Information Technology Reference
In-Depth Information
Within each NLRI, label mapping and VPNv4 prefix are carried in the following order (see
RFC 3107, Carrying Label Information in BGP-4 ):
Label
RD
IPv4 prefix
Note that the Label field can carry one or more labels. The following sections go into detail
about how labels are generated and exchanged.
Understanding MPLS Fundamentals
MPLS is an IETF standard, as defined in a set of RFCs. Currently, MPLS is defined only for
IP. Using a connection-oriented approach, packet forwarding in MPLS relies on preexisting
paths. An MPLS forwarding path can also be thought of as a tunnel that goes from an MPLS
ingress point to an MPLS egress point. Compared to conventional IP routing and forward-
ing, path selection and packet forwarding are separated in MPLS.
Figure 10-1 shows an example of an MPLS network. It consists of two types of devices:
Edge Label Switch Router or Label Edge Router (LER)
Label Switch Router (LSR)
Figure 10-1 MPLS Network
IP
IP
MPLS
LSR
R3
R1
R2
R5
R6
LER
LER
LSR
R4
The function of an LER is to switch packets between the IP network and the MPLS net-
work. In other words, switching can occur from an IP packet to a labeled packet, or from a
labeled packet to an IP packet. The function of an LSR is to switch the packet with one label
to a packet with another label.
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