Multi-Protocol Label Switching (MPLS) Conform…

Label Information Base. As the network is established and signaled, each MPLS router builds a Label Information Base (LIB)梐 table that specifies how to forward a packet. This table associates each label with its corresponding FEC and the outbound port to forward the packet to. This LIB is typically established in addition to the routing table and Forwarding Information Base (FIB) that traditional routers maintain.

Signaling and label distribution

Connections are signaled and labels are distributed among nodes in an MPLS network using one of several signaling protocols, including Label Distribution Protocol (LDP) and Resource reSerVation Protocol with Tunneling Extensions (RSVP璗E). Alternatively, label assignment can be piggybacked onto existing IP routing protocols such as BGP.

The most commonly used MPLS signaling protocol is LDP. LDP defines a set of procedures used by MPLS routers to exchange label and stream mapping information. It is used to establish LSPs, mapping routing information directly to Layer 2 switched paths. It is also commonly used to signal at the edge of the MPLS network ?the critical point where non-MPLS traffic enters. Such signaling is required when establishing MPLS VPNs, for example.

RSVP-TE is also used for label distribution, most commonly in the core of networks that require traffic engineering and QoS. A set of extensions to the original RSVP protocol, RSVP-TE provides additional functionality beyond label distribution, such as explicit LSP routing, dynamic rerouting around network failures, preemption of LSPs, and loop detection. RSVP-TE can distribute traffic engineering parameters such as bandwidth reservations and QoS requirements.

Multi-protocol extensions have been defined for BGP, enabling the protocol to also be used to distribute MPLS labels. MPLS labels are piggybacked onto the same BGP messages used to distribute the associated routes.

MPLS allows multiple labels (called a label stack) to be carried on a packet. Label stacking enables MPLS nodes to differentiate between types of data flows, and to set up and distribute LSPs accordingly. A common use of label stacking is for establishing tunnels through MPLS networks for VPN applications.

Figure 2. MPLS network.
Data flow in an MPLS network

Figure 2 shows a typical MPLS network and its associated elements. The central cloud represents the MPLS network itself. All data traffic within this cloud is MPLS-labeled. All traffic between the cloud and the customer networks is not MPLS-labeled (IP for example). The customer-owned Customer Edge (CE) routers interface with the Provider Edge (PE) routers (also called Label Edge Routers, or LERs) owned by the service provider. At the ingress (incoming) side of the MPLS network, PE routers add MPLS labels to packets. At the egress (outgoing) side of the MPLS network, the PE routers remove the labels. Within the MPLS cloud, P (Provider) routers (also called Label Switching Routers, or LSRs), switch traffic hop-by-hop based on the MPLS labels.

To demonstrate an MPLS network in operation, we will follow the flow of data through the network in Figure 2:

1. Before traffic is forwarded on the MPLS network, the PE routers first establish LSPs through the MPLS network to remote PE routers.
2. Non-MPLS traffic (Frame Relay, ATM, Ethernet, etc.) is sent from a customer network, through its CE router, to the ingress PE router operating at the edge of the provider抯 MPLS network.
3. The PE router performs a lookup on information in the packet to associate it with a FEC, then adds the appropriate MPLS label(s) to the packet.
4. The packet proceeds along its LSP, with each intermediary P router swapping labels as specified by the information in its LIB to direct the packet to the next hop.
5. At the egress PE, the last MPLS label is removed and the packet is forwarded by traditional routing mechanisms.
6. The packet proceeds to the destination CE and into the customer抯 network.

How Is MPLS Used?

One of the primary original goals of MPLS, boosting the performance of software-based IP routers, has been superseded as advances in silicon technology have enabled line-rate routing performance implemented in router hardware. In the meantime, additional benefits of MPLS have been realized, notably VPN services and traffic engineering.

Virtual Private Networks

A Virtual Private Network (VPN) is a private network service delivered over a public (shared) network. VPNs benefit end customers by allowing remote locations to be securely connected over a public network, without the expense of buying or leasing dedicated network lines. MPLS enables VPNs by providing a circuit-like, connection-oriented framework, allowing carriers to deploy VPNs over the traditionally connectionless IP network infrastructure.

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