First, the essential ideas
- Rapid PVST+
- Cisco's rapid per-VLAN spanning tree runs a separate loop-free tree for each VLAN using rapid spanning-tree behavior. A physical link can have different roles in different VLANs.
- BPDU
- A bridge protocol data unit carries spanning-tree information between switches. Receiving these control messages is compatible with discarding ordinary user frames.
- Root bridge and bridge ID
- The root bridge is the reference point for one tree, not automatically the default gateway. The lowest bridge ID wins: compare priority including the VLAN system-ID extension, then the MAC address.
- Root and designated ports
- A non-root switch chooses one root port offering its best path toward the root. A designated port offers the best information onto its attached segment. These are roles, not physical cable types.
- Alternate and backup ports
- An alternate port offers another path toward the root through a different bridge. A backup port backs up a designated port on the same shared segment. This point-to-point triangle demonstrates alternate, not backup.
- Discarding, learning and forwarding
- Discarding does not learn source MAC addresses or forward ordinary frames. Learning learns source addresses but does not forward ordinary frames. Forwarding does both. IOS may display BLK for discarding.
- Path cost
- A numeric preference accumulated along a path to the root; lower is preferred. It is not a hop count or a measured delay. Explicit costs in this lab remove dependence on default speed-to-cost methods.
- Convergence and topology change
- Convergence establishes consistent roles after a change. A rapid topology change can cause MAC relearning. Exact packet loss and convergence time require measurement, not an assumption from the word rapid.
First choose the root of one VLAN, then work outward to choose each switch's best path and each segment's designated port. Finally ask what each port currently does with data. Keeping election, role and state separate makes a blocked-but-healthy link understandable.
See the idea
One VLAN, three links, one alternate
The physical triangle stays intact while one endpoint discards data. Losing SW3's direct link activates its path through SW2.
Baseline tree
SW1 is root. SW2 and SW3 use their direct cost-10 root links. SW3 Gi0/1 discards on the third physical segment.
What this model leaves out: All three lab switches and cables are shown. SVIs are on these switches, not hidden hosts. Highlighted links show predicted data paths, not measured frames.
Full visual explanation and references
Three switches form a VLAN 10 triangle. SW1 is root; SW3's alternate bottom link becomes its root path through SW2 when the direct link fails.
All three lab switches and cables are shown. SVIs are on these switches, not hidden hosts. Highlighted links show predicted data paths, not measured frames.
- Baseline tree
SW1 is root. SW2 and SW3 use their direct cost-10 root links. SW3 Gi0/1 discards on the third physical segment.
www.cisco.com - Direct link fails
Shut SW3 Gi0/0 only. SW3 Gi0/1 becomes its root port, reaching SW1 through SW2 with cumulative cost 20.
www.cisco.com - Original tree returns
Enable SW3 Gi0/0 again. Verify direct root cost 10, alternate Gi0/1 and restored SVI reachability.
www.cisco.com
One idea at a time
Start with something familiar.
Draw SW1 above SW2 and SW3, with all three sides connected. Write cost 10 at every port, then mark SW1 as the intended root and SW2 as the intended successor.
Why learn this?
A redundant physical design needs one loop-free forwarding topology. Removing a healthy alternate link to make every port appear forwarding throws away the recovery path you deliberately installed.
An everyday comparison
A building has several exits, but a temporary route plan directs normal movement through selected corridors while retaining a usable alternate.
What happens in a network
The selected corridors resemble forwarding ports and the retained alternate resembles a discarding alternate port. A route change can use that previously idle path.
Where the comparison stopsSpanning tree exchanges protocol messages and compares numeric information. It does not understand people, emergency signage or the shortest walking distance.
Follow one worked example
- Elect the intended root.
SW1's lower bridge priority beats SW2 and SW3 for VLAN 10. SW1 has no root port for this tree.
Why: Root identity is determined before comparing each non-root switch's paths.
- Compare cost and segment information.
SW2 and SW3 each prefer their direct cost-10 link to SW1 over the cost-20 path through the other switch.
Why: Lower cumulative cost wins before later tie-breakers are needed.
- Identify the alternate on the bottom link.
SW2's lower bridge ID wins the equal-cost segment contest. SW3 Gi0/1 becomes alternate/discarding in the stable baseline.
Why: One discarding endpoint prevents the physical triangle from becoming a data loop.
