6.2.2. Fallback

If a requestor detects that the remote end does not support EDNS(0), it MAY issue queries without an OPT record. It MAY cache this knowledge for a brief time in order to avoid fallback delays in the future. However, if EDNS0 is a requirement to carry information necessary for a transaction, no fallback should be performed as these transactions are likely to fail in the absence of EDNS0.