Trac supports a convenient way to refer to resources of other Trac servers, from within the Wiki markup, since version 0.10.
List of Active InterTrac Prefixes
Simply use the name of the other Trac environment as a prefix, followed by a colon, ending with the resource located in the other environment.
The other resource is specified using a regular TracLinks, of any flavor.
That target environment name is either the real name of the environment, or an alias for it. The aliases are defined in trac.ini (see below). The prefix is case insensitive.
For convenience, there's also some alternative short-hand form, where one can use an alias as an immediate prefix for the identifier of a ticket, changeset or report: (e.g. #T234, [T1508], [trac 1508], ...)