Discussion

(RSVP), originally designed as a general protocol for reserving bandwidth for network
flows, was extended to set up LSPs, assign and manage labels, and reserve
bandwidth for the LSP.
Since the introduction of MPLS, many new services have been developed that use
MPLS, including Layer 3 VPNs, Virtual Private LAN Services (VPLS), and Differentiated
Services Traffic Engineering (DiffServ TE).
For more detailed information about how MPLS works, see MPLS: Technology and
Applications (Morgan Kaufmann) and MPLS-Enabled Applications (Wiley UK).
LSPs
MPLS assigns labels to network packets that describe how to forward them through
the network. A label is a short, fixed-length numeric identifier. The labeled traffic is
forwarded along LSPs, which are unidirectional tunnels through the IP network.
LSPs are connections similar to ATM or Frame Relay virtual circuits. LSPs have an
entry point, called an ingress router, and an exit point, called an egress router. LDPsignaled
LSPs have multiple ingress points and a single egress point. RSVP-signaled
LSPs have one ingress point and one egress point and optional intermediate routers
called transit routers.
A signaling protocol establishes the physical path taken by the LSP between the
ingress and egress routers. Once the paths are established, the ingress router pushes a
fixed-length label onto packets traveling through the LSP. Each transit router swaps
the label, removing the incoming label and replacing it with an outgoing label, and
forwards the packets to the next hop. At the egress router (or typically at the penultimate
hop, which is the router immediately prior to the egress router), the label is
popped, or removed, from the packet, and the egress router continues forwarding the
packet using the standard IP routing longest-match lookup algorithm. MPLS maintains
a label forwarding table that it uses to determine the next hop in an LSP. The
next-to-last router in the LSP, the penultimate router, usually performs a label pop
operation, removing the MPLS label before sending the packet to the egress router.
This is called penultimate hop popping (PHP).
LSPs are established either manually or dynamically using a signaling protocol.
MPLS Header and Labels
Packets traveling along an LSP are identified by a label, which is part of a 4-byte
MPLS header that is inserted as a shim between the packet’s link-level (Layer 2)
header and its network layer (Layer 3) data (see Figure 14-1).
Multiple MPLS headers can be stacked in the packet’s header. The newest header is
placed at the beginning (top) of the stack.
The first 20 bits of the MPLS header are the MPLS label value, a numeric identifier
that the LSP uses to forward packets. The label itself has no internal structure, unlike
480 | Chapter 14: MPLS
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