Refer to the exhibit. While troubleshooting a connectivity issue on router R2, a network engineer with an employee id:3876.13.497 notices that although it detects three OSPF links from R1, the OSPF…

Different MTUs can prevent OSPF adjacency from forming or cause packet fragmentation issues, but a missing IP address is a more fundamental reason for complete failure to establish an adjacency and exchange routes.

Loopback interfaces configured as stub on R1 would not prevent R2 from learning other OSPF prefixes advertised by R1 through its serial interfaces; loopbacks are typically used for router IDs and network advertisement, not directly for neighbor adjacency formation on serial links.

If R1's Serial 1/0 interface, which R2 is trying to form an OSPF adjacency with, is configured as part of the OSPF process but lacks an IP address, then a full OSPF adjacency cannot form, and R1 will not exchange its OSPF prefixes with R2, leading to missing routes in R2's routing table.

Mismatched subnet masks on directly connected serial interfaces will prevent OSPF adjacency from forming, similar to MTU issues, but a complete absence of an IP address on an interface is a more primary and absolute barrier to adjacency formation and route exchange.