Your company has a campus network with two distribution switches (DS1 and DS2) each connected to two access switches (AS1-AS4). All switches run Rapid PVST+. The root bridge for VLAN 10 is DS1. Recently, users on AS2 (VLAN 10) report intermittent connectivity. You notice that AS2's root port for VLAN 10 is flapping between two uplinks to DS1 and DS2. The link from AS2 to DS1 is a 1 Gbps fiber, and the link to DS2 is a 1 Gbps copper. The cost of both links is 4 (default for 1 Gbps). The network administrator previously configured PortFast on all access ports but did not configure any other spanning-tree parameters. Which action should you take to stabilize the topology?
Trap 1: Disable PortFast on all access ports to force proper RSTP…
Disabling PortFast on access ports does not affect root port selection and would only slow down convergence for end devices; it does not solve the equal-cost path flapping.
Trap 2: Configure spanning-tree vlan 10 priority 4096 on DS1 to ensure it…
Lowering DS1's priority to 4096 is unnecessary because DS1 is already the root bridge (priority 32768 by default). This does not change the fact that both uplinks from AS2 have equal cost, so flapping continues.
Trap 3: Increase DS2's priority to 8192 to make it less likely to become…
Increasing DS2's priority to 8192 makes it even less likely to become root, but since it is not the root, this does not affect the equal-cost path selection from AS2; the tie remains.
- A
Disable PortFast on all access ports to force proper RSTP convergence.
Why wrong: Disabling PortFast on access ports does not affect root port selection and would only slow down convergence for end devices; it does not solve the equal-cost path flapping.
- B
Configure spanning-tree vlan 10 priority 4096 on DS1 to ensure it remains the root and that AS2 selects the uplink to DS1 as root port.
Why wrong: Lowering DS1's priority to 4096 is unnecessary because DS1 is already the root bridge (priority 32768 by default). This does not change the fact that both uplinks from AS2 have equal cost, so flapping continues.
- C
Increase DS2's priority to 8192 to make it less likely to become the root.
Why wrong: Increasing DS2's priority to 8192 makes it even less likely to become root, but since it is not the root, this does not affect the equal-cost path selection from AS2; the tie remains.
- D
Change the fiber link between AS2 and DS1 to operate at 10 Gbps to reduce its cost.
Changing the fiber link to 10 Gbps reduces its port cost to 2 (from 4), breaking the tie and making the path via DS1 the only best path. This stabilizes the root port and resolves the flapping.