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.
Explanation:
First we should understand what will happen if nothing is configured (use default values). Because R is the root bridge so all of its ports will forward. D will need to block one of its ports to avoid a bridging loop between the two switches. But how does D select its blocked port? Well, the answer is based on the BPDUs it receives from R. A BPDU is superior than another if it has:
1. A lower Root Bridge ID
2. A lower path cost to the Root
3. A lower Sending Bridge ID
4. A lower Sending Port ID
These four parameters are examined in order. In this case, all the BPDUs sent by R have the same Root Bridge ID, same path cost to the Root and same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). If using default values, the default port prioritys value is 32 or 128 (128 is much more popular today), so D will compare port index values, which are unique to each port on the switch, and because port R2 is inferior to port R1 (the ports number of R2 is higher than that of R1, for example port Fa0/2 is inferior to port Fa0/1), D will select the port connected with port R1 as its root port and block the other port.
The problem here is port D2 is blocked for both VLAN 1 & 2 and that means we cant use the underneath link for load-sharing. The underneath link is just used in the case the above link fails.
Now as you can guess, the easiest way to load-share traffic across both trunks is decreasing the port priority on R2 for VLAN 2 on Catalyst R so that port D1 will be blocked for VLAN 2. Notice that decreasing here means make that port ID superior to the other port.