P1
| 
R1---R2--R3
|        |
X        |
|        |
R5-------R4

Same metric everywhere.


R5 is trying to reach P1

- LFA won't work, as R4 would send back towards R5

R5 knows R3 would not use broken path
R5 knows R4 would not use broken path towards R3

So R5 can impose label R4 told to use to reach R3.

R3 will pop, do lookup, and continue to forward towards P1.



But what if we can't do lookup. What if it wasn't INET but VPN?