Been a little busy with interviews and getting set for two of my last four
classes (Graduate Accounting and Strategic Management). Meanwhile just
read a cool article by Stretch at Packetlife. Give it a read:
By default, any network interface advertised by an IGP is considered active for that protocol. This means that the router will send information out that interface in an attempt to advertise routes or form adjacencies with neighboring routers. While this provides a bit of convenience to the administrator, it also presents a gaping security hole if neighbor adjacencies are not securely authenticated...