When the TE metric is selected for an LSP, the shortest path computation will select an LSP path based on the TE metric constraints instead of the IGP metric (for OSPF and IS-IS), which is the default metric. The user configures the TE metric under the router>mpls>interface context and the IGP metric under the router>ospf>area> interface context (for OSPF) and the router>isis>if>level context (for IS-IS). 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 traffic engineering extensions of the IGP protocols. For more information about the OSPF and IS-IS routing protocols, see the 7705 SAR Routing Protocols Guide.
Typically, the TE metric is used to allow Constrained Shortest Path First (CSPF) to represent a dual TE topology for the purpose of computing LSP paths, where one TE topology is based on the RSVP-TE database and the other is based on the IGP-TE database.
An LSP dedicated to 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 amount of delay, or combined delay and jitter, of the link. In this case, the shortest path satisfying the constraints of the LSP path will effectively represent the shortest-delay path.
An LSP dedicated to 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 value as required.
When the use of the TE metric is enabled for an LSP, the CSPF process will first eliminate all links in the network topology that do not meet the constraints specified for the LSP path; the constraints include bandwidth, admin-groups, and hop limit. CSPF will then run the SPF algorithm on the remaining links. The shortest path among all the SPF paths will be selected based on the TE metric instead of the IGP metric. The TE metric is only used in CSPF computations for MPLS paths and not in the regular SPF computation for IP reachability.
Operational metrics of LSPs that use the TE metric in CSPF path calculations can be overridden with the user-configured administrative LSP metric.