mediumMultiple Choice
350-401 Practice Question: A large enterprise is redesigning its campus…
A large enterprise is redesigning its campus network to support 5000 users across three buildings. The design must provide high availability and fast convergence in case of a link failure. The network engineer is considering using Spanning Tree Protocol (STP) in the access layer. What is the primary design concern with using STP in this scenario?
⚠ Common exam trap
Cisco often tests the misconception that STP is a suitable high-availability solution, when in fact its slow convergence and blocked link inefficiency make it a poor choice for modern campus networks; candidates may overlook the need for RSTP or MST in the design.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
STP will cause slow convergence and inefficient use of redundant links.
STP (802.1D) converges slowly, typically taking 30-50 seconds (listening + learning states) after a topology change. In a large campus network with 5000 users, this delay causes unacceptable downtime. Additionally, STP blocks redundant links to prevent loops, wasting bandwidth that could be used for load balancing. Modern alternatives like Rapid PVST+ (802.1w) or MST (802.1s) offer sub-second convergence, making classic STP a poor choice for high-availability designs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
STP will cause slow convergence and inefficient use of redundant links.
Why this is correct
STP (802.1D) prevents loops by placing redundant switch ports in a blocking state, leaving only one active path to a given root bridge. During a topology change, the listening and learning forward-delay timers (default 15 seconds each) plus the max-age timer can cause convergence to take 30–50 seconds, far too slow for high-availability designs. Meanwhile, the blocked redundant links remain physically connected but carry no user traffic, wasting available bandwidth and forcing traffic over a single, possibly congested path. This is why modern designs often use RSTP or link aggregation, which either converge faster or utilize all links in an EtherChannel.
- ✗
STP requires all switches to be in the same VLAN to function correctly.
Why it's wrong here
The claim conflates VLAN membership with spanning-tree operation. On Cisco Catalyst switches, PVST+ runs a separate instance of STP per VLAN, so switches can belong to completely different VLAN sets and still exchange BPDUs over 802.1Q trunks. A switch only needs to have at least one interface participating in the spanning-tree domain; it does not need to be in the same VLAN as every other switch. In fact, 802.1D runs a single common spanning tree across all VLANs, but even that instance requires no uniformity of VLAN membership—it simply requires that all bridges are connected by links that carry the relevant VLANs.
- ✗
STP cannot be used with 5000 users due to MAC address table limitations.
Why it's wrong here
STP is a Layer 2 loop-prevention protocol that depends on the number of switches and VLANs, not on the number of end hosts. A switch's ability to learn 5000 MAC addresses is a hardware resource limitation (CAM/TCAM capacity), completely independent of the spanning-tree algorithm. Modern enterprise switches support tens of thousands of MAC entries, so 5000 users would be well within normal operating limits. The misconceived idea that STP cannot be used with a given user count typically stems from confusing CPU/memory pressure from BPDU processing (which scales with switch and VLAN count) with MAC address table exhaustion.
- ✗
STP will cause broadcast storms in a three-building design.
Why it's wrong here
A broadcast storm is caused by loops in a bridged network: a broadcast frame is repeatedly flooded by switches that receive the same frame from multiple ports and re-flood it everywhere. STP is precisely the mechanism that prevents such storms by blocking redundant ports and creating a loop-free logical topology. In a three-building design with multiple inter-building links, STP blocks all but one forwarding path per VLAN, so a broadcast frame is never re-circulated. Without STP (or loop protection like loop guard/BPDU filter misconfiguration), those redundant links would indeed produce storms; STP itself is the cure, not the cause.
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Related to this question
Learn chapter
Network Access Control and AAA
Key term
High Availability in Enterprise Networks
High Availability in Enterprise Networks is the design principle that ensures network services remain accessible and operational with minimal downtime, even when individual components fail.
Key term
MSTP
MSTP (Multiple Spanning Tree Protocol) allows multiple VLANs to be grouped into a single spanning tree instance, reducing the number of spanning tree instances needed in a network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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