Redundancy is not covered by the PROFIBUS PA specification, but vendors have jumped in with their own solutions. There are several choices including these two from PI North America members:
1. MooreHawk provided a nice video showing how their solution works for both Foundation Fieldbus and PROFIBUS PA.
2. Siemens has a solution as well, but they did not provide a fancy video. So here is an animation from the PROFIBUS PA Certified Network Engineer class.