[...]
One would need to obtain a licence from all the copyright holders
concerned. According to advice, I am one of those copyright holders. And
that includes having a copyright interest in the code in the ldpd/ and
babeld/ directories of FRR, being code which depends explicitly and
heavily on the GPL code in the other directories and which can not be
compiled, comprehended or function without reference to the GPL source
code.
I'll admit to having trouble following.
Can you point at a specific example of a derived file which they're distributing under the BSD license, and the GPL-licensed file it was derived from?