Question : Create Relationship Fails

With SQL Server 2000,

TableA.ColA1 PK  ->  TableB.ColB1 FK  relationship with "Cascade Update Related Fields"  (rel 1)
TableC.ColC1 PK  ->  TableB.ColB2 FK  relationship with "Cascade Update Related Fields"  (rel 2)

No errors. But then I tried this:

TableC.ColC1 PK  ->  TableA.ColA2 FK  relationship with "Cascade Update Related Fields"  (rel 3)
this caused error "...may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints..."

Indeed, one may think that when TableC.ColC1 PK is updated, cascading may take 2 paths to TableB :  
1. rel 2
2. rel 3, then rel 1

However, rel 3 cascading only can update TableA.ColA2, which is not PK. Because TableA.ColA1 (the PK) was not modified by rel 3, further cascading through rel 1 is not possible. Thus, in reality there are NOT multiple cascading paths to TableB...

What am I missing?

Answer : Create Relationship Fails

It seems it is not possible for a table to appear more than once in the cascading tree :  

From SQL Server Book's online:

Multiple Cascading Actions
Individual DELETE or UPDATE statements can start a series of cascading referential actions. For example, a database contains three tables, TableA, TableB, and TableC. A foreign key in TableB is defined with ON DELETE CASCADE against the primary key in TableA. A foreign key in TableC is defined with ON DELETE CASCADE against the primary key in TableB. If a DELETE statement deletes rows in TableA, the operation also deletes any rows in TableB that have foreign keys matching the deleted primary keys in TableA, and then deletes any rows in TableC that have foreign keys that match the deleted primary keys in TableB.

The series of cascading referential actions triggered by a single DELETE or UPDATE must form a tree containing no circular references. No table can appear more than once in the list of all cascading referential actions that result from the DELETE or UPDATE. The tree of cascading referential actions must not have more than one path to any given table. Any branch of the tree is terminated when it encounters a table for which NO ACTION has been specified or is the default.