Terms used for defining tunnel as below:

A tunnel established for the purpose of providing a virtual neighbor that is a Loop-Free Alternate.

The P-space of a router with respect to a protected link is the set of routers reachable from that specific router using the pre-convergence shortest paths, without any of those paths (including equal cost path splits) transiting that protected link.

For example, the P-space of S with respect to link S-E is the set of routers that S can reach without using the protected link S-E.

Consider the set of neighbors of a router protecting a link. Exclude the router reachable over the protected link from that set of routers. The extended P-space of the protecting router with respect to the protected link is the union of the P spaces of the neighbors in that set of neighbors with respect to the protected link.

Q-space of a router with respect to a protected link is the set of routers from which that specific router that can be reached without any path (including ECMP Splits) transiting that protected link.

A PQ node of a node S with respect to a protected link S-E is a node that is a member of both the P-space (or the extended P-space) of S with respect to that protected link S-E and the Q-space of E with respect to that protected link S-E. A repair tunnel endpoint is chosen from the set of PQ-nodes.

The use of a PQ node rather than a neighbor of the repairing node as the next hop in an LFA repair.

In Figure 39-18, S can reach A, B, and C without going via S-E; these form S's extended P-space with respect to S-E. The routers that can reach E without going through S-E will be in E's Q-space with respect to link S-E; these are D and C.B has equal-cost paths to E via B-A-S-E and B-C-D-E, and so the forwarder at S might choose to send a packet to E via link S-E. Hence, B is not in the Q-space of E with respect to link S-E. The single node in both S's extended P-space and E's Q-space is C; thus, node C is selected as the repair tunnel's endpoint. Thus, if a tunnel is provided between S and C as shown in Figure 2, then C, now being a direct neighbor of S, would become an LFA for D and E.