問題文
A receiver's designated router has just joined the shortest path tree for one sender of a group that it also receives over the shared tree, and copies from that sender are now arriving over both trees. The note has to record what the router does with the copies from the shared tree and how far the message it sends towards the meeting point reaches. Which statement is correct?
選択肢
- It drops the copies from the shared tree and sends a prune for that one sender towards the meeting point, and the prune travels all the way to the meeting point in every case, with the state cleared along the whole path.
- It drops the copies from the shared tree and sends a prune for the whole group towards the meeting point, and the prune stops at the first router that still needs that group for other receivers.
- It drops the copies from the shared tree and sends a prune for that one sender towards the meeting point, and the prune stops at the meeting point or at the first router that still needs that sender's traffic for other receivers of the group.
- It keeps forwarding both copies until the shared tree state times out and sends no prune at all, the meeting point stopping the feed of that sender down the shared tree as soon as it sees the join for the sender go by.