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?
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.
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Correct link for the explanation is as below –
https://kb.juniper.net/InfoCenter/index?page=content&id=KB7848
B.
From the reference page:
Every interface that participates in an ISIS routing domain must be capable of transmitting packets at least as large as 1492 bytes. When sent over a GRE tunnel this means that the actual physical interface MTU has to support a minimum of 1516 bytes ( 1492 ISO + 24 GRE overhead ). Compare this to the default Ethernet MTU of 1500 and we come up 16 bytes short. Luckily this is a logical interface rather than a physical one and it is possible to simulate a larger MTU by relying on IP fragmentation to fit the GRE packet onto the lower MTU physical links.