MPLS TE Overview 1349For a new path to preempt an existing path, the setup priority of the new pathmust be greater than the holding priority of the existing path. To initiate apreemption, the RESV message of RSVP-TE is sent.To avoid flapping caused by improper preemptions between CR-LSPs, the setuppriority of a CR-LSP should not be set higher than its holding priority.Route pinningRoute pinning prevents an established CR-LSP from changing upon route changes.If a network does not run IGP TE extension, the network administrator will beunable to identify from which part of the network the required bandwidth shouldbe obtained when setting up a CR-LSP. In this case, loose explicit route (ER-hop)with required resources is used. The CR-LSP thus established however, may changewhen the route changes, for example, when a better next hop becomes available.If this is undesirable, the network administrator can set up the CR-LSP using routeunderpinning to make it a permanent path.Administrative group and affinity attributeThe affinity attribute of an MPLS TE tunnel identifies the properties of the linksthat the tunnel can use. Together with the link administrative group, it decideswhich links the MPLS TE tunnel can use.ReoptimizationTraffic engineering is a process of allocating/reallocating network resources. Youmay configure it to meet desired QoS.Normally, service providers use some mechanism to optimize CR-LSPs for best useof network resources. They can do this manually but CR-LSP measurement andtuning are required. Alternatively, they can use MPLS TE where CR-LSPs aredynamically optimized.Dynamic CR-LSP optimization involves periodic calculation of paths that traffictrunks should traverse. If a better route is found for an existing CR-LSP, a newCR-LSP will be established to replace the old one, and services will be switched tothe new CR-LSP.CR-LDP Constraint-based routed label distribution protocol (CR-LDP) is an extension toLDP. It is used in MPLS TE to create an explicit path with resource reservationbetween the ingress node and the egress node.When initiating an LSP at the ingress, CR-LDP appends some constraints in thelabel request message.RSVP-TE This section covers these topics:■ “Overview” on page 1350■ “Basic concepts of RSVP-TE” on page 1350■ “Make-before-break” on page 1351■ “RSVP-TE messages” on page 1351■ “Setting up an LSP tunnel” on page 1352
