Pre-MPLS TE techniques involved multiple mechanisms:
Altering IGP routing metrics
Offline design of virtual circuit overlays
IGPs have many limitations when used to achieve traffic engineering. IGPs rely on metrics that do not reflect actual network resources and constraints. IGPs based on Shortest Path First (SPF) algorithms contribute significantly to congestion problems in IP networks. SPF algorithms generally optimize based on a simple additive metric. These protocols are topology-driven, so real-time bandwidth availability and traffic characteristics are not factors considered in routing decisions. As such, congestion occurs when the ...