A network administrator is troubleshooting a connectivity issue for a host connected to switch SW1, which is part of a Rapid PVST+ topology. The administrator suspects a loop might be causing the issue. The 'show spanning-tree vlan 10' command output on SW1 shows the following for interface Gi0/1: `Port Gi0/1 (GigabitEthernet0/1) of VLAN0010 is designated forwarding` The administrator also observes that SW1's Bridge ID for VLAN 10 is 32778. SW2, connected to SW1 via Gi0/1, has a Bridge ID of 32778 for VLAN 10 as well. Both switches have the same default priority for VLAN 10 (32768 + VLAN ID). What is the most likely reason for Gi0/1 on SW1 being the designated port for VLAN 10, assuming no manual port priority or cost configuration?
- ASW2 has a higher MAC address than SW1.
- BSW1 has a lower MAC address than SW2.
- CSW1's Gi0/1 has a lower port cost than SW2's connected port.
- DSW1 has a higher port priority on Gi0/1 than SW2's connected port.
Show answer & explanationAnswer & explanation
Correct answer: B. SW1 has a lower MAC address than SW2.
When two switches have the same Bridge ID (due to identical priority and VLAN ID), and are connected by a link for which the root path cost is equal from both ends (as implied by 'designated forwarding' on SW1 and identical bridge IDs, meaning both are equally 'close' to the root or are the root themselves), the tie-breaker for determining the designated port is the MAC address of the sending bridge. The switch with the lower MAC address will become the designated bridge for that segment.
Why the other options are wrong
- A. A higher MAC address would cause SW2 to lose the designated port election to SW1, not win it.
- C. Port cost is considered before port priority but after Bridge ID; if costs were different, it would have been the deciding factor.
- D. Port priority is considered after Bridge ID and root path cost; it would be used if both switches had equal MAC addresses.
STP Designated Port Election
In Spanning Tree Protocol (STP), the designated port for a segment is the port on the designated bridge that sends BPDUs for that segment. The designated bridge is chosen based on a series of tie-breakers.
- Lowest Root Bridge ID wins.
- Lowest Root Path Cost to Root Bridge wins.
- Lowest Designated Bridge ID wins (for a segment).
- Lowest Port ID (port priority then port number) wins if all above are equal.
Memory trick: R.P.D.P. - Root, Path, Designated, Port (ID)