200-301 - Network Access - Section 2.5

Interpret the basic operation of Rapid PVST+ Spanning Tree Protocol, including root bridge and root port election, port roles and states (forwarding/blocking), and PortFast.

Determine which switch becomes the root bridge from the lowest bridge ID (priority then MAC address), then identify each switch's root port by lowest cumulative path cost and the designated and blocking ports that prevent Layer 2 loops. Recognise that PortFast moves an access port straight to forwarding for end devices, and that Rapid PVST+ converges faster than legacy 802.1D.

Rapid PVST+Root bridge electionRoot port and path costPort states (forwarding/blocking)PortFast

Practice question for this objective

Free sampleNetwork Accesshard

Three switches run Rapid PVST+ for VLAN 20. Their bridge priorities and MAC addresses are as follows: Switch Priority MAC Address SW1 32768 0011.1111.1111 SW2 28672 0022.2222.2222 SW3 28672 0033.3333.3333 Spanning tree adds the VLAN id to the configured priority to form the bridge priority value used in the bridge ID. Which switch becomes the root bridge for VLAN 20?

SW1  configured priority 32768  MAC 0011.1111.1111
SW2  configured priority 28672  MAC 0022.2222.2222
SW3  configured priority 28672  MAC 0033.3333.3333

ASW1, because 32768 is the IEEE default and the default-priority switch is preferred as root.
BSW3, because the higher MAC address 0033.3333.3333 wins the priority tie-breaker for the root role.
CSW2, because SW2 and SW3 share the lowest priority value and SW2 has the lower MAC address tie-breaker. Correct
DSW2, because adding VLAN 20 makes its bridge priority 28652, the lowest value present.

The root bridge is the switch with the lowest bridge ID, formed from configured priority plus VLAN id, with the lowest MAC as tie-breaker. Each switch builds its bridge ID from the configured priority plus the extended system id (the VLAN number). SW2 and SW3 reach 28692, lower than SW1 at 32788, so one of them is root; the equal-priority tie is broken by the numerically lower MAC address, which is SW2.

Why A is wrong: The default priority is not preferred; the lowest bridge ID wins, and 32768 + 20 is higher than the others, so SW1 loses.

Why B is wrong: The tie-breaker selects the lower MAC, not the higher one, so SW3 with 0033 loses to SW2 with 0022.

Why C is correct: SW2 and SW3 both have bridge priority 28672 + 20 = 28692, beating SW1; the tie is broken by the lower MAC, and 0022 is lower than 0033.

Why D is wrong: SW2 is the right switch but the arithmetic is wrong: 28672 + 20 is 28692, not 28652, so the stated reason is incorrect.

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