求助：VLAN在干道之间使用STP端口优先级达到负载均衡的问题

xzx1943

9. Refer to the exhibit. Catalyst R is the root bridge for both VLAN 1 and VLAN 2. What is the
easiest way to load-share traffic across both trunks and maintain redundancy in case a link
fails, without using any type of EtherChannel link-bundling?

A. Increase the root bridge priority (increasing the numerical priority number) for VLAN 2 on
Catalyst D so that port D2 becomes the root port on Catalyst D for VLAN 2.
B. Decrease the port priority on R2 for VLAN 2 on Catalyst R so that port D1 will be blocked
for VLAN 2 and port D2 will remain blocked for VLAN 1.
C. Decrease the path cost on R2 on Catalyst R for VLAN 2 so that port D1 will be blocked for
VLAN 2 and port D2 will remain blocked for VLAN 1.
D. Increase the root bridge priority (decreasing the numerical priority number) for VLAN 2 on
Catalyst R so that R2 becomes the root port on Catalyst D for VLAN 2.


Answer: B
求助，为什么这题是选B？

王晓强

这题是考察生成树根端口的选举。对CCNA来说，还是有点难度的。
根端口是指非根桥上离根“最近”的端口。
在题目的前提下，交换机R是根桥，交换机D是非根桥。
根端口选择的原则为：
1.离根桥最近的cost
2.不同发送方的BID，BID=Bridge 优先级+ Bridge MAC
3.相同发送方的PID，PID=Port 优先级+Port ID
这个题目的环境，只能使用第三个原则。
PortID已经固定好，因此只能修改Port优先级。
按照越低越优先的原则，要让交换机D的某个vlan在D1被block，就需要D2成为根端口。
D2成为根端口就需要D2收到的PID的Port优先级要小于D1，因此要修改交换机R上的R2端口优先级改小。

xzx1943

回复 2# andersen


    明白了，谢谢强哥....

zssw2008

学习了!!感谢

wangzhao428

谢谢 分享啊

SSDD126

学习学习。。。