Sorry, I saw this and was planning on relpying but just didn't here yet.  First off I guess I would ask just how much more life you want to get out of the 4900s. You don't have much more time to use them and still be supported. But if you are OK with that then I would strongly suggest sticking with a consistent FOS version across entire fabrics. So if you are talking about mixing the 5100s and 4900s in the same fabric I would stick with the v6.4.3d. If the 5100s are going to be isolated in a different fabric then I don't see as compelling a reason to keep them at the lower version.  That being said, there is no good technical reason I have seen NOT to run a fabric with those different FOS versions at the same time. I just prefer not to since any really complex issues that might come up are easier to troubleshoot when you have consistency in the environment. Also, even if the switches are in separate fabrics I would likely keep the same versions on them unless there was something I really wanted to take advantage of in the newer FOS.  The most important consideration I can think of regarding the use of ANY FOS v7.1.x is that there is no longer support for McData mode fabrics, or even FC Routing native EX port connectivity to McData mode fabrics. It's one of those things you have to read the release notes carefully to find, but could have a huge impact if you aren't paying attention.
Hi friends,  I have 4 Brocade 4900 and 2 slightly newer 5100 switches connected using FOS 6.4.2 and I'm trying to stretch their use a bit by upgrading them to the latest possible FOS.  I found out that the 4900 supports FOS 6.4.3d and the 5100 supports the 7.1.1 version.  Does anyone have any experience or advice to what FOS to upgrade? I could go to version 6.4.3d, so each of my 6 switches runs the same version, or I could go even further with the two 5100 switches, but I would be running a mixed FOS 6 and 7 environment.  Recommendations anyone?
Upgrading some old switches to latest FOS. Which FOS?