ccna(640-802)V104.4 296题求助

afrous · 发表于 2011-7-5 11:25:27

本帖最后由 fulltimes 于 2011-7-7 10:28 编辑

Q296
Refer to the exhibit. Each of these four switches has been configured with a hostname, as well as being
configured to run RSTP. No other configuration changes have been made. Which three of these show the
correct RSTP port roles for the indicated switches and interfaces? (Choose three.)

A. SwitchA, Fa0/2, designated
B. SwitchA, Fa0/1, root
C. SwitchB, Gi0/2, root
D. SwitchB, Gi0/1, designated
E. SwitchC, Fa0/2, root
F. SwitchD, Gi0/2, root
Answer: ABF

答案解释：
stp的问题，我们可以通过mac地址确定跟桥为switchC，那么C上的端口都为dp，A的f0/1，d的g0/2为rp，那
么B哪一个是RP呢。从D过来的bpdu的cost都为23，从a过来的为19+19=38，所以B的rp为gi0/1，那么阻塞
端口应该在A和B之间的网段，A的f0/2这个网段收到的BPDUcost最小为19，而B的g0/2收到的BPDUcost最
小应该为23，所以B的 g0/2应为BLK.1G链路的cost为4,100M链路cost为19.
100M链路cost为19.1G链路cost为4.

不明白的地方：
A到B网段上，A的f0/2 和 B的g0/2到 C的路径花费值，是如何计算出来的。谢谢啦……