Other MPLS Applications
This last relatively short section of the chapter introduces the general idea about the protocols used
by several other MPLS applications. To that end, this section introduces and explains the concept
of a Forwarding Equivalence Class (FEC) and summarizes the concept of an FEC as used by
various MPLS applications.
Frankly, this chapter has already covered all the concepts surrounding the term FEC. However, it
is helpful to know the term and the FEC concept as an end to itself, because it helps when comparing
various MPLS applications.
Generally speaking, an FEC is a set of packets that receives the same forwarding treatment by a
single LSR. For simple MPLS unicast IP forwarding, each IPv4 prefix is an FEC. For MPLS
VPNs, each prefix in each VRF is an FEC—making the prefix 10.3.3.0/24 in VRF-A a different
FEC from the 10.3.3.0/24 prefix in VRF-B. Alternately, with QoS implemented, one FEC might
be the set of packets in VRF-A, destined to 10.3.3.0/24, with DSCP EF in the packet, and another
FEC might be packets in the same VPN, to the same subnet, but with a different DSCP value.
For each FEC, each LSR needs a label, or label stack, to use when forwarding packets in that FEC.
By using a unique label or set of labels for each FEC, a router has the ability to assign different
forwarding details (outgoing interface and next-hop router.)
Each of the MPLS applications can be compared by focusing on the information used to determine
an FEC. For example, MPLS traffic engineering (TE) allows MPLS networks to choose to send
some packets over one LSP and other packets over another LSP, based on traffic loading—even
though the true end destination might be in the same location. By doing so, SPs can manage the
flow of data over their high-speed core networks and prevent the problem of overloading the best
route as determined by a routing protocol, while barely using alternate routes. To achieve this
function, MPLS TE bases the FEC concept in part on the definition of an MPLS TE tunnel.
You can also compare different MPLS applications by listing the control plane protocols used to
learn label information. For example, this chapter explained how MPLS VPN uses both LDP and
MP-BGP to exchange label information, whereas other MPLS applications use LDP and
something else—or do not even use LDP at all. Table 19-5 lists many of the common MPLS
applications, the information that determines an FEC, and the control plane protocol that is used
to advertise FEC-to-label bindings.
Control Protocols Used in Various MPLS Applications
Application FEC
Control Protocol Used to Exchange FECto-
Label Binding
Unicast IP routing Unicast IP routes in the
global IP routing table
Tag Distribution Protocol (TDP) or Label
Distribution Protocol (LDP)
Multicast IP
routing
Multicast routes in the
global multicast IP
routing table
PIM version 2 extensions
VPN Unicast IP routes in the
per-VRF routing table
MP-BGP
Traffic engineering MPLS TE tunnels
(configured)
RSVP or CR-LDP
MPLS QoS IP routing table and the
ToS byte
Extensions to TDP and LDP