When routing neighbors don't form, check these in order:
Layer /: Can you ping the neighbor? Check cables and interfaces.
Subnet match: Are both routers on the same IP subnet?
Protocol parameters:
- OSPF: Area ID, Hello/Dead intervals, authentication
- EIGRP: AS number, K-values, authentication
ACLs and firewalls: Routing protocols use specific ports and multicast. Ensure they're permitted.
MTU mismatches: OSPF can stall at ExStart due to MTU differences.