Your first small task
Use the explicit costs and priorities to mark both SW1 ports, SW2 Gi0/0 and Gi0/1, and SW3 Gi0/0 and Gi0/1. Then erase only the SW1-to-SW3 link and predict SW3's new root port.
Show a hint
Compare direct cost 10 with indirect cost 20, then use SW2's lower bridge ID on the bottom segment.
Check the expected result
SW1 ports are designated; SW2 Gi0/0 is root and Gi0/1 designated; SW3 Gi0/0 is root and Gi0/1 alternate. After failure, SW3 Gi0/1 becomes root.
Pause and explain it in your own words
Does an alternate port discard BPDUs as well as ordinary data?
Show a hint
How would the port know that its alternate path still exists?
Show the explanation
No. A functioning alternate must process spanning-tree information. Discarding describes ordinary forwarding and MAC learning behavior; it does not mean the control protocol is deaf.
Short answer
What should you understand today?
Rapid PVST+ keeps a separate loop-free tree for each VLAN. The lowest bridge ID becomes root; other switches select root ports using their best path toward it. Designated ports serve segments, while alternate ports can discard user frames yet remain useful recovery paths. Configure root intent explicitly, then verify actual root IDs, costs, roles and states. A blocked alternate is not automatically a broken cable.
Before you begin
Learning targets
- Understand VLANs, Ethernet loops and local SVI addressing.
- Have an existing CML 2.10 entitlement with three fresh IOSvL2 nodes.
- Know that read-only show commands describe operational state, not just intended configuration.
- Distinguish root election from root-port selection.
- Separate rapid spanning-tree roles from forwarding states.
- Configure and verify primary and secondary root priorities.
- Predict the alternate port using path cost and bridge-ID comparisons.
- Correlate role changes, traffic tests and actual logs during a safe fault.
Words you will meet
Open a word when you need its meaning. You do not need to memorize the whole list before reading.
Rapid PVST+
Cisco's rapid per-VLAN spanning tree runs a separate loop-free tree for each VLAN using rapid spanning-tree behavior. A physical link can have different roles in different VLANs.
BPDU
A bridge protocol data unit carries spanning-tree information between switches. Receiving these control messages is compatible with discarding ordinary user frames.
Root bridge and bridge ID
The root bridge is the reference point for one tree, not automatically the default gateway. The lowest bridge ID wins: compare priority including the VLAN system-ID extension, then the MAC address.
Root and designated ports
A non-root switch chooses one root port offering its best path toward the root. A designated port offers the best information onto its attached segment. These are roles, not physical cable types.
Alternate and backup ports
An alternate port offers another path toward the root through a different bridge. A backup port backs up a designated port on the same shared segment. This point-to-point triangle demonstrates alternate, not backup.
Discarding, learning and forwarding
Discarding does not learn source MAC addresses or forward ordinary frames. Learning learns source addresses but does not forward ordinary frames. Forwarding does both. IOS may display BLK for discarding.
Path cost
A numeric preference accumulated along a path to the root; lower is preferred. It is not a hop count or a measured delay. Explicit costs in this lab remove dependence on default speed-to-cost methods.
Convergence and topology change
Convergence establishes consistent roles after a change. A rapid topology change can cause MAC relearning. Exact packet loss and convergence time require measurement, not an assumption from the word rapid.
A root is elected separately for each VLAN
Rapid PVST+ runs per VLAN, so always name the VLAN when reading root output. Bridge priority is considered before the MAC-address tie-breaker. The displayed priority commonly includes the VLAN system-ID extension: configured 24576 with VLAN 10 appears as 24586. An IP address does not decide the election.
- Verify the actual root ID on every switch after configuration.
- The root bridge is not necessarily an IP gateway or the busiest device.
Cost selects a path; later tie-breakers resolve equality
A non-root switch compares received root information and cumulative root path cost. When candidates tie, sender bridge ID, sender port ID and finally local port ID can distinguish them. Do not jump straight to the lowest local interface number. The designated-port comparison also considers advertised root-path information.
- Cost is a preference, not a measured latency or traffic load.
- Explicit per-VLAN cost makes this example independent of short versus long default cost tables.
A role answers why; a state answers what happens
Root, designated, alternate and backup describe a port's function in the tree. Discarding, learning and forwarding describe data behavior. A designated port can temporarily be discarding during convergence. In a stable ordinary point-to-point tree, root and designated ports forward and alternate ports discard.
