ASBR reachability
LSA Type 4 knows a guy
Networking, in both senses of the word.
Okay, but what does it mean?
The joke separates two pieces of information: Type 5 describes external destinations, while Type 4 advertises inter-area reachability to an ASBR. Type 4 does carry useful reachability information; “knows nothing” is the personality joke, not the protocol definition.
Protocol reference: RFC 2328 (OSPFv2). NSSA reference: RFC 3101.