Thanks to both of you. For full clarification, here's what I get with PFGW's "-tp" switch:

Code:

$ pfgw64.exe -f0 -a1 -tp -q"63998*297^21888-1"
PFGW Version 3.7.10.64BIT.20150809.Win_Dev [GWNUM 28.6]
No factoring at all, not even trivial division
Primality testing 63998*297^21888-1 [N+1, Brillhart-Lehmer-Selfridge]
Running N+1 test using discriminant 2, base 2+sqrt(2)
63998*297^21888-1 is composite (240.1252s+0.0018s)

I went ahead and used LLR to confirm that it's prime.