You have two routers unable to form an IS-IS Level 1 adjacency.
Which two statements describe a reason for this problem? (Choose two.)
You have two routers unable to form an IS-IS Level 1 adjacency.
Which two statements describe a reason for this problem? (Choose two.)
A common reason for two routers being unable to form an IS-IS Level 1 adjacency is if their area numbers are different, as IS-IS Level 1 routers must be in the same area to establish an adjacency. Another possible reason is if the loopback interface (lo0) is not included as an IS-IS interface since this can prevent IS-IS from functioning correctly, as the loopback interface is typically used for router ID purposes in IS-IS.
B and C make sense
Typo BC is correct https://www.juniper.net/documentation/us/en/software/junos/is-is/topics/task/isis-verifying.html Another common configuration error is to omit the loopback interface (lo0) from the configuration at the [edit protocols isis] hierarchy level. IS-IS does not function correctly if the loopback interface (lo0) is not configured at this level.
https://www.juniper.net/documentation/us/en/software/junos/is-is/topics/task/isis-verifying.html Another common configuration error is to omit the loopback interface (lo0) from the configuration at the [edit protocols isis] hierarchy level. IS-IS does not function correctly if the loopback interface (lo0) is not configured at this level.
Answer should be B and C. ISIS adjacency can possibly come up even if the routers on a link do not agree on the IP subnet to which they belong
BD https://www.juniper.net/documentation/us/en/software/junos/is-is/topics/task/isis-verifying.html
B and C: ISIS does NOT use IP for adjacencies. So subnets are irrelevant. ISIS can even exchange routes without IP.