4.1.e Describe IPv6 tunneling techniques

4.1.e [iii] 6RD

IPv6 Rapid Deployment (6rd) is a stateless tunneling mechanism which allows a Service Provider to rapidly deploy IPv6 in a lightweight and secure manner without requiring upgrades to existing IPv4 access network infrastructure. While there are a number of methods for carrying IPv6 over IPv4, 6rd has been particularly successful due to its stateless mode of operation which is lightweight and naturally scalable, resilient, and simple to provision.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4683-4686).  . Kindle Edition.

http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6553/whitepaper_c11-665758.html