[j-nsp] SPF with Pseudonode

[Alex]

Hello YiFeng Zhou
I have this book and the correct page number is 254, not 267.
You can also use Google books to illustrate what you are talking about
http://books.google.co.uk/books?id=NxIadsCKZxMC&printsec=frontcover&dq=the+complete+is-is#PPA254,M1
Back to your question:
At step (5) AMS tries to work out if he can reach Stockholm.00 from 
Stockholm.02. Of course, AMS can and the full cost of such path is 10+0=10. 
Then AMS is about to put path "Stockholm.00 via Stockholm.02, cost 10" into 
PATH list but another path "Stockholm.00 via Stockholm.00, cost 10" is 
already there, having been put there at step (3) after RANDOM choice (which 
was a mistake). Therefore, work on the path "Stockholm.00 via Stockholm.02, 
cost 10" is discontinued.
Hope this makes sense. I realise that figures 10.8 and 10.9 may have added 
to your confusion since they don't tally up with figures 10.3-10.6.
Rgds
Alex


----- Original Message ----- 
From: "zhuifeng0426" <zhuifeng0426 at gmail.com>
To: "juniper-nsp" <juniper-nsp at puck.nether.net>
Sent: Sunday, March 29, 2009 8:53 AM
Subject: [j-nsp] SPF with Pseudonode


> 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
> Mobile:+86 (0)15905171724
>
>
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