How MTU mismatches are addressed in the IS-IS neighbor-formation process.

D.

The correct answer is A. IS-IS checks the locally configured MTU against the MTU advertised in neighbor hello packets.

IS-IS (Intermediate System-to-Intermediate System) is a link-state interior gateway protocol used to exchange routing information within a single autonomous system (AS). It is based on the OSI reference model and operates at the OSI Layer 2 (Data Link Layer) and Layer 3 (Network Layer). When IS-IS routers are connected, they form neighbor relationships to exchange routing information.

The Maximum Transmission Unit (MTU) is the largest size of a data packet that can be transmitted over a network without being fragmented. If there is a mismatch in the MTU between two IS-IS routers, it can cause issues in the transmission of data packets, such as fragmentation and dropped packets.

To avoid these issues, IS-IS checks for MTU mismatches during the neighbor formation process. When an IS-IS router sends a hello packet to a potential neighbor, it includes the MTU value in the packet. The receiving router compares the MTU value in the hello packet to its locally configured MTU. If the MTU values match, the routers can form a neighbor relationship.

Therefore, option A is correct: IS-IS checks the locally configured MTU against the MTU advertised in neighbor hello packets. Option B is incorrect because the MTU value is not included in LSPs. Option C is incorrect because IS-IS does check for MTU mismatches. Option D is incorrect because IS-IS does not pad hello packets to accommodate for MTU mismatches.