JNCIP-SP: Juniper Networks Certified Professional – Service Provider (2021) — Question 27

You want to use IS-IS on a GRE interface where the underlying Layer 3 MTU is 1500.
Which statement is correct in this scenario?

Answer options

• A. IS-IS can be used because every IS-IS interface must be capable of transmitting packets at least as large as 1476 bytes, and the GRE header is 24 bytes.
• B. IS-IS can be used, but the networking device directly attached to the circuit must be capable of fragmentation.
• C. IS-IS cannot be used, but the router can enable a GRE key that serves the same function as IS-IS.
• D. IS-IS cannot be used because the IS-IS hello is not allowed to be fragmented and has the DF bit set.

Correct answer: B

Explanation

The correct answer is B because IS-IS requires that the networking device be able to handle fragmentation due to the overhead introduced by the GRE header. Option A is incorrect because while IS-IS interfaces must support a minimum packet size, the fragmentation capability of the device is still necessary. Options C and D are incorrect as they misrepresent the capabilities and constraints of IS-IS in this scenario.