- Do not confuse an alternate port with an administratively shut interface.
- Learning learns source MAC addresses without forwarding user frames.
Rapid transition is conditional, not a stopwatch promise
Rapid spanning tree can coordinate safe transitions using proposal/agreement on suitable point-to-point links and can promote an alternate when the root path fails. Legacy neighbors and shared links can change behavior. Keep default timers in this lesson and verify the actual protocol and link type rather than tuning timers to chase a promised number.
- Do not apply PortFast to these inter-switch links to force quick forwarding.
- Rapid PVST+ changes convergence behavior, not the need for a loop-free final tree.
Use logs as corroboration, not as the whole diagnosis
Capture show spanning-tree vlan 10 and its detail output before and after the controlled event. Compare root identity, root cost, port role and topology-change information. Read show logging for relevant interface events and preserve the time context; absence of a retained message does not prove absence of a change.
- Read the actual log buffer and note clock accuracy or missing history.
- Root-primary/secondary configuration is directly practiced; whole-root-device failover remains a paper prediction.
Real-world walkthrough
The healthy link that looks blocked
A hypothetical technician sees a physically up campus link marked Altn BLK and plans to disable spanning tree to make it forward. The topology contains a redundant triangle. The port may be performing exactly the loop-prevention role required by the design.
- Name the affected VLAN and trace all three physical segments before changing state.
- Identify the actual root and compare it with the intended primary switch.
- Calculate direct and indirect costs, then compare the bridge IDs on the equal-cost segment.
- Confirm that the alternate port is physically up and processing protocol information.
- Use a maintenance-safe single-link failure to verify recovery, then restore and retest the original tree.
Guided GNS3 lab
Predict and recover a three-switch VLAN tree
Build an isolated cost-controlled triangle, set primary/secondary root intent, and record the alternate-path transition after a single uplink shutdown. Expected outputs are predictions, not author runtime evidence.
Addressing plan
| Device | Interface | Address | Purpose |
|---|---|---|---|
| SW1 | Vlan10 | 192.0.2.1/24 | Root-switch test endpoint |
| SW2 | Vlan10 | 192.0.2.2/24 | Secondary-switch test endpoint |
| SW3 | Vlan10 | 192.0.2.3/24 | Alternate-path test source |
Set up the lab
- Use exactly three fresh entitled CML nodes and the three declared links, without external connectors.
- Open each node's Console, enter enable, then show version and show running-config. Record the build and confirm no prior guards, filtering, EtherChannels or nondefault root priorities.
- Preserve a starting lab copy. Leave spanning tree enabled at all times and do not enable PortFast on any triangle port.
- Without entitled supported images, perform the role calculation on paper only; do not report a successful recovery or a measured time.
Build SW1 VLAN triangle
In CML, select SW1, open its Console tab and connect before typing. Use only the two named triangle cables, with no PortFast, guards, filtering or EtherChannel. Set equal per-port VLAN costs so the prediction does not depend on virtual speed defaults.
enable
configure terminal
spanning-tree mode rapid-pvst
vlan 10
exit
interface range GigabitEthernet0/0 - 1
switchport mode access
switchport access vlan 10
spanning-tree vlan 10 cost 10
no shutdown
endWhat each command means
enable- Enter privileged EXEC on the named lab node; stop if authorization is unavailable.
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
spanning-tree mode rapid-pvst- Select rapid per-VLAN spanning-tree operation.
vlan 10- Create the exercise's single data VLAN.
exit- Return to global configuration.
interface range GigabitEthernet0/0 - 1- Select exactly this switch's two inter-switch links.
switchport mode access- Use a single-VLAN access link on each side.
switchport access vlan 10- Place both links in the same VLAN 10 tree.
spanning-tree vlan 10 cost 10- Set the receiving port cost to ten on each link.
no shutdown- Enable the links while leaving spanning tree active.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- VLAN 10 has a redundant physical triangle with spanning tree enabled.
- Why this step matters
- Explicit equal costs make the later role prediction reproducible without pretending a virtual interface is measured hardware.
