Bad. Upgrade the Root server first. There are very good reasons for doing so. Depending on how many servers are in your tree, start about two week prior to the upgrade and make sure that NDS is clean on all of the servers. Follow the directions and you should be okay. I would place any WAN linked servers into their own replica ring that did not inlcude the root server until the upgrade was over.
On your NW51 CD, there should be a Deployment Manager. It autoruns if you insert the CD in a Windows workstation. Run through that, prior to your upgrade, and it gives you both the needed upgrades, and the information you need to properly install NW51 in a 4.x tree. By the way, using NDS8 and NDS7 makes a big difference in choosing an installation strategy.
Good. Sp9 on all 4.11's. Perform dsrepair's and timesynchronization until everything is fine. Use deployment manager to check if your tree is pre-configured enough for NW5.1. Then Install NW5.1 server in the tree with NDS 7 (not 8 !). Add Sp2a and acouple of postfixes. Do dsrepair unattended + time synch. Make NW5.1 primary (or single) time server and master replica. Perform upgrade on the other NW4.11's. Don't upgrade to NDS 8 until you are clean of NDS6 (so clean of NW4.x). Check coolsolutions for more.
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First NetWare 5.1 Server in a 4.11 Tree
What were your results?