Preparation

Increase the metric on the P2 router toward P3 on the P2-P3 link and shut down the P8-P4 interface on the P8 router. This adjustment causes the P8 router to use the P2-P3 link to reach the PE5 router.

On router P2, increase the metric toward router P3 from 10 to 100:

P2

router isis IGP
interface GigabitEthernet0/0/0/0
  address-family ipv4 unicast
   metric 100

Additionally, shut down the interface on P8 toward P4:

P8

router isis IGP
interface GigabitEthernet0/0/0/1
  shutdown

This configuration should result in no backup path from P2 to PE5, as shown below.