This question's explanation and your answers so far.
問90
頻出度★★★
Mさんの部署では，図に示すような，PC2台（PC1とPC2），サーバ1台とプリンタ1台を，ハブ1台と4本のケーブルa～dでつないだLANで構成されるシステムを使っている。
このLANに接続されているプリンタは，プリントサーバ機能を有しており，PCから直接プリンタに出力できる設定になっている。ある日，MさんはPC1からプリンタに出力を行ったが，出力できなかった。前日に同じ処理を行ったときは，出力できていた。
なお，各機器と各ケーブルの接続箇所については，ソケットの不具合は機器側の障害，コネクタの不具合はケーブル側の障害として切り分けられるものとする。また，障害が複数箇所で同時に発生する可能性はなく，PC1では障害が発生していないものとする。
M さんは，障害の原因を追及するため，PC2 からプリンタに出力できるかを確認した。また，PC1 と PC2 をハブにつないでいるケーブル c と d をハブ側で差し替えた場合，PC2 からプリンタに出力できるかをも確認した。どちらの場合も，PC2 から出力できるようであれば，障害の原因と考えられる構成要素は，LAN を構成する PC2，サーバ，プリンタ，ハブとケーブル a ～ d の八つの中に幾つあると考えられるか。
出典 平成21年度 春期 ITパスポート試験 問90
解説
問 90
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×Incorrectア
It is not zero. Output worked the previous day but fails today, so a fault must exist somewhere in the LAN. Even though the two checks show that PC2, the hub, the printer cable, and the printer are normal, cable c alone is not used for communication in either check 1 or check 2 (in check 2 as well, c stays attached on the PC1 side and PC2 sends its output through d), so it has not been confirmed to be normal. The candidate that remains untested is cable c, so the answer is not 0 but 1.
○イ
正解There are two premises: a fault occurs in only one place, and PC1 has no fault. Since output cannot be sent from PC1 to the printer, the cause must lie on the communication path from PC1 to the printer. That path is PC1 → cable c → the hub (the port PC1 is plugged into) → the printer cable → the printer. PC1 is excluded by the premise, and the server and the server cable are outside this path, so they are not candidates for the cause from the start. The starting candidates are therefore these four: cable c, the hub, the printer cable, and the printer.
In check 1, output succeeded over the path PC2 → cable d → the hub (PC2's port) → the printer cable → the printer, so PC2, the hub itself, the printer cable, and the printer on that path are all known to be normal. Two candidates remain: cable c and the hub port that PC1 had been using.
In check 2, cables c and d were swapped at the hub side and output from PC2 still succeeded. At that moment PC2's cable d is plugged into the port PC1 had been using, so that port is known to be normal as well. This removes the hub entirely from the candidates.
Cable c, on the other hand, stayed attached on the PC1 side in both check 1 and check 2 and was never used for communication, so it is left untested. In other words, the only candidate that remains untested is cable c, and the number of components that can be considered the cause of the fault is one. The correct answer is therefore イ.
×Incorrectウ
Two is the count you get if you stop after check 1. The path in check 1 is PC2 → cable d → the hub (PC2's port) → the printer cable → the printer, and it does not include the port that PC1 had been using. At that point, therefore, two candidates remain: cable c and PC1's port. However, in check 2 output from PC2 still succeeded after cables c and d were swapped at the hub side, which confirms that this port is normal and removes it from the candidates. In the end only one remains.
×Incorrectエ
Three is a number that barely uses the narrowing done by the two checks. The printer, the printer cable, and the hub were already shown to be normal the moment check 1 succeeded; the server and the server cable are outside the path from PC1 to the printer and were never candidates to begin with; and PC1 is stated in the premises to have no fault. The only component that can remain is cable c, so there is no room for three.
ここがポイント
Fault isolation eliminates candidates using two principles: a fault occurs in only one place, and every component on a path over which communication succeeded is normal. The cause always lies on the path where the trouble occurred, so devices and cables outside that path (here the server and the server cable) can be ruled out from the start.
The point of a swap test is to exchange parts and see whether the symptom moves with them, separating a fault on the part side from one on the side it connects to. Here, swapping the cables at the hub side tests the hub ports themselves and narrows the remaining suspicion down to cable c alone.