Forwarding database entries indicate the outgoing port and any
label(s) to be applied to forwarded frames. Thus, forwarding may consist of a simple
lookup and replacement of the incoming label with the appropriate outgoing label
(otherwise known as label swapping).
Figure 3-1 illustrates an MPLS network.
Ingress
LER
Source IP
network
Figure 3-1: MPLS network
MPLS Terms and Acronyms
Table 3-1 defines common MPLS terms and acronyms.
Table 3-1: MPLS Terms and Acronyms
Term or Acronym
CSPF
DoD
3-2
LSR
MPLS cloud
Description
Constrained Shortest Path First. Route selection determined by an
algorithm based on available link bandwidth and path cost.
Downstream-on-Demand. Distribution of labels as a result of explicit
upstream label requests.
Egress
LER
Destination IP
network
LSR
LSP
MPLS Module Installation and User Guide
MPLS_05
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