When you use a Windows 2000 member computer to perform administrative tasks on trusts in a Windows Server 2003 domain, instead of a trust type of Forest, the trust may appear as External or Unknown.
If the Windows 2000 member has NOT had Service Pack 3 (SP3) or greater applied, the trust type may be listed as External. If the Windows 2000 member has SP3 or greater applied, the trust type may be listed as Unknown.
NOTE: A Windows Server 2003 Forest trust is transitive to all domains in the trusted forest, but an External trust is non-transitive and applies only to the domain that is trusting or being trusted.
This problem occurs because the Windows 2000 administrative tools, specifically Active Directory Domains and Trusts (Domain.msc), does NOT correctly interpret the Forest trust type.
NOTE: Microsoft is aware of this problem. A fix is NOT currently available.