Suppose we have 192.167.0.0/16 and 192.169.0.0/16 in NB. A lookup for 192.168.0.1 then generates a negative entry for 192.168.0.0/16 in SB. A new positive for 192.168.1.0/24 is added to NB. Now, a request for 192.168.0.1 should return 192.168.0.0/24. However, NB still returns null, and SB has the 192.168.0.0/16 large prefix as a negative, which will be returned. If an xTR installs the large prefix, it will not send requests for 192.168.1.0/24, thus completely masking/blackholing that prefix.