Build SW2 VLAN triangle
In CML, select SW2, open its Console tab and connect before typing. Use only the two named triangle cables, with no PortFast, guards, filtering or EtherChannel. Set equal per-port VLAN costs so the prediction does not depend on virtual speed defaults.
enable
configure terminal
spanning-tree mode rapid-pvst
vlan 10
exit
interface range GigabitEthernet0/0 - 1
switchport mode access
switchport access vlan 10
spanning-tree vlan 10 cost 10
no shutdown
endWhat each command means
enable- Enter privileged EXEC on the named lab node; stop if authorization is unavailable.
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
spanning-tree mode rapid-pvst- Select rapid per-VLAN spanning-tree operation.
vlan 10- Create the exercise's single data VLAN.
exit- Return to global configuration.
interface range GigabitEthernet0/0 - 1- Select exactly this switch's two inter-switch links.
switchport mode access- Use a single-VLAN access link on each side.
switchport access vlan 10- Place both links in the same VLAN 10 tree.
spanning-tree vlan 10 cost 10- Set the receiving port cost to ten on each link.
no shutdown- Enable the links while leaving spanning tree active.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- VLAN 10 has a redundant physical triangle with spanning tree enabled.
- Why this step matters
- Explicit equal costs make the later role prediction reproducible without pretending a virtual interface is measured hardware.
Build SW3 VLAN triangle
In CML, select SW3, open its Console tab and connect before typing. Use only the two named triangle cables, with no PortFast, guards, filtering or EtherChannel. Set equal per-port VLAN costs so the prediction does not depend on virtual speed defaults.
enable
configure terminal
spanning-tree mode rapid-pvst
vlan 10
exit
interface range GigabitEthernet0/0 - 1
switchport mode access
switchport access vlan 10
spanning-tree vlan 10 cost 10
no shutdown
endWhat each command means
enable- Enter privileged EXEC on the named lab node; stop if authorization is unavailable.
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
spanning-tree mode rapid-pvst- Select rapid per-VLAN spanning-tree operation.
vlan 10- Create the exercise's single data VLAN.
exit- Return to global configuration.
interface range GigabitEthernet0/0 - 1- Select exactly this switch's two inter-switch links.
switchport mode access- Use a single-VLAN access link on each side.
switchport access vlan 10- Place both links in the same VLAN 10 tree.
spanning-tree vlan 10 cost 10- Set the receiving port cost to ten on each link.
no shutdown- Enable the links while leaving spanning tree active.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- VLAN 10 has a redundant physical triangle with spanning tree enabled.
- Why this step matters
- Explicit equal costs make the later role prediction reproducible without pretending a virtual interface is measured hardware.
Address SW1 test endpoint
In CML, select SW1, open its Console tab and connect before typing. Use an SVI only as a local traffic endpoint. No inter-VLAN routing, default gateway or external network is needed for these same-subnet tests.
configure terminal
interface Vlan10
ip address 192.0.2.1 255.255.255.0
no shutdown
endWhat each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
interface Vlan10- Select the switch's logical attachment to VLAN 10.
ip address 192.0.2.1 255.255.255.0- Assign this switch's distinct same-subnet test address.
no shutdown- Enable the SVI, subject to active forwarding VLAN membership.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The SVI can operate when VLAN 10 has an active forwarding member.
- Why this step matters
- A traffic endpoint makes forwarding testable without adding an unlisted host or router.
Address SW2 test endpoint
In CML, select SW2, open its Console tab and connect before typing. Use an SVI only as a local traffic endpoint. No inter-VLAN routing, default gateway or external network is needed for these same-subnet tests.
configure terminal
interface Vlan10
ip address 192.0.2.2 255.255.255.0
no shutdown
endWhat each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
interface Vlan10- Select the switch's logical attachment to VLAN 10.
ip address 192.0.2.2 255.255.255.0- Assign this switch's distinct same-subnet test address.
no shutdown- Enable the SVI, subject to active forwarding VLAN membership.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The SVI can operate when VLAN 10 has an active forwarding member.
- Why this step matters
- A traffic endpoint makes forwarding testable without adding an unlisted host or router.
Address SW3 test endpoint
In CML, select SW3, open its Console tab and connect before typing. Use an SVI only as a local traffic endpoint. No inter-VLAN routing, default gateway or external network is needed for these same-subnet tests.
configure terminal
interface Vlan10
ip address 192.0.2.3 255.255.255.0
no shutdown
endWhat each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
interface Vlan10- Select the switch's logical attachment to VLAN 10.
ip address 192.0.2.3 255.255.255.0- Assign this switch's distinct same-subnet test address.
no shutdown- Enable the SVI, subject to active forwarding VLAN membership.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The SVI can operate when VLAN 10 has an active forwarding member.
