Content Overview Service providers face many challenges when trying to meet customer demand. One of these challenges is an ongoing need for value-added services. Service providers (SPs) must be concerned with both protecting their existing infrastructure and finding ways to generate new services not currently supported with existing technologies. One area of current need is packet forwarding. Conventional Internet Protocol (IP) packet forwarding has a number of limitations, and more and more SPs realize that a new method is needed. Cisco IOSŪ Multiprotocol Label Switching (MPLS) fuses the intelligence of routing with the performance of switching and provides significant benefits to networks with a pure IP architecture as well as those with IP and ATM or a mix of other Layer 2 technologies. MPLS technology is key to scalable virtual private networks (VPNs) and end-to-end quality of service (QoS), enabling efficient utilization of existing networks to meet future growth and rapid fault correction of link and node failure. The technology also helps deliver highly scalable, differentiated end-to-end IP services with simpler configuration, management, and provisioning for both Internet providers and subscribers. This module describes the MPLS conceptual model data and control planes and the function of the MPLS label. A description of how labels are allocated and distributed in a frame mode MPLS network as IP packets cross an MPLS network follows. MPLS implementation and MPLS Virtual Private Network (VPN) routing and packet flow are both explained. Web Links MPLS/Tag Switching
http://www.cisco.com/univercd/cc/td/doc/
cisintwk/ito_doc/mpls_tsw.htm MPLS Overview
http://cisco.com/en/US/products/ps6350/
products_configuration_guide_chapter091
86a00800ca7fa.html#wp1000871 Configuring CEF
http://cisco.com/en/US/products/sw/iosswrel/
ps1835/products_configuration_guide_chapter
09186a00800ca7cc.html#wp46064 RFC 3031: Multiprotocol Label Switching Architecture
http://www.ietf.org/rfc/rfc3031.txt Configuring MPLS
http://cisco.com/en/US/products/ps6350/
products_configuration_guide_chapter091
86a00800ca7fb.html RFC 3032: MPLS Label Stack Encoding
http://www.ietf.org/rfc/rfc3032.txt RFC 3036: LDP Specification
http://www.ietf.org/rfc/rfc3036.txt
Content 4.1 Introducing MPLS Networks 4.1.1 The MPLS Conceptual Model Figure shows a range of possible WAN topologies. While sites can be connected using any of these topologies, a full mesh topology is required for optimal routing between the sites. The full mesh topology provides a dedicated virtual circuit between any two customer edge (CE) routers in the network to support the best routing solution, but using the full mesh configuration is very expensive. A partial mesh topology or hub-and-spoke topology is a less expensive solution. These topologies use a central point to coordinate activities. However, these solutions do not provide optimal routing. Using the partial mesh topology reduces the number of virtual circuits to the minimum number of circuits that are needed to provide optimum transport between major sites. The hub-and-spoke topology allows for the ultimate reduction in circuits within the partial mesh topology. Many sites, or spokes, connect directly to the central site or sites, or hubs, with no direct connectivity occurring between them. To prevent single points of failure, the hub-and-spoke topology sometimes extends to a redundant hub-and-spoke topology. MPLS provides optimal routing between sites. With MPLS, a site requires only one connection to the MPLS SP. MPLS provides a blend of Layer 2 switching and Layer 3 routing to forward packets using short, fixed-length labels. Using MPLS in a wide area network adds many useful features: Figure illustrates how MPLS provides fast routing for large networks. Only the edge routers perform a routing lookup, and core routers forward packets based on the labels. These two functions mean faster forwarding of packets through the SP network. The example illustrates a situation in which the intermediary router, or core router, does not have to perform a time-consuming routing lookup. Instead, the core router simply swaps label 25 with label 23. The core router then forwards the packet to the Edge-1 router based on receiving label 23 from the Edge-1 router. In the example, the routing table tells the Edge-2 router that to reach the 10.1.1.1 network, the Edge-2 router should assign a label of 25 to the packet. The edge router then forwards the packet to the core router. The label tells the core router that when the core router receives a packet with label 25, the router should swap that label with label 23 and then forward the packet to the Edge-1 router. Later in the course, you will read about the actual method that is used to inform the routers of these label allocations. Router switching mechanisms are a key component of the MPLS conceptual model. The next topic explains router switching mechanisms in more detail.
Content 4.1 Introducing MPLS Networks 4.1.2 Router Switching Mechanisms Cisco IOS Platform Switching Mechanisms
The Cisco IOS platform supports three IP switching mechanisms as listed in Figure : The original switching mechanism available on Cisco routers was process switching. However, process switching is very slow because it must find a destination in the routing table. This process can possibly result in a recursive lookup. Process switching must also construct a new Layer 2 frame header for every packet. As a result, process switching is no longer widely used in modern