MPLS creates a connection-based model overlaid onto the traditionally connectionless framework of IP routed networks. This connection-oriented architecture opens the door to a wealth of new possibilities for managing traffic on an IP network. MPLS builds on IP, combining the intelligence of routing, which is fundamental to the operation of the Internet and today抯 IP networks, with the high performance of switching. Beyond its applicability to IP networking, MPLS is being expanded for more general applications in the form of Generalized MPLS (GMPLS), with applications in optical and Time-Division Multiplexing (TDM) networks.
Advantages of MPLS
- MPLS enables a single converged network to support both new and legacy services, creating an efficient migration path to an IP-based infrastructure. MPLS operates over both legacy (DS3, SONET) and new infrastructure (10/100/1000/10G Ethernet) and networks (IP, ATM, Frame Relay, Ethernet, and TDM).
- MPLS enables traffic engineering. Explicit traffic routing and engineering help squeeze more data into available bandwidth.
- MPLS supports the delivery of services with Quality of Service (QoS) guarantees. Packets can be marked for high quality, enabling providers to maintain a specified low end-to-end latency for voice and video.
- MPLS reduces router processing requirements, since routers simply forward packets based on fixed labels.
- MPLS provides the appropriate level of security to make IP as secure as Frame Relay in the WAN, while reducing the need for encryption on public IP networks.
- MPLS VPNs scale better than customer-based VPNs since they are provider-network-based, reducing the configuration and management requirements for the customer.
How Does MPLS Work?
MPLS is a technology used for optimizing traffic forwarding through a network. Though MPLS can be applied in many different network environments, this discussion will focus primarily on MPLS in IP packet networks ?by far the most common application of MPLS today.
MPLS assigns labels to packets for transport across a network. The labels are contained in an MPLS header inserted into the data packet (Figure 1).
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. They have significance only on a local node-to-node connection. 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 combines the best of both Layer 3 IP routing and Layer 2 switching. In fact, it is sometimes called a 揕ayer 2綌 protocol. While routers require network-level intelligence to determine where to send traffic, switches only send data to the next hop, and so are inherently simpler, faster, and less costly. MPLS relies on traditional IP routing protocols to advertise and establish the network topology. MPLS is then overlaid on top of this topology. MPLS predetermines the path data takes across a network and encodes that information into a label that the network抯 routers understand. This is the connection-oriented approach previously discussed. Since route planning occurs ahead of time and at the edge of the network (where the customer and service provider network meet), MPLS-labeled data requires less router horsepower to traverse the core of the service provider's network.
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抯 path from source to destination. LSPs direct packets in one of two ways: hop-by-hop routing or explicit routing.
Hop-by-hop routing. In hop-by-hop routing, each MPLS router independently selects the next hop for a given Forwarding Equivalency Class (FEC). A FEC describes a group of packets of the same type; all packets assigned to a FEC receive the same routing treatment. FECs can be based on an IP address route or the service requirements for a packet, such as low latency.
Figure 1. MPLS header format on an MPLS packet.
In the case of hop-by-hop routing, MPLS uses the network topology information distributed by traditional Interior Gateway Protocols (IGPs) ?routing protocols such as OSPF or IS-IS. This process is similar to traditional routing in IP networks, and the LSPs follow the routes the IGPs dictate.
Explicit routing. In explicit routing, the entire list of nodes traversed by the LSP is specified in advance. The path specified could be optimal or not, but is based on the overall view of the network topology and, potentially, on additional constraints. This is called Constraint-Based Routing. Along the path, resources may be reserved to ensure QoS. This permits traffic engineering to be deployed in the network to optimize use of bandwidth.
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