- Why this step matters
- A traffic endpoint makes forwarding testable without adding an unlisted host or router.
Set the primary root
In CML, select SW1, open its Console tab and connect before typing. Configure root intent for VLAN 10 only. The macro changes priority based on current information; verify the resulting actual values.
configure terminal
spanning-tree vlan 10 root primary
end
show spanning-tree vlan 10What each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
spanning-tree vlan 10 root primary- Lower this switch's priority to make it the intended VLAN 10 root.
end- Return to privileged EXEC without saving or reversing the configuration.
show spanning-tree vlan 10- Compare this bridge ID with the root ID and read the local priority.
- What you should see
- In this fresh topology SW1 should become root with no root port.
- Why this step matters
- The command expresses intent, while the operational root comparison proves the election.
Set the secondary root
In CML, select SW2, open its Console tab and connect before typing. Configure the designated successor without changing the primary. This does not create a separately elected standby role in the protocol.
configure terminal
spanning-tree vlan 10 root secondary
end
show spanning-tree vlan 10What each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
spanning-tree vlan 10 root secondary- Set the secondary-root priority for VLAN 10.
end- Return to privileged EXEC without saving or reversing the configuration.
show spanning-tree vlan 10- Confirm SW1 remains root and SW2 has the intended lower-than-default priority.
- What you should see
- Expect SW2's direct Gi0/0 root port and Gi0/1 designated port.
- Why this step matters
- Secondary intent is meaningful only relative to every other eligible bridge ID.
Record the complete baseline
In CML, select SW3, open its Console tab and connect before typing. Wait for stable roles and record the outputs before creating a fault. Stop if the physical map or costs disagree with the predicted tree.
show spanning-tree vlan 10
show spanning-tree vlan 10 detail
show logging
ping 192.0.2.1What each command means
show spanning-tree vlan 10- Read root ID, root cost and the roles/states of both ports.
show spanning-tree vlan 10 detail- Record per-port detail and topology-change context.
show logging- Preserve retained messages and their time context.
ping 192.0.2.1- Establish successful reachability to SW1's SVI.
- What you should see
- Expect Gi0/0 root/forwarding and Gi0/1 alternate/discarding, with direct cost 10.
- Why this step matters
- A baseline separates an intentional recovery experiment from a pre-existing fault.
Remove only the direct root link
In CML, select SW3, open its Console tab and connect before typing. Shut Gi0/0 only after the baseline succeeds. Do not shut Gi0/1, change costs or disable the protocol during this experiment.
configure terminal
interface GigabitEthernet0/0
shutdown
end
show spanning-tree vlan 10
ping 192.0.2.1
show loggingWhat each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
interface GigabitEthernet0/0- Select SW3's direct cable to SW1.
shutdown- Introduce one bounded administrative link failure.
end- Return to privileged EXEC without saving or reversing the configuration.
show spanning-tree vlan 10- Observe Gi0/1 becoming the new root port.
ping 192.0.2.1- Retest the same SVI through the surviving two-link path.
show logging- Look for actual messages corresponding to the changed interface.
- What you should see
- Expect SW1 still root, SW3 root cost 20 and Gi0/1 forwarding after convergence.
- Why this step matters
- A root-port failure can change the path without changing the elected root bridge.
Verify the intermediate switch
In CML, select SW2, open its Console tab and connect before typing. Inspect the surviving path from its middle switch. This validates the actual SW3-to-SW2-to-SW1 interpretation instead of relying on one endpoint's ping alone.
show spanning-tree vlan 10
show spanning-tree vlan 10 detail
ping 192.0.2.3What each command means
show spanning-tree vlan 10- Check Gi0/0 root and Gi0/1 designated roles.
show spanning-tree vlan 10 detail- Correlate the new topology with available detail counters.
ping 192.0.2.3- Confirm the SW3 endpoint is reachable through the bottom link.
- What you should see
- Expect SW2 to retain its root and designated roles while carrying SW3 traffic.
- Why this step matters
- Traffic restoration and coherent roles on the intermediate device support the same diagnosis.
