2.5 Interpret basic operations of Rapid PVST+ Spanning Tree Protocol.
Objective 2.5 sits in Network Access, which carries 20% of the CCNA exam. The questions below are original, written from the official objective title above, and each explanation cites the Cisco page it rests on.
Objective title verbatim from the official objectives. Cisco exam page ↗
A worked example
Shown solved, with the whole explanation open: this is what every question here carries.
Two switches run Rapid PVST+ in VLAN 10 and no priority was ever configured on either.
SW1# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 32778
Address 0c29.3a11.4000
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0c29.3a11.4000
SW2# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 32778
Address 0c29.3a11.4000
Cost 4
Port 24 (GigabitEthernet1/0/24)
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 4c77.6d22.8000What settled the election?
The concept
The identifier a switch advertises for a VLAN is built from a configurable field and a hardware field, and it is compared as one number. When nobody has changed the configurable half, every candidate advertises the same leading value and the comparison falls through to the hardware half, which is unique per switch.
Why this answer
Both bridges advertise 32778, so the configurable half is identical and cannot separate them. The remaining field decides, and 0c29.3a11.4000 sorts below 4c77.6d22.8000, which is why that switch reports itself as the root and its peer reports a cost and a root port.
- APort count never enters the comparison. A switch with one link can be the root.
- BTiming has no standing here. Every switch claims the role at power-up and yields once it hears a better advertisement.
- CCost selects a root port on a switch that has already lost the election. It does not choose the winner.
- Correct. With the priorities equal, the lower hardware address wins.
Now you: objective 2.5 questions
No account needed. The explanation opens when you answer.
Sample question 1 of 2
An access switch has two uplinks into the distribution layer and one client-facing port in VLAN 20.
SW3# show spanning-tree vlan 20
VLAN0020
Spanning tree enabled protocol rstp
Root ID Priority 24596
Address 0c29.3a11.4000
Cost 4
Port 23 (GigabitEthernet1/0/23)
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------
Gi1/0/23 Root FWD 4 128.23 P2p
Gi1/0/24 Altn BLK 4 128.24 P2p
Gi1/0/5 Desg FWD 4 128.5 P2pWhat does the Gi1/0/24 role mean?
Sample question 2 of 2
An engineer set one non-default spanning-tree value on this switch for VLAN 40 and now reads the output back.
SW1# show spanning-tree vlan 40
VLAN0040
Spanning tree enabled protocol rstp
Root ID Priority 24616
Address 0c29.3a11.4000
This bridge is the root
Bridge ID Priority 24616 (priority 24576 sys-id-ext 40)
Address 0c29.3a11.4000What produces the 24616 figure?
That’s 2 of the full CCNA bank.
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Continue practicingRead the sources
These are the official pages the questions above cite. Reading them is studying the objective from the primary source, which is what the explanations point you toward anyway.