[Isis-wg] integrated is-is question

[He, James]

To IS-IS fellows:

I was reading the book "CCIE Professional Development: Routing TCP/IP,
Volume 1" by Jeff Doyle from ciscopress. 
In the Integrated IS-IS chapter, there is an example which discusses IS-IS
adjacency issues. 
The following is the illustration from the book:

There are six L1/L2 routers in two areas, area1 and area2, and we assume
area2 is a level 2 area (IS-IS backbone).
The illustration is as follows:


 
Subnet1--------R1---------------Subnet2-------------R2------------Subnet3
		  |			           |
		  |			           |
		 R3                                        R4
		  |			           |	
 
|---------------------------------------------------------------------------
|
				|
				|
			           R5
				|
				|
	          Subnet4-------------R6------------Subnet5



The top section which consists of R1-R2-R3-R4 is area1 and the lower section
which has R5-R6 constitutes area2.
All routers are L1/L2 routers (the default configguration in cisco routers).

On page 655 of the book,  it says that R6 will form level 1 adjacency with
R5 as R5 is the only router in area2. 
However, R6 will form level 2 adjacency with every other router including R1
to R4.  

Is this statement true? R6 is not physically connected to R1 or R2. How can
it form level 2 adjacenct to them.
Why can't R6 be level 1 adjacenct to R1 if we adopt the same logic.  


Many thanks in advance. Please forward answer to jameshe@ctron.com.


James He