Restore and retest the original tree
In CML, select SW3, open its Console tab and connect before typing. Restore only Gi0/0, then wait for the cost-preferred direct path to return. Keep the same SVI destination for comparison.
configure terminal
interface GigabitEthernet0/0
no shutdown
end
show spanning-tree vlan 10
show spanning-tree vlan 10 detail
ping 192.0.2.1What each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
interface GigabitEthernet0/0- Select the interface disabled by this experiment.
no shutdown- Restore the direct SW1 link.
end- Return to privileged EXEC without saving or reversing the configuration.
show spanning-tree vlan 10- Verify Gi0/0 root and Gi0/1 alternate again.
show spanning-tree vlan 10 detail- Compare role and topology-change detail with the baseline.
ping 192.0.2.1- Repeat the original successful traffic test.
- What you should see
- Expect root cost 10 and the original alternate/discarding port after stabilization.
- Why this step matters
- Rollback is complete only when the intended tree and service both return.
Close the root-intent check
In CML, select SW1, open its Console tab and connect before typing. Confirm the primary stayed elected throughout this link-only test. On paper, predict SW2's election if SW1 were entirely absent; do not claim that separate event was executed.
show spanning-tree vlan 10
ping 192.0.2.3
show loggingWhat each command means
show spanning-tree vlan 10- Confirm the local bridge is still the actual VLAN 10 root.
ping 192.0.2.3- Verify reverse-direction endpoint reachability after restoration.
show logging- Retain any real link messages without inventing missing records.
- What you should see
- SW1 should still be root and SW3 should reply through the restored tree.
- Why this step matters
- Primary-root configuration and secondary priority are verified; whole-device failover is explicitly outside the runtime exercise.
Verify
- Match all six stable baseline roles to the actual three cables.
- Verify SW1's root ID on SW2 and SW3, and SW2's configured successor priority.
- Compare explicit direct cost 10 with indirect cost 20.
- During the fault, confirm SW1 remains root while SW3's root port changes.
- After restoration, require original roles and the original destination test to recover.
- Preserve real logs and note absent messages or unsynchronized clock information.
Troubleshoot
- If the wrong switch wins, compare full bridge IDs and actual configured priorities rather than hostnames or IP addresses.
- If the predicted alternate differs, inspect cable mapping, VLAN membership and per-port costs before considering tie-breakers.
- If a port is physically down, fix its actual connectivity; that is different from an up alternate/discarding port.
- If traffic does not recover, verify the surviving VLAN path and both endpoint SVIs. Do not disable STP to force forwarding.
- If the image lacks the documented mode or syntax, stop and label that branch unsupported; paper predictions are not runtime.
Use an existing Cisco Modeling Labs 2.10 entitlement and its bundled reference images inside CML only. GNS3 does not provide Cisco images; use images there only when a separate applicable license permits that environment. Do not share or redistribute image files. A CML subscription alone does not authorize exporting its Cisco images to GNS3. Without entitlement, use the explicitly labeled paper exercises; they do not establish hands-on competence.
- Leave SW3 Gi0/0 enabled and verify the restored role table before stopping the lab.
- Preserve desired root intent in the isolated lab only; no production timer, guard or mode changes are authorized.
Practice set
Answer before you reveal
01Which bridge wins when priority values differ?
The root election is not based on a switch's IP address, hostname or traffic volume.
02Why can VLAN 10 priority show 24586?
Compare like-for-like bridge IDs and distinguish the configured base priority from the displayed extended value.
03What is SW3's baseline root cost in this lab?
The indirect path adds SW3's receiving-port cost ten to SW2's advertised root cost ten.
04What does Altn BLK imply here?
The port can remain up and process BPDUs; an administrative shutdown is a different condition.
05Does root secondary guarantee the next root in every network?
Another bridge with a better ID can win, so the macro is not a reservation or a dynamic standby election.
06What evidence completes rollback?
Merely entering no shutdown does not establish that the desired loop-free forwarding topology recovered.
Knowledge check
Quiz: prove the reasoning
Keep these
Technical takeaways
- Name the VLAN before identifying its root and port roles.
- A lower bridge ID wins the root election.
- Path cost is cumulative and does not measure delay.
- An alternate/discarding link can be healthy and useful.
- Verify primary and secondary intent against actual priorities.
- Correlate roles, traffic and retained logs through fault and rollback.