(I'm not sure if that could be relevant in any way. However, as I noted it, I thought I could as well report it ...) The zebra startup scripts heavily use /usr (binaries are in /usr/sbin, some of them linked with readline and ncurses which are in /usr too). If /usr is on a NFS server, these files are not yet available (S1[56] vs S25).
zebra is not necessary to boot up a diskless workstation, nor does it make much sense to run zebra on one. But if you need it anyways, I could move the zebra initscript to later in the startup sequence.
Well, I actually don't use zebra, I just noted this when checking that kind of things ... I don't really know when this should be started. If it's not necessary to start it before netfs, I'd move it. If there's a good reason to start it before, then this issue should definitely not be considered a bug because using zebra that way is not really useful (as you stated, and that's what I meant with "I'm not sure if that could be relevant").