MPLS Network Components
An MPLS-based network consists of routers and switches interconnected via transport facilities such as fiber links (see Figure 1-1). Customers connect to the backbone (core) network through multiservice edge (MSE) routers. The backbone comprises the core routers that provide high-speed transport and connectivity between the MSE routers. An MSE router contains different types of line cards and physical interfaces to provide Layer 2 and Layer 3 services, including ATM, FR, Ethernet, and IP/MPLS VPNs.
Figure 1-1 Converged IP/MPLS Network Architecture
In the incoming direction, line cards receive packets from external interfaces and forward them to the switching fabric (see Figure 1-2). In the outgoing direction, line cards receive packets from the switching fabric and forward them to the outgoing interfaces. The switching fabric, the heart of the router, is used for switching packets between line cards. The IP/MPLS control-plane software, the brain of a router, resides in the control processor card. The phrase IP/MPLS control plane refers to the set of tasks performed by IP routing and MPLS signaling protocols. IP routing protocols are used to advertise network topology, exchange routing information, and calculate forwarding paths between routers within (intra) and between (inter) network routing domains. Examples of IP routing protocols include Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS-IS), and Border Gateway Protocol (BGP). MPLS signaling protocols are used to establish, maintain, and release label-switched paths (LSP). Examples of MPLS signaling protocols include BGP, Label Distribution Protocol (LDP), and Resource Reservation Protocol (RSVP). The IP control plane may also contain tunneling protocols such as Layer 2 Tunneling Protocol (L2TP) and Generic Routing Encapsulation (GRE), but these protocols are not covered in this book.
Because redundant network elements add to the overall network cost, service providers typically employ different levels and types of fault tolerance in the edge and core network. For example, the core network is generally designed to protect against core router failures through mesh connectivity. This allows alternative paths to be quickly established and used in the face of a failure. In the core, additional routers and links are used to provide fault tolerance. In contrast, on the edge, often thousands of customers are connected through a single router, and the edge router usually represents a single point of failure. The edge router is what most service providers consider the most vulnerable point of their network after the core is protected. On the edge, instead of using additional routers and links as in the core, redundancy within the edge router via redundant control processor cards, redundant line cards, and redundant links (such as SONET/SDH Automatic Protection Switching [APS]) are commonly used to provide fault tolerance.
Figure 1-2 Functional Components of Router Architecture
In summary, service (to a customer) downtime can result from failure of the access port, edge links, the edge router, backbone transport facilities, or the core routers. Generally, the core network offers a higher level of fault tolerance than the edge network. The edge router is an important network element because it routes traffic to/from multiple customers to the core network. Therefore, improving the availability of edge routers is extremely important. In short, service providers are looking for truly edge-to-edge reliability, and this includes all of the edge routers as well as the core routers.