A reminder to myself and others who are scheduling upgrades to version 11 of the BigIP software: Although the matchclass (DevCentral login required) command is deprecated (but will still work as of v11.2.0), data groups referenced in your irules using the $::data_group_name syntax will result in TCL errors written to your ltm logs and Connection Resets to your end-users.
If you come across some error messages in the ltm logs like this:
MMM DD HH:MM:SS tmm err tmm[PID]: 01220001:3: TCL error: /Partition/the_offending_irule - can't read "::your_data_group": no such variable while executing "matchclass [IP::client_addr] equals $::your_data_group"
The quickest way to fix and restore service is to remove the '$::' from in front of your Data Group in your irule and then wrap it in double quotes. (For example, $::your_data_group becomes "your_data_group")
To avoid future matchclass incompatibilities, it might be wise to modify those rules using matchclass to the new class match functions. Migrating matchclass functions to class match syntactically looks to be just as easy—replace matchclass with class match (although I personally haven't done this yet in my rules using matchclass).