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.
Table 19-5. Control Protocols Used in Various MPLS Applications
|
Application |
FEC |
Control Protocol Used to Exchange FEC-to-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 |