Multi-Topology (MT) IS-IS allows a single IGP instance to maintain multiple independent topologies (e.g., one for base IPv4, one for VPN IPv4). The key advantage is that each topology can have its own Traffic Engineering parameters (like metrics, admin-groups) on the same physical links. This is optimal for separating VPN traffic (requiring specific TE paths) from other traffic without the overhead of running multiple full IGP instances.

CSPF requires that all strict hops in an Explicit Route Object (ERO) are reachable via the TE database. If the address specified as a strict hop is not advertised as a Router ID or a TE-enabled interface address (via the Router Address TLV or IPv4 Interface Address TLV in IS-IS/OSPF), CSPF cannot find a path to it, resulting in this specific error. The hop must be a TE node or link identifier.

CSPF uses the TE metric (administrative weight) for its shortest path calculation by default, not the IGP metric. If the links on the longer path have a lower TE metric than the links on the direct path, CSPF will choose the longer path. Adjusting the TE metrics is a primary tool for traffic engineering. While admin-groups (C) are also a key steering tool, the scenario describes a simple sub-optimal path, making TE metric the most direct first check.

A path configured with the ‘standby’ flag is a true cold standby. When the primary is active, the standby path’s route is computed (CSPF runs) and stored locally, but no RSVP signaling messages are sent. Therefore, no resources are reserved in the network. Upon primary failure, the standby path is then signaled, which introduces a delay during switchover but saves network resources until needed.

The TED is built from the TE information flooded by the IGP (OSPF-TE or IS-IS-TE). If the flooding of TE Link State Advertisements (in OSPF) or TE TLVs (in IS-IS) is not reliable, fast, and consistent across the network, nodes will have different views of the network topology and resources. This ‘out of sync’ state is a direct result of flooding problems. All other options relate to SPF calculation or other protocols, not the fundamental synchronization of TE data.

In a hierarchical GMPLS model (e.g., for inter-area), the network is divided into areas. The Routing Controller (RC) maintains a summarized Traffic Engineering Database (TED) of its own area and abstracted information about other areas. Its key role is to use this summarized view to compute a viable end-to-end path across area boundaries, which is then decomposed into per-area segments for detailed path computation and signaling.

MPLS-TP OAM packets are carried in-band with user traffic using the Generic Associated Channel (G-ACh) and a special label, the G-ACh Label (GAL). This channel carries OAM protocols like Continuity Check (CC) and Connectivity Verification (CV) based on ITU-T Y.1731. This is distinct from BFD for MPLS or LSP Ping, which are more common in dynamic MPLS networks but don’t fully align with the static, connection-oriented OAM model mandated by MPLS-TP.

To avoid links marked with a specific attribute (an admin-group or ‘link color’), you must use an `exclude` constraint. This tells CSPF to compute a path that does *not* traverse any link associated with that admin-group name. An `include` constraint would force the path to use those links, which is the opposite of the requirement.

The ‘Subscription’ limit on an interface defines the maximum bandwidth that can be reserved by RSVP-TE. If ‘Active Resv’ is at this limit, the available bandwidth for new reservations is effectively zero. CSPF, consulting the TED which includes bandwidth information, will see zero available bandwidth and exclude the link. If no alternative path exists, CSPF will fail, and the LSP will remain down.

Multi-Topology (MT) IS-IS allows a single IGP instance to maintain multiple independent topologies (e.g., one for base IPv4, one for VPN IPv4). The key advantage is that each topology can have its own Traffic Engineering parameters (like metrics, admin-groups) on the same physical links. This is optimal for separating VPN traffic (requiring specific TE paths) from other traffic without the overhead of running multiple full IGP instances.0