Index Pages (page type 2)
Index pages (type 2) hold the non-leaf level(s) of the clustered index b-tree. The leaf level of the clustered index is just the underlying object's data pages themselves.

In the special case where the whole object (table in your case) fits in a single page, SQL Server does not create a separate index page, because there is no need.

In your example, the first index page is created when the page split results in the table consisting of more than one data page for the first time.

Index pages are also used for all levels of nonclustered b-tree indexes.

Index pages can also be inspected with DBCC PAGE. Dump style 3 is the most informative, as it shows the child file and page pointers for index key ranges. This is the information needed to navigate down (potentially multiple levels of) the b-tree index.

As the number of rows in the index increases, the number of levels in the index will also increase.

See:


Clustered Index Structures
Nonclustered Index Structures
Heap Structures
Table and Index Organization


Comprehensive information about table and index internals can be found in the Microsoft SQL Server Internals books by Kalen Delaney et al.

A new index page will be created whenever an existing index page needs to split. If that page was the root page (top of the index tree), the index gains a new level as a result. Index pages can split if the index key is widened, or if another entry is needed on that page (e.g. to refer to a new child page) and there isn't enough room.