What value is primarily used to determine which port becomes the root port on each non-root switch in a spanning-tree topology? A. path cost B. lowest port MAC address C. VTP revision number D. highest port priority number E. port priority number and MAC address
Correct Answer: A
Explanation:
The path cost to the root bridge is the most important value to determine which port will become the root port on each non-root switch. In particular, the port with lowest cost to the root bridge will become root port (on non-root switch).
At the end of an RSTP election process, which access layer switch port will assume the discarding role?
A. Switch3, port fa0/1 B. Switch3, port fa0/12 C. Switch4, port fa0/11 D. Switch4, port fa0/2 E. Switch3, port Gi0/1 F. Switch3, port Gi0/2
Correct Answer: C
Explanation:
In this question, we only care about the Access Layer switches (Switch3 & 4). Switch 3 has a lower bridge ID than Switch 4 (because the MAC of Switch3 is smaller than that of Switch4) so both ports of Switch3 will be in forwarding state. The alternative port will surely belong to Switch4.
Switch4 will need to block one of its ports to avoid a bridging loop between the two switches. But how does Switch4 select its blocked port? Well, the answer is based on the BPDUs it receives from Switch3. A BPDU is superior to 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 specific case, all the BPDUs sent by Switch3 have the same Root Bridge ID, the same path cost to the Root and the same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). In this case the port priorities are equal because they use the default value, so Switch4 will compare port index values, which are unique to each port on the switch, and because Fa0/12 is inferior to Fa0/1, Switch4 will select the port connected with Fa0/1 (of Switch3) as its root port and block the other port -> Port fa0/11 of Switch4 will be blocked (discarding role).
