[j-nsp] SPF with Pseudonode

[zhuifeng0426]

HI, list:
I have a question about SPF with Pseudonode. If you guys have the book named " The Complete IS-IS Routing Protocol", pls trun to page 267, if you don't have one, pls see my description as below:
Two Routers connected with both Ethernet and Serial links.

and here is the descripiton of book with 2 different SPF calculation methods:
1:The SPF calculation starts with moving all reported adjacencies to the UNKNOWN list. In this small, sample network there are six reported adjacencies between the three nodes. Next the calculating router (Amsterdam) puts all local adjacencies into the TENTative list (1). Both adjacencies pass the two-way check and the links are removed from the UNKNOWN list (2). Next, the Amsterdam router randomly decides to move a node from the TENTative to the PATH list, as both have equal cost. In the example, the Amsterdam.00 to Stockholm.00 element is moved onto the PATH list (3). (We will see later that this random decision was a mistake.) The immediate successors of Stockholm.00, which is now the node under consideration in the PATH list, are moved from the UNKNOWN list onto the TENTative list (4). Stockholm.02 passes the two-way check and its links are removed from the UNKNOWN list (5). Now the Amsterdam router realizes it already has a path to Stockholm.00 with a cost of 10, so this link is discarded (6). As there are no further ele- ments in the UNKNOWN list, the SPF calculation terminates and as a result just one path (the point-to-point link) is used between the two routers.


2:this time no  random decision is made when moving a node from the TENTative list to the PATH list. Steps (1) and (2) are processed exactly as in the previous example. The difference now is that the system prefers the pseudonode (3) when moving an equal cost node from the TENTative to the PATH list. The router knows that the pseudonode will connect to a real node with a cost of zero, and so is a path of interest. Next, the router evaluates immediate successors from Stockholm.02 and puts them onto the TENTative list (4). After passing the two-way check, the links are removed (5). Next, the router evaluates the TENTative list and moves Stockholm.00 onto the PATH list (6). The remaining node in the TENTative list has a cost of 0 to a node (Stockholm.02) that is already on the PATH list. After summing up 0 plus the cost to reach Stockholm.02, it turns out that there is another path at cost 10 to Amsterdam.00 available, and this one moved into the PATH list (7). The TENTative and UNKNOWN lists are empty, which is the terminating condition for the SPF calculation. The result this time is that both paths are available, which is the desired result.

I can understand method 2, but I can't understand method 1 at step six. why can't it add A.00-->S.02 into PATH list?
can anyone tell me why? Thanks in advance!
due to size problems, I can't sent pictures to you. if you have interested in this issue and do not have the book, pls add my MSN and we can discuss it further: zhuifeng0426 at hotmail.com
2009-03-29 




Best regards！         
YiFeng Zhou
Mail:zhuifeng0426 at gmail.com
MSN:zhuifeng0426 at hotmail.com
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