When the use of the TE metric is selected for an LSP, the shortest path computation after the TE constraints are applied selects an LSP path based on the TE metric instead of the IGP metric. The user configures the TE metric under the MPLS interface. Both the TE and IGP metrics are advertised by OSPF and IS-IS for each link in the network. The TE metric is part of the TE extensions of both IGP protocols.
A typical application of the TE metric is to allow CSPF to represent a dual TE topology for the purpose of computing LSP paths.
An LSP dedicated for real-time and delay sensitive user and control traffic has its path computed by CSPF using the TE metric. The user configures the TE metric to represent the delay figure, or a combined delay/jitter figure, of the link. In this case, the shortest path satisfying the constraints of the LSP path effectively represents the shortest delay path.
An LSP dedicated for non-delay sensitive user and control traffic has its path computed by CSPF using the IGP metric. The IGP metric could represent the link bandwidth or some other figure as required.
When the use of the TE metric is enabled for an LSP, CSPF first prunes all links in the network topology that do not meet the constraints specified for the LSP path. These constraints include bandwidth, admin-groups, and hop limit. CSPF then runs an SPF on the remaining links. The shortest path among the all SPF paths is selected based on the TE metric instead of the IGP metric which is used by default. The TE metric is only used in CSPF computations for MPLS paths and not in the regular SPF computation for IP reachability.