MPLS-Enabled Applications: Emerging Developments and New Technologies

2.5. USING THE TRAFFIC-ENGINEERED PATHS

The simplest, most basic way to map traffic to LSPs is through static routing. The LSR can be configured to send traffic to a destination by sending it over the LSP. However, the fact that the route must be manually configured to use the LSP is both restrictive and unscalable from an operational point of view, thus limiting widespread use.

To reap the benefits of the traffic-engineered paths, it is necessary for the routing protocols to become aware of the LSPs. From the routing protocol's point of view, an LSP is treated as an interface (a tunnel) and has a metric associated with it. The metric can be the same as that of the underlying IP path or it can be configured to a different value to influence the routing decision. Different routing protocols have different properties and therefore their use of the LSP is different.

The rule for LSP usage in BGP is that when an LSP is available to the BGP next-hop of a route, the LSP can be used to forward traffic to that destination. This property is crucial for the implementation of Layer 3 BGP/MPLS VPNs, as will be seen in the chapter discussing the basics of VPNs (Chapter 7). In a plain IP/MPLS network (non-VPN), this means that if an LSP is set up between the AS border routers (ASBRs), all traffic transiting the AS uses the LSP, with the following consequences:

1. Forwarding for transit traffic is done based on MPLS labels. Thus, none of the routers except the ASBRs need to have knowledge of the ...