1. http://www.informit.com/library/content.aspx?b=CCIE_Practical_Studies_I&seqNum=131
2. http://www.firewall.cx/networking-topics/routing/routing-protocols/186-routing-protocol-ospf.html
3. https://tools.ietf.org/html/rfc2328
4. http://overapi.com/static/cs/OSPF.pdf