You are troubleshooting an issue where R1 is no longer receiving the default IS-IS route from R3. Referring to the exhibit, which action would you take to solve the problem?
A.
Delete the protocols isis ignore-attached-bit configuration statement on R1.
B.
Delete the protocols isis level 2 disable configuration statement on R3.
C.
Delete the protocols isis import configuration statement on R1.
D.
Delete the protocols isis ignore-attached-bit configuration statement on R3.
A is correct:
Ignore the attached bit on IS-IS Level 1 routers. Configuring this statement enables the routing device to ignore the attached bit on incoming Level 1 link-state PDUs. If the attached bit is ignored, no default route, which points to the routing device which has set the attached bit, is installed.
A seems to be correct, I never configured this but :
Ignore the attached bit on IS-IS Level 1 routers. Configuring this statement enables the routing device to ignore the attached bit on incoming Level 1 link-state PDUs. If the attached bit is ignored, no default route, which points to the routing device which has set the attached bit, is installed.
There might be times, such as during a denial-of-service (DoS) attack, that you do not want a Level 1 router to be able to forward traffic based on a default route.
To prevent a routing device from being able to reach interarea destinations, you can prevent the routing device from installing the default route without affecting the status of its IS-IS adjacencies. The ignore-attached-bit statement is used to tell the routing device to ignore the presence of the attached bit in Level 1 link-state PDUs, which blocks the installation of the IS-IS default route.
https://www.juniper.net/documentation/us/en/software/junos/is-is/topics/ref/statement/ignore-attached-bit-edit-protocols-isis.html
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