A. When TE FRR in FA mode is enabled, nodes except the node on which FA is configured can sense the TE tunnel interface. When the primary tunnel link becomes faulty, the PLR switches traffic to the bypass tunnel through protection switching. The upstream node assumes the TE tunnel link changes and therefore it triggers an FRR.

B. When TE FRR in IGP Shortcut mode is enabled, nodes except the node on which FA is configured cannot sense the TE tunnel interface. When the primary tunnel link becomes faulty, the PLR switches traffic to the bypass tunnel through protection switching. On the PLR, traffic is still forwarded through the bypass tunnel. The upstream node of the PLR cannot sense the TE tunnel interface and does not trigger an FRR.

C. When TE FRR in FA mode is enabled, nodes except the node on which FA is configured can sense the TE tunnel interface. When the primary tunnel link becomes faulty, the PLR switches traffic to the bypass tunnel through protection switching. The upstream node assumes the TE tunnel link is normal and therefore it continues traffic forwarding. In this case, the route does not change and an FRR is not triggered.

D. When TE FRR in IGP Shortcut mode is enabled, nodes except the node on which FA is configured cannot sense the TE tunnel interface. When the primary tunnel link becomes faulty, the PLR switches traffic to the bypass tunnel through protection switching. Though the upstream node of the PLR cannot sense the TE tunnel interface, IGP can sense the faulty link. Therefore, the upstream node triggers an FRR.