Multi Protocol Label Switching (MPLS)

Multiprotocol Label Switching (MPLS)　is a mechanism in high-performance telecommunication layer which directs and carries data from one network node to the next. MPLS operates at an OSI Model layer that is generally considered to lie between traditional definitions of Layer 2 (Data Link Layer ) and Layer 3 (Network Layer), and thus is often referred to as a "Layer 2.5" protocol.

WORKING :

Labeling packets are the main concept behind MPLS. These short, fixed-length labels carry the information that tells each switching node (router) how to process and forward the packets, from source to destination. As each node forwards the packet, it swaps the current label for the appropriate label to route the packet to the next node. This mechanism enables very-high-speed switching of the packets through the core MPLS network. MPLS predetermines the path data takes across a network and encodes that information into a label that the network’s routers understand.

MPLS routing

MPLS networks establish Label-Switched Paths (LSPs) for data crossing the network. An LSP is defined by a sequence of labels assigned to nodes on the packet’s path from source to destination.

As the network is established and signaled, each MPLS router builds a Label Information Base (LIB)—a 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 (RSVPTE). 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.

Data Flow in MPLS Network


Figure 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’s 